Long, long ago, there were no flowers.
It wasn’t until the Cretaceous period, some 130 million years ago, that a handful of renegade cone-bearing gymnosperms started protecting their naked seeds with a new structure. This new, flimsy bit of color was so successful at boosting pollination rates that it spread far and wide, making flowering plants (angiosperms) one of the most successful types of plant life on Earth.
That structure is the petal.
Of course, that’s a pretty big claim for such a delicate flap of plant tissue. Too frequently discounted as an unimportant fashion accessory to more vital, functional parts of plant anatomy, there is far more to a flower petal than meets the eye!
Before we get to the really astounding stuff, let’s make sure we know what we are talking about when we talk about petals.
What are petals?
You may be surprised to learn that petals are modified leaves. In fact, sepals, stamens, and carpels are all genetic twists on the leaf. As a modified leaf, a petal has a broad, flat area called a blade. At the narrow end, where the petal attaches to the plant, is the claw, which is very similar to a petiole, or leaf stem. Where petals are attached to one another is called the limb. The petals that make up a flower are called its corolla.
Just under a collection of petals is another set of modified leaves, called sepals. Sepals are usually green. When discussing the combined petals and sepals of a flower, it is called the perianth. When sepals and petals are indistinguishable from one another, they are called tepals. [Aloes and tulips are tepals.] Sometimes, sepals look more like petals than leaves. When that occurs, they are said to be petaloid.
Petals of parentage
The number of petals present in a flower, the way the petals are arranged, whether or not they are fused to neighboring petals, or how much they are fused, as well as color are used by pollinators to find the pollen and nectar they seek. We can use the same information to identify unknown plants
First, flowers with 3 or 6 petals tend to be monocots, while flowers with 4 or 5 petals, or groups thereof, are most often eudicots, though not always. Petal arrangement, or floral symmetry, can also help with plant identification:
Petals, in particular, evolved to protect the reproductive part of a flower and to attract or repel specific pollinators. We know that flowers come in every color imaginable, but did you know that they also feature colors we cannot see, with glowing flight lines, traffic patterns, and welcome mats? It’s true! Flowers exist to attract the type of pollinator that will help them to procreate their species. Not all pollinators are created equally. It is a waste of resources for a plant (or any living thing) to attract the wrong sort.
The story of floral scent
Orchids produce floral scent in specialized sacs, but most flowers get their scent from chemicals produced by the petals. While many of us, along with most insect and bat pollinators, find floral aromas appealing, herbivores and disease-carrying insects often disagree with that evaluation. Combined with the colors, petal arrangement, and floral placement, floral scent works to increase a flower’s chance at becoming pollinated and/or fertilized.
Did you know that plants use floral scent to communicate with each other? It’s true! The volatile chemicals that give a flower its fragrance trigger a behavioral response in a surprising number of neighboring life forms and no two floral scents are identical, sort of like snowflakes.
Sensing a reproductively fertile neighbor, another flower may shift its chemical production to attract pollinators of its own. On the other hand, a fertilized flower will often release ethylene, a ripening agent, to discontinue the scent so that local pollinators will turn their attentions to neighboring flowers in need of pollinating. Also, injured flowers produce different scents than those being chewed on by herbivores. We can’t see it or smell it, but it’s going on all the time.
The Daily Garden is all about plant vocabulary. Today, we are looking at overall plant anatomy because it can be difficult to talk about something if you don’t know the words.
By taking a closer look at plant anatomy, we will be better able to understand each other, we can get more out of plant descriptions, and be better able to identify those mystery plants that always seem to pop up in the yard.
Plant anatomy, or phytotomy, starts with simple descriptions of the outside and inside of plants. Remember those black-line masters from grade school used to teach parts of a plant? Well, let’s start there.
Basic plant systems
Plants are have two basic systems: roots and shoots, with the root system below ground and the shoot system above ground. Roots provide anchorage and often store nutrients. Roots can develop as a taproot or fibrous root system. Roots have hairs that absorb water and nutrients. The shoot system consists of vegetative parts (leaves and stems) and reproductive parts (buds, fruits, seeds, and flowers or cones). Let’s take a closer look at each of those parts.
Leaves are the sugar factories of the plant world, absorbing sunlight and converting it into sugar through photosynthesis. The wide, flat part of a leaf is called the blade, or lamina. The shape of the leaf blade is very useful in plant identification, as is the way those leaves are arranged along a stem and the pattern of veins within a leaf. The edge of the leaf is called its margin. Leaves are coated with a waxy protective cuticle. There are tiny holes, usually found on the underside of a leaf, called stoma, that allow plants to exchange gases with the environment and to regulate water flow within the plant. The stem that connects a leaf to a stem is the petiole. Leaflike structures seen at the base of the petiole are called stipules,
Stems support leaves, flowers and buds. These structures are attached at nodes. The spaces between nodes are called internodes. Herbaceous stems have waxy cuticles for protection while woody plants have bark. Stems contain a vascular system that consists of the xylem, phloem and may include a cambium layer. This system carries food, water, and minerals throughout the plant. That vascular system is arranged in a circular pattern in dicots and eudicots, while it is more scattered in monocots. Twigs are woody stems from the previous year. Branches are more than one year old and may have lateral stems. Trunks are the main stem of woody plants, such as trees and shrubs. Canes are a type of stem filled with spongy pith. Canes generally only live for a year or two. Modified stems occur both above and below ground. Bulbs, corms, rhizomes, and tubers, such as potatoes, are below ground modified stems. Crowns, spurs, and stolons are aboveground modified stems. Thorns are also modified stems, but rose thorns are not really thorns. They are prickles, which are modified epidural, or skin cells. Stubby stems, called spurs, produce fruit buds.
Buds are shoots that may develop into leaves or flowers. Buds are identified by their location on a stem: lateral buds are found along the sides of a stem, while terminal buds are found at the end. Lateral buds usually grow where leaves meet the stem and are called axillary buds. Renegade adventitious buds may show up at injury sites, on roots, or even at the edge of a leaf. The place where buds fall off leave a mark called a bud scar. Tree leaf buds have scales, while leaf buds of annuals and herbaceous perennials have delicate naked buds. Potato eyes are clusters of buds.
Fruits are ripened plant ovaries. Fruits can be simple (formed with one ovary), as in the case of stone fruits, or compound (formed with several fused ovaries). Compound fruits can be multiple or aggregate. Apples and other pomes are multiple compound fruits. You can tell by the 5-pointed star shape in the center of the fruit. Raspberries, which are drupes, not berries, along with pineapples and figs are formed by many flowers fusing together and are called aggregate fruits. By the way, strawberries are not berries, either. They are ripened receptacles. Berries, such as pumpkins, cantaloupes, cucumbers, eggplants, and tomatoes, all have many seeds inside an outer shell of varying thicknesses and hardnesses. Dry fruits, such as peas and beans, grow in pods that either open down a seam (dehiscent), or stay closed (indehiscent), as in the case of peanuts and most cereal grains, such as wheat and barley.
Seeds have three parts: the embryonic plant, stored food, called endosperm, and a protective seed coat. As temperatures rise and moisture is absorbed through the sed coat, a primary root, called the radicle, will emerge, followed by the first stem, or hypocotyl. First leaves, or cotyledons often look very different from adult leaves.
Flowers exist solely to attract pollinators. Only angiosperms make flowers. Gymnosperms, such as conifers, ginkgo trees, and cycads make cones, or strobili. The colors, patterns, showy displays, and sweet aromas we associate with flowers are all in place to attract insects, bats, and birds. Flowers are supported by a stem called a peduncle. Small green leaf-like structure, called sepals, are often seen at the base of a flower. A collection of sepals is called a calyx. Individual petals may produce nectar or scent. All of the petals together are called the corolla. The combined corolla and calyx are called the perianth. The tip of a flower stalk, called the receptacle, contains the plant’s reproductive organs. Flowers can be male, female, or both, though not always at the same time. The female part, or pistil, consists of a pollen-receiving stigma, supportive style, and the ovary. [When you eat saffron, you are eating the style and stigma of an autumn crocus flower.] The male part, or stamen, consists of a pollen-producing anther and a supporting filament. Flowers are very useful in plant identification.
Genetic research and electron microscopes have brought plant anatomy to exciting new levels. Assumptions about kinship have been wrecked asunder and colorized scanning electron microscope (SEM) images can be breathtaking. Different types of plant cells gather together to create tissues. Those tissues come together to create the functional parts of a plant.
Ultimately, all those functional parts grow into delicious, nutritious foods that we can cultivate in our yards for decades. For me, feet up in the yard with a nice glass of wine beats standing in line at a grocery store any day!
What do wedges of citrus, hard walnut shells, the white bits inside a pomegranate, and the paper coating around avocado pits have in common?
They are all endocarps.
How can this be? How can structures so very different be the same part? Let’s find out by starting with some basic fruit facts.
The fruits and seeds we eat are plant ovaries. When a flower is pollinated and fertilized, three new structures form: seeds, pericarp, and placentae. Embryonic seeds attach to the placenta, and pericarp begins to grow, to feed and protect the embryonic seed, and to attract seed-spreading herbivores. There are three different types of pericarp tissue: exocarp (outer skin), mesocarp (flesh), and endocarp (inner layer). So, endocarp is the interior fruit that surrounds seeds. But what about all those differences?
Types of endocarp
Endocarp is generally not fruit in the way you would expect, unless you are talking about peppers or citrus. The fleshy parts of sweet peppers and chili peppers is the endocarp, as are those membranous wedges of fruity goodness found inside lemons, limes, and oranges. If you look inside an apple, the endocarp is the hard clear plate-shaped bits close to the seeds.
If you take a close look at a stone fruit, such as a nectarine or cherry, the endocarp is very hard and inedible. To us, it looks more like the shell of a nut. And guess what? The hard outer shell of walnuts, pecans, and almonds, that shell is the endocarp, even though, to us, it looks as though it is on the outside.
Confused? Read on!
Nuts about endocarp
When a nut develops on a tree, the exterior rarely looks like what you see in the grocery store. Many nut species have smooth or furry green exteriors (exocarp). That exocarp coats a hard, familiar shell. That shell is the endocarp of a nut.
Stamens are the male aspect of a flower.
Flowering plants, or angiosperms, have flowers that can be male, female, or both, though not usually at the same time.
The word stamen comes to us from the Latin word for ‘thread’. This is because the stamen is a threadlike stalk, called a filament, which has a pollen-producing anther on top. The stamen usually surrounds the female part, or pistil, though not always
Different plant families have different arrangements of pistils and stamens. For example:
When eating edible flowers, it is a good idea to remove the stamen and pistils and just eat the petals and other parts. The only exceptions are violas and Johnny-jump-ups. In these cases, the other parts add good flavor. Saffron threads are the dried [female] styles and stigmas of a specific crocus flower species, not the stamen.
Melons, zucchini and other squashes can easily be hand-pollinated by breaking off a pollen-carrying stamen and touching each of the flowers flowers with it.
Now you know.
You’ve heard of tannins, but what are they?
The word tannin comes to us from Medieval Latin and it refers to oak bark. Oak, chestnut, and other tanbarks were used in tanning leather. Now, I do not mean some cow slathered itself with cocoa butter and lounged on the beach. Hardly. The process of tanning a raw animal skin and converting it into durable leather requires a lot of hard work and some powerful chemicals.
Tannins are large acidic molecules that bind to and alter proteins, which is why they were used in tanning leather. Tannins also bind to starches, minerals, and cellulose. This binding action slows decomposition. You may have seen ponds in forest environments with brownish water. That brown color is likely caused by tannins leaching out of nearby plants and into the water. In the plant world, tannins are used as pesticides, to protect against predators, and to regulate growth.
Plants produce tannins to make themselves less palatable and harder to digest. This discourages feeding by some herbivores. To counteract the presence of those tannins, some plant eaters have evolved to include a tannin-binding protein in their saliva. [Isn’t the world amazing?] The latex produced by dandelions contains tannins.
Tannins as growth regulators
Tannins also have antimicrobial and allelopathic actions. Allelopathy is a type of plant chemical warfare in which one plants releases chemicals that inhibit the growth of neighboring plants. This growth regulation can occur by reducing the available nitrogen or oxygen in the soil, killing nearby beneficial soil microorganisms that support plant life.
If you bite into an unripe fruit, it is the tannins that cause your mouth to pucker. As fruits approach maturity, the level of tannins decreases. Many popular garden plants contain tannins, to one degree or another, including:
In autumn, when leaves turn color, the golds and yellows you see are the result of tannins.
Now you know.
Botanical stigmata are part of the female reproductive system.
Tiny stigmata may not grab your attention at first glance, but maybe they should.
Before we learn why, let’s do a quick review of flower anatomy.
Stigmas and pollination
Carried by insects, bats, or wind, pollen is received at the stigma by sticky, specialized cells (stigmatic papillae). Once the pollen has been captured, the stigma, which is often quite moist, helps to rehydrate the pollen after its lengthy travels. Once hydrated, the pollen grain germinates, sending a pollen tube down the style to the ovary. To ensure that the proper pollen is collected, stigmas have evolved some very fancy attraction and capture methods.
You may be surprised to learn, as I was, that high temperatures, usually above 104°F, for 2 or more days prior to pollination, can exhaust the stigma of tomato plants to the point they cannot capture pollen. This may explain why, during particularly hot summers, we see lots of tomato blossoms, but no fruit. High temperatures (above 100°F) also reduces pollen germination.
Besides being sticky, stigmas use various shapes, flaps, and hair arrangements to help ensure that the correct pollen is captured and all others are rejected. These shapes can be simple tubes, truncated tubes, threadlike, bulbous, conical, lobed, feathery, hairy, beaked, fan-shaped, brush-like, leaflike, or disc-shaped. The familiar threads found on ears of corn, called silk, are stigmata.
How many different stigmata shapes are there in your garden?
When you cut flowers for a bouquet, you are generally cutting the peduncle. Peduncles are simply flowering stems, but they may surprise you.
Peduncles can occur in plants without stems, they may continue to grow indefinitely, and some peduncles grow underground.
The peduncle of a simple flower is easy to recognize. It is the classic stem you hold, cut, or put into a vase to admire. Its job is to support the flower. An artichoke stem is a peduncle, and for the same reason.
Now you know.
A flower is a flower, unless it is a bunch of flowers growing on the same stem, then it’s an inflorescence.
Anatomy of an inflorescence
A singular flower appears at the end of a stem, called a peduncle, nestled in a (normally) green cup, called the receptacle, and surrounded by modified leaves, called sepals. When there are multiple stems or branching stems (rachis), or flowers that occur on a disk, it is an inflorescence. The stalks of individual flowers within an inflorescence are called pedicels. These flowers are called florets, and their leaves are called bracts.
Types of inflorescences
Inflorescences can be determinate or indeterminate. The oldest flowers of a determinate (cymose) inflorescence are found at the end of the stem, as other flowers bloom in succession, down the stem, with the youngest flowers at the base. Indeterminate inflorescences are just the opposite, with older flowers at the base and younger flowers occurring closer to the tip.
There are also catkins (mulberry), spadix (cobra plant), and many subdivisions of each category, but this is a good start.
When an inflorescence produces fruit, such as sunflower seeds, it is called an infructescence.
Now you know.
We’ve all heard of a “hill of beans”, but did you know that beans have hilums?
Beans, peas, and other legumes produce fruits, called pulses, in pods. If you look closely, you can see where the seed attaches to the pod. Once the fruit or seed is mature, the pod opens along a seam, which means they are dehiscent. After the pod opens, the seeds fall to the ground where they are protected by a hard, water-resistant seed coat.
Seed coats have scars. When the seed separates from its pod, one scar is formed. This scar is called the hilum. On beans, the hilum is called the “eye”. Another scar, called the raphe, is a seed’s bellybutton. This is the scar that forms when the seed was separated from its placenta, within the pod. If you look even closer, you can see a tiny opening, called the micropyle, at one end of the hilum. This opening is where water is absorbed to allow germination to occur.
Chestnuts have hilums, too.
Now you know.
Galls are the warts or tumors of the plant world.
Not really. Galls are neither warts nor tumors, but that’s how many of them appear. The word gall comes to us from the Latin galla, for ‘oak-apple’. Oak apples are not fruits. They are a plant’s reaction to the presence of a foreign substance. The study of plant galls is called cecidology [see-SID-ology]. Most commonly associated with baseball-sized knobs seen on oak trees, galls come in all sizes, and can be found on a variety of plants.
Galls are swellings that occur in response to invasion. That invasion may be in the form of bacteria, fungi, insect larvae, eriophyid mites, nematodes, or other pests, and even other plants. Mistletoe is one example of a gall-forming plant. Unlike fungal cankers, which involve plant tissue death, galls, fungal or otherwise, are cases of extra tissue growth.
Galls are nearly always woody knobs that may occur on stems, branches, roots, buds, petioles, flowers, fruits, or leaves. Galls may be simple, with a single chamber (unilocular), or highly complex, with many chambers (plurilocular). Galls can also look like a sphere, a saucer, pineapples, pinecones, pouches, pods, or fantastic, tiny red spikes. It just depends on the host plant and the cause of the gall.
Where they occur and what they look like inside can tell you a lot about what caused it.
If you cut a gall open, you will see distinctly arranged vascular tissues, depending on the cause of the gall, and an enlarged cambium layer. These distortions interfere with the flow of water and nutrients, which can lead to wilting and stunting. Or, you may see a large, open area, perfect for use as a larval nursery, with no noticeable impact to the host plant.
Insect, mite, and nematode galls
When insects invade a plant, the galls that form are built by the insect. These galls can act as food or shelter for the insects. This is different from the plant-produced domatia (tiny apartments) found in some thorns for beneficial insects. Insect galls are made when an insect injects chemicals (pseudo plant hormones) into the plant, causing the gall to develop. Very often, eggs are laid in these galls, providing developing larvae with food and protection. Gall wasps, sawflies, gall flies, scale insects, some aphid species, weevils, psyllids, and gall midges can all cause insect galls, but it is nearly always gall wasps or gall midges.
Nematodes are microscopic round soil worms that can cause small galls on roots. Root knot nematodes are one such pest. In each of these cases, the gall is made up entirely of plant tissue, unlike fungal and bacterial galls, which incorporate fungal or bacterial tissues, respectively. Insect galls may also house interlopers, technically called inquilines.
When a fungi infects a plant, it grows alongside plant cells, creating swollen areas that can develop into galls. Several varieties of rust can cause galls to form. When these galls form on conifers, as in the case of cedar apple rust, the galls look like gelatinous fingers, called telial horns.
Fungal galls on other types of leaves tend to look more spherical.
Bacterial and viral galls
Bacterial and viral galls develop because the bacteria or virus reprograms plant cells into producing more bacteria or viruses, or other supportive cells. When galls are found at or just below the soil level, it is, most likely, crown gall. Crown gall is a bacterial disease that can occur on blackberries, sunflowers, grapes, and roses, along with almond, apple, apricot, cherry, and pear trees.
Galls on roots may mean clubroot, a disease caused by an entirely separate group of parasites, known as Phytomyxea. Root galls may also mean the presence of beneficial, nitrogen fixing Rhizobium bacteria.
Galls have long been used in tanning, to make ink, and as astringents. Most galls contains high levels of tannic acid and resin. There even a few edible galls, but that is beyond my skill set.
Sometimes, what looks like a gall is actually caused by herbicide overspray.
Galls are most commonly formed when plant tissue is new and undifferentiated. This meristem tissue is most often seen in spring, so that’s when you should start looking for galls. Once gall development begins, the tissues have been reprogrammed and cannot go back to normal.
In a word, you can’t. Insect and mite galls rarely harm plants and you can’t completely control these pests anyway. Once they are inside the plant, there is nothing you can spray or apply that will even reach them. Anyway, the gall is already in place. Fungal and bacterial galls may, possibly, if you are really lucky, and can time it perfectly, be prevented or reduced with fungicide treatments. Or not.
If you are galled by galls, take them off. Otherwise, recognize that galls are just another amazing aspect of playing with plants.
Boron isn’t nearly as boring as it sounds, once you know what it does for your plants.
Members of the cabbage family use a lot of boron, while peas and beans, peppers, and sweet potatoes need very little. Before you start adding boron to your garden soil, let’s take a closer look at what this element does for and to our plants.
Boron (B) is a micronutrient. In the world of plant food, micronutrients are only used in tiny amounts, but they are very important to plant growth. The optimal range for boron found in a soil sample is 0.1-0.5 parts per million (ppm). The only way you can determine how much boron is in your soil is with a laboratory soil test. Take my word for it, it’s the best investment you can make in your garden, next to mulching. But back to boron.
How do plants use boron?
Boron is critical for cell wall development and function, making those cell walls both strong and porous. The plasma membrane that allows molecules of sugar, water, wastes, and gases to move in and out of a cell rely heavily on boron to function properly. Research has also shown that boron is used by plants to produce and transport sugars within the plant, in protein synthesis, seed and pollen grain development, pollen tube growth, and flower growth and retention. Boron also plays important roles in nitrogen metabolism and fixation, the accumulation of the chemicals that affect taste (phenols), and in root development.
Boron, is most easily absorbed when soil pH is 5.5 to 7.5. First absorbed through root cells, boron then moves into the xylem, where it is taken to new leaves and shoots, or into the phloem, where it is taken to reproductive tissues, as well as vegetative tissues. Once boron is absorbed by a plant, it stays where it was placed. This is because boron is not a mobile plant nutrient. This is useful information because it means boron deficiencies will tend to show up in new growth before being seen in older leaves.
Helping plants get the boron they need
Boron is commonly leached out of the soil, leading to deficiencies, in areas with heavy rainfall. In drought-prone regions with very little rainfall, boron can build up in the soil, leading to potential toxicities. This is especially true for alkaline soil, or when too much fertilizer has been applied. [Just because a plant looks unhealthy does not mean it needs more food.]
Nutrient imbalances can make it difficult for a plant to absorb the nutrients it needs, even when those nutrients are present in the soil. For example, too much potassium in the soil can interfere with a plant’s ability to absorb boron, along with several other important nutrients. [The optimal range for potassium is 100-160 ppm.] Calcium and boron ratios are also very important to plant health. We will take a closer look at a tool, called Mulder’s Chart, that shows how these interactions work, in my next post. For now, we will look at what too little or too much boron can do.
Boron toxicity occurs when boron levels are at or above 1.8 ppm. Too much boron negatively impacts plant metabolism, and it reduces root and shoot development, chlorophyll production, rates of photosynthesis, and the lignin and suberin needed for structure and protection.
Toxic levels of boron can often be identified by looking at plant leaves. Too much boron will appear as either necrosis (death) or chlorosis (yellowing) of leaf tips and edges (margins). These damaged areas are believed to occur because the overabundance of boron interferes with several life processes, all at the same time. Unfortunately, these are the same symptoms as caused by magnesium deficiencies. [Can you say laboratory soil test?]
Adding extra boron is easy, when more is needed. Getting rid of excess boron requires more effort in the form of improved drainage through the addition of more organic material. Obviously, this takes time.
Insufficient or unavailable boron in the soil is the world’s most widespread micronutrient deficiency. It is a common problem in soils with low levels of organic matter (<1.5%). Boron deficiencies lead to reduced crop size and quality but symptoms can vary, depending on the crop:
Too much, too little, or no way for plants to get to the boron they need can all cause problems. Getting a laboratory soil test is the only way to know what’s eating your plants, or rather, what your plants are eating.
Lenticels are porous tissues used in plant respiration.
Plant respiration involves exchanging oxygen, carbon dioxide, and water vapor as part of photosynthesis and other cellular functions to generate or release energy.
The words ‘lenticel’ and ‘lenticular’ refer to the more common lentil-shape of these openings, but theses raised areas can be round, oval, or elongated. In some cases, such as silver birch, lenticels appear as horizontal cracks.
There are two types of lenticels: those found in the stems, trunks, and roots of woody plants and trees, and those found in the skin of certain fruits, such as apples.
Many apples and pears, in particular, have fruit skin lenticels. These are the tiny nicks of color seen on the skin. These lenticels start out light colored and then darken as the fruit reaches maturity and is ripe for picking. This darkening occurs because of the formation of cork cells. These openings are often the site of failing stoma, broken off trichomes, or other points of early damage, rather than planned growth. The number of lenticels seen on pome fruits can vary by species and by the availability of water during early development.
Bacterial and fungal disease can enter the fruit through these openings. There is a global skin disorder of pome fruits, called ‘lenticel breakdown’, in which 1-8 mm pits develop at the lenticels just after processing.
Trees and other woody plants have lenticels in their bark (periderm), both above and below ground. These openings facilitate the necessary exchange of oxygen, carbon dioxide, and water vapor. Since different species have uniquely shaped lenticels, knowing the characteristic shape of a tree’s lenticels can help in identification
Trees growing in low oxygen environments, such as mangroves, have lenticels on specialized roots. Grapes, on the other hand, have lenticels on their pedicels, or flower stems. Grape lenticels react to changes in temperature, rather than oxygen levels.
Did you know that potatoes have lenticels?
Now you know.
Where are your tree’s roots? Are they deep-rooted or shallow-rooted? What difference does it make?
We used to think that all trees had huge taproots that went deep into the soil. Then, we thought some trees had evolved shallow root systems that spread out through the upper layers of soil. We were wrong. There are no “deep-rooted” or “shallow-rooted” trees, per se.
Tree roots are not genetically programmed to move in a specific direction. Instead, they move through the soil the way ants scout for food - first one way, then the other. Tree roots are opportunistic, going wherever the food, water, beneficial microorganisms, warmth, and oxygen are available.
How much is root?
A single tree can have hundreds of miles of roots, and hundreds of thousands of root tips. Approximately 20% of a tree’s weight is found in the root system. That means an 80’ hardwood tree, with a 24 inch diameter, which weighs approximately 10 tons, can have 4,000 lbs. of roots. [Your average new car weighs 2,871 lbs.] That’s a lot of roots.
Function of tree roots
Tree roots serve several functions. They anchor the tree in place. Tree roots absorb and store water, oxygen, and nutrients from the soil, compete with neighboring plants, and maintain incredible relationships with beneficial fungi and bacteria found in the soil. You may not think of tree roots when it comes to photosynthesis, but you should. According to Thomas O. Perry, in a report published by the Harvard Arboretum, tree roots produce produce nitrogenous compounds that are essential to photosynthesis. Knowing what your trees’ roots are doing, and what they need, can help you keep your trees healthy and productive. But it all starts with the first root.
The first root
The first root that emerges from a tree seed, be it an acorn, peach pit, or pine nut, is called the radicle. The radicle nearly always grows straight down, pulled by gravity. Once the radicle is established in a place where it can absorb water, oxygen, and nutrients, further root development continues.
Taproots and oxygen
In some cases, such as walnuts, pines, and oaks, the taproot persists, growing down 3 to 6 feet. This taproot can grow extremely deep, under ideal conditions, but that's rare. Other trees develop a system of fibrous roots, and the taproot is not maintained. Tree roots move through the macropores and micropores found in the soil. If the soil is too hard or compacted, roots cannot move through it. Tree root growth generally stops when insufficient oxygen levels are encountered. This can be caused by compaction, the presence of hardpan, or by flooding. Cherry trees are particularly sensitive to insufficient oxygen in the soil, which is why they tend to be difficult to grow in compaction-prone clay. [Their roots contain chemicals that turn into cyanide gas when oxygen levels are too low! (Rowe and Catlin, 1971)]
The place where major roots emerge from the subterranean trunk is called the root collar. Most trees put out 4 to 11 major roots that grow horizontally from the root collar. These roots are generally found in the top 12 inches of soil, though they can range 3 to 7 feet deep, depending on age, species, and local conditions. These roots are can be 3 to 15 feet long, and up to 1 inch in diameter. Like trunks and branches, these woody, perennial roots develop growth rings, and they provide anchoring support from which all the transport roots emerge.
Transport roots normally fill a circular area that can be 4 to 7 times greater than the drip line. Transport roots do exactly what their name implies: they transport resources collected by the root hairs into the vascular bundles that feed the rest of the tree. To take good care of your trees, you really need to know where these roots might be.
GARDEN CHALLENGE: Where are your roots?
I challenge you to learn where your trees’ roots right be. You can do this outside, with a tape measure and a long rope, or you can do it on your computer, or with paper and pencil. However you do it, it will probably surprise you just how far these roots go. Regardless of the method you use, these steps will help you learn where your trees’ root might be:
Note how the root systems of all your trees (and large shrubs and everything else) overlap in significant ways. Also note where those root systems might end up covered by your house, the street, or some other dead zone. [Printouts of current satellite views of your property are very handy for activities like this one.]
But there is a lot more to tree roots than perennial and transport roots. There are some roots you’ve never seen, and some you’ve probably never even heard of!
In particularly dry, sandy soil, trees will put out striker roots. Striker roots grow straight down, from the perennial roots, until they encounter a barrier or insufficient oxygen. These striker roots, as well as taproots, then start branching out horizontally, creating an entirely new layer of root system.
Feeder roots are where all the action happens. These are the microscopic root hairs (which aren’t actually hairs at all) that interact with water, oxygen, and mineral molecules found in the soil, along with billions of soil microorganisms that make everything possible. These feeder roots grow upward into the top soil to collect (and disperse) nutrients, oxygen, and water.
Causes of tree root damage
You may be surprised to learn that fully 99% of a tree’s roots are found in the top 3 feet of soil, and that it is a lot easier than you might expect to damage those roots. Tree roots are frequently damaged by drought, flooding, extreme temperatures, the presence of rocks or hardpan, nematodes, springtails, and root-eating vermin, such as voles. Tree roots can also be damaged by human actions, even when our intentions are good. These actions include:
Signs of root damage
Tree roots can be seen as a reflection of the aboveground portion of the tree. Not necessarily in terms of size or shape, but in overall health. For example, if the leaves are repeatedly removed from a tree, some of the roots will die, as well. In the same way, if a portion of the root system is damaged or drowned, a corresponding dieback of the aboveground portion of the tree can be seen.
The vascular systems of some trees, such as oaks, are tied directly to branches on the same side of the tree. Damage to the roots will be reflected in poor health or death of branches on the same side of the tree. In other cases, portions of roots are tied to branches on the opposite side of the tree, while others have more of a spiral or zig-zag vascular system that serves the entire tree. These patterns can vary between species and individual trees, but arborists use this information to sort out problems related to irrigation, pesticides, fertilizers, insecticides, and herbicides.
Helping tree roots
You can help your trees’ roots stay healthy by aerating the soil, avoiding compaction, irrigating properly, mulching, and top dressing with organic material. These actions will help the worms, microorganisms, and other processes “fluff” the soil, improving soil structure, and provide important nutrients.
Unlike the crown shyness seen above ground, where the leaves of individual trees avoid touching, the root systems of different trees, shrubs, and other plants can intertwine in complex networks that are made evermore astounding when you learn how they use soil microorganisms to share nutrients and to communicate. [We’ll talk more about that later.] In situations where several of the same species of tree are growing near each other, the roots of one tree will graft to the roots of another tree. [I’ve said it before, I’ll say it again - the more I learn, the weirder the world gets!]
I hope that you can now see your trees with new eyes, eyes that can better imagine what is happening underground and under your feet.
Fragrant pine needles are modified leaves.
Unlike the soft, flat leaves of broadleaved plants, many evergreens have evolved an entirely different sort of leaf, the needle.
Most plants with needles need very little water, which means they are called xerophytes. These plants also tend to grow at higher elevations. Each tree produces millions of needles over the course of its life. The physiological characteristics of needles are an adaptation that allow evergreens to thrive where other plants would perish.
Benefits of needles
Conifer needles have many characteristics that allow them to hang on to the water absorbed by the roots:
The dark green color of most needles also aids in collecting the sun’s energy.
Anatomy of a needle
Even though needles look very different from classical leaves, they still have many of the same structures and functions. For example, needles perform photosynthesis. The stomata, used in gas exchanges and moisture level control, are arranged in lines, down the length of the needle, or in small patches.
Needles often grow in clusters, called fascicles. There are also single needle fascicles. The number of needles found in a single facile can help you identify the species. At the base of needles, you will see a sheath, called the fascicle sheath. This sheath can be persistent, as in hard pines, or deciduous, as with soft pines.
Most needle-bearing trees are evergreen, though there are a few deciduous species. Just as the leaves of deciduous trees change colors and fall in autumn, the needles of many evergreens also change color and fall, it’s just not as obvious. This is a natural occurrence, much like citrus June drop. Most evergreens hold onto their needles for 2 or 3 years. Discarded needles are usually those found closer to the trunk. If needles are lost elsewhere on the tree, or if needles are discolored, it can indicate fungal disease.
Needle pests and diseases
Pine wilt can also cause discolored needles. Pine wilt is caused by pinewood nematodes, which attack the vascular tissue. Pinewood nematodes move from tree to tree by catching a ride on pine sawyer beetles, in a behavior called phoresy.
Pine needles are a favorite food of some moth and butterfly species, as well as the pine sawfly. Goats will eat pine needles, too, but you wouldn’t want to drink the milk they produce after that snack. It’s nasty.
Acidic pine needles
There is a popular misconception that pine needles can be used to acidify soil. While it is true that fresh pine needles are slightly acidic, and a thick layer of needles on the ground can interfere with the growth of competing hardwoods, the dried needles, often called pine straw, are not acidic. While they will improve soil structure by adding organic material as they decompose, pine needles will not, I’m sorry to say, acidify your soil.
Pine needles can be steeped in boiling water for a refreshing tea that is high in vitamins A and C. Pine needles have long been used to make baskets, trays, and other crafts. Here is a pine needle basket made by my grandmother.
If extrafloral nectaries are not super-sized nectarine flowers, what are they?
While most plants produce nectar in their flowers to attract pollinators, there are over 2,000 plants that produce nectar in other places, and for entirely different reasons.
What are extrafloral nectaries?
Nectar is the currency used by plants to attract beneficial insects. Nectar is manufactured in glands, called nectaries. Nectaries are usually found in flowers. When these glands occur elsewhere, usually on leaves or stems, they are called extrafloral. So, extrafloral nectaries (EFN)s) are knob-shaped, nectar-producing glands found on leaves and stems.
These glands can take many different forms. Some are very primitive in structure, while others are highly complex. Regardless of the form, the nectar produced by EFNs is surprisingly consistent across species, and around the globe. This is in direct contrast to the wide ranging differences found in the nectar produced by flowers.
Why do plants have extrafloral nectaries?
If nectar is supposed to attract pollinators, why would it occur on stems and leaves? The most popular theory asserts that extrafloral nectar attracts insects, spiders, and crustaceans that protect the plant from sap-sucking, plant nibbling, seed eating pests. There is another theory that claims extrafloral nectaries may also serve a waste elimination function, but that theory is not nearly as popular, or as appetizing. Many beneficial insects (and some not so beneficial insects) are attracted to EFNs, regardless of the reason.
Insects attracted by extrafloral nectaries
Scientists believe this structure evolved on vining plants, due to ant traffic. Ants are one of the most frequent visitors to extrafloral nectaries. Since ants frequently carry diseases from one plant to another, and they farm aphids, I don’t usually count them as beneficial insects, even though they do help aerate the soil. Recent research, however, has also shown that ants serve a valuable function to trees by feeding on nectar and harmful insects, and then pooping those nutrients onto leaves. Those nutrients are then absorbed through the leaf, providing valuable plant food, right where it is needed.
A few, full-blown pests, such as Florida’s lovebugs, also tap into this food resource. For the most part, it is beneficial insects, such as ladybugs, lacewings, praying mantids, and wasps, who are attracted to extrafloral nectaries. Some plants provide sheltered chambers, called domatia, for similar benefits.
Plants that feature extrafloral nectaries
There are over 2,000 plants that have extrafloral nectaries. All cucurbits and many members of the Prunus and legume families feature extrafloral nectaries. This means that your squash, melons and gourds have these knobby glands, as do your peach, apricot, nectarine, cherry, and plum trees. Cowpeas and elderberries do, too. Common vetch, willow, peonies, and many ferns, vines, and carnivorous plants also feature extrafloral nectaries. [Some scientists disagree with ferns being included in this list, since ferns do not produce flowers. Those scientists call these glands ‘extrasoral’.]
As more botanical research is conducted, we are learning than the food provided through extrafloral nectaries is critical to biodiversity, especially during times of drought.
Which plants in your garden have extrafloral nectaries?
Flowers come in many shapes and sizes. When a flower cluster has a flat or dome-shaped profile, it is said to be corymb [kor-im].
Corymb comes to us from the Greek word (korumbos) for ‘cluster’. The only reason this information is important, besides helping you win more often in word games, is that it can help you to identify plants of mysterious parentage. So, let’s find out more about corymbs and flower clusters. [And don’t let all the new words scare you off.]
Umbels and corymbs
First, we need to differentiate between umbels and corymbs. Umbels are flower clusters that look like umbrellas. The tiny stems, called pedicels, all emerge from a central stalk. Carrot, dill, and parsley flowers are all umbels.
If a flower cluster has many branches, instead of a single point of contact, it is called a panicle. [But don’t panic! You can do this!]
Flower stems are called peduncles. As soon as the tiny stems of a flower cluster begin to emerge, that main stem changes its name to rachis [ray-kiss]. Each individual stalk within a flower cluster is called the pedicel. Each pedicel holds a floret. Pedicels can be arranged in pairs (parallel), or they can take turns (alternate).
Types of corymbs
Corymbs may be flat-topped or convex. This is because the tiny stems, or pedicles, get progressively longer as they move away from the center. If the pedicels of a corymb all emerge from the central rachis, it is said to be racemose. If there are several layers of branching rachis, it is called cymose.
Cymose corymbs are said to be determinate. Determinate inflorescences have a flower on the top that halts further growth. This top (apical) flower is the oldest one in the bunch. Younger flowers develop below this primary flower. Forget-me-nots, jasmine, and figs are all cymose.
Racemose corymbs, or racemes, are said to be indeterminate. Indeterminate inflorescences are those with the oldest florets at the base and newer growth at the top. They just keep on growing. Cherries and other stone fruits all have racemose corymbs. Snapdragons and yerba maté are also racemes.
The next time you look at a flower cluster, take a moment to see if it is built like an umbrella (umbel), if its branches are all connected to a central stem (raceme), or if there is a complex system of branches (cymose). This can help you make better use of the many plant identification tools available online.
Accessory fruits are not designer handbags or the latest fad. In the word of botany, accessory fruits are more familiar that you might expect.
What is fruit?
Fruit is the tissue that surrounds the seeds of angiosperms (flowering plants). Fruit tissue is made from the ovary. Except when it isn’t. In some cases, a fruit develops from both the ovary and nearby tissue, found outside of the carpel. These neighborly tissues can be either the perianth, the flower whorls, or the hypanthium, the flower base. When this occurs, the part we eat is called an accessory fruit.
Popular accessory fruits
Using our botanical definition of an accessory fruit, we learn that pineapples are accessory fruits because the fruit is made from the ovary plus tissue from the pistils and sepals. We also learn that strawberries are accessory fruits. [The seeds you see on a strawberry fruit are actually achenes, a type of dried fruit. Each achene develops from a single pistil.] Other popular accessory fruits include apples, figs, mulberries, and pears. And those delicious cashew nuts? Those are the seeds of the cashew apple, another accessory fruit.
Now you know.
The truth about nuts may surprise you.
While you probably already know that peanuts are not nuts (they’re legumes), many of the other foods you have come to know as nuts are not true nuts at all. Let’s begin by learning the botanical definition of nuts.
True nuts are hard-shelled, inedible pods that hold both the fruit and the seed of a plant. These pods do not open of their own accord, which means they are indehiscent. The pod, or shell, of a nut is made from the ovary wall, which hardens over time. Hazelnuts, chestnuts, and acorns are true nuts. So are kola nuts, which gives “cola” soft drinks their signature flavor.
[Did you know that small nuts are called ‘nutlets”? To me, that sounds like the perfect name for a little chihuahua.]
So, when is a nut not a nut?
A nut is not a nut when it is a fruit seed. Fruit seeds can be angiosperm, drupe, or gymnosperm seeds:
These not-nut nuts are commonly referred to as culinary nuts.
[Did you know that cashews are the seeds of an accessory fruit, which means they share characteristics with strawberries and poison ivy. Isn’t botany amazing?]
Of course, you can call any of these delicious morsels "nuts" whenever you want to. True nut or culinary nut, many of these yummy snacks find their way into our gardens and foodscapes. Which ones are you growing?
Plants do not chow down on rocks like they were burgers and fries. Instead, their menu reads more like the Periodic Table.
Plants absorb water from the soil. Minerals are in that water. Those minerals are plant food. Plants also produce their own food using the sun’s energy to create sugar.
There are 16 chemical elements critical to plant health. Depending on how much is needed, they are labeled as micronutrients (tiny amounts) or macronutrients (large amounts). Macronutrients are further divided between primary and secondary nutrients. Primary nutrients are the NPK of fertilizer bags. Plants use nitrogen, phosphorus, and potassium more than any other plant food, which is why they are the ones most often needing replacement. They are the rice and beans of a plant’s diet. Secondary nutrients, calcium, magnesium, and sulfur, rarely need to be supplemented, but they are very important to plant health. Micronutrients include boron, copper, iron, chloride, manganese, molybdenum, and zinc. [These used to be called trace elements.]
If you count up all those nutrients, you will only find 13. That is because plants also have non-mineral nutrients. These non-mineral nutrients are hydrogen, oxygen, and carbon. All of these nutrients work together to provide your plants with the energy and materials needed to grow. Some of those nutrients are mobile, while others are immobile.
Highly mobile nutrients go where they are needed within a plant. Nitrogen, potassium, phosphorus, magnesium, chloride, and molybdenum are all mobile plant nutrients. All the other nutrients are considered immobile because they stay where they were initially placed. Problems with mobile nutrients tend to appear in older leaves, while problems with immobile nutrients are seen in new growth. This is important to know because it can help you narrow down deficiencies and toxicities.
What is in your soil?
Before we take a closer look at each of these important factors to plant health, let me remind you that you cannot know what nutrients are in your soil without a soil test from a reputable lab. I wish those colorful plastic tubes from the store could do the job accurately, but they can’t. Not yet, anyway. Contact a local soil test lab and find out what you are working with. Not knowing the facts can lead to toxic levels of these nutrients, which can backfire. [For a hysterical read about the effects of too much fertilizer, check out Don Mitchell’s Moving/Living/Growing Up Country series.]
Just because your plants are not thriving does not mean they need to be fed. All too often, plant problems are caused by inhospitable soil conditions, unhealthy roots, irrigation problems, pests, or disease.
So, let’s see what each of these nutrients do for your plants:
Not enough of a plant nutrient, or too much, can cause problems. The tricky part comes in when the balance of nutrients is out of whack.
A man named Mulder created a chart that shows us the interactions between plant nutrients. While there are limits to the usefulness of this overly simplistic view, it can help you understand what might be happening to your plants.
According to Mulder's Chart, synergistic elements help each other to be absorbed by plants, while antagonistic elements get in each other’s way. Using the chart above, you can see that proper levels of potassium help plants absorb iron and manganese, but too much potassium interferes with a plant’s ability to absorb boron, calcium, magnesium, nitrogen, and phosphorus. This interference can take the form of competition for space on water molecules, or it can alter soil pH, making some nutrients unavailable.
Plant food and soil pH
Soil pH ranges from 0 to 14, with lower numbers indicating acidity and higher numbers indicating alkalinity. Using the chart below, you can see that more nutrients are available, and there is greater microbe activity, when soil pH is between 6.0 and 7.0. Most plants can survive in soil pH from 5.2 to 7.8, but the narrower range allows plants to thrive. This is because the minerals used as food are ions. Ions are atoms and molecules that have a positive or negative charge. These cations and anions, respectively, attach themselves to water molecules and are pulled into the plant by root hairs. The wrong soil pH can cut your plants off from a bounty of nutrients.
Soil is given a cation exchange capacity rating to describe its ability to hold nutrients. [Did you know that root hairs knock cations (unbalanced atoms or molecules) loose with a hydrogen canon? Stay tuned for more on that!]
How to feed your plants
While there is no chemical difference between nitrogen from compost and nitrogen formulated in a lab, I prefer feeding my plants with composted yard and kitchen scraps and chicken bedding. Not only does this mix have excellent nutrients, it also improves soil structure. If you decide fertilizer really is necessary: READ THE BAG. Seriously. Federal law requires that important information is printed on the container and for good reason. Follow directions carefully and wash your hands when you’re done.
What are your plants hungry for?
Plant prickles are skin spikes.
Unlike thorns, which are modified shoots, and spines, made from modified leaves, prickles are spiked skin extensions.
Because prickles are made out of epidermis and cortex tissue, they can occur anywhere on a plant. This is also what differentiates them from the hairs (trichomes) growing on your squash plant leaves. Trichomes only contain epidermis tissue, whereas prickles contain both epidermis and cortex.
The purpose of prickles
The most obvious purpose of prickles is to make plants less palatable to herbivores. Chewing on a stem covered with prickles can’t be very appealing. In extreme cases, such as the silk floss tree, the entire trunk is covered with massive prickles. I suppose that’s the level of protection needed in rural South America.
While prickles are generally meant to keep herbivores away, most species specific pollinators have learned to maneuver around the prickles without too much trouble. Prickles can also provide limited amounts of shade or insulation from temperature extremes.
A rose by any other pokey bit
Everyone calls the sharp bits on rose stems thorns, but they are actually prickles. One easy way to tell if a protuberance is a prickle, thorn, or spine, is by how easy it is to remove. Spines and thorns contain vascular bundles, but prickles do not.
This is why it is so easy to flick a rose thorn from its stem, while trying the same trick on your orange tree won’t work. Orange tree thorns have added strength from the phloem and xylem, carrying water and nutrients into the pointy protuberance. Thorns do not have that type of attachment, so they are easier to remove.
So, now you know the difference between thorns and prickles.
You can grow a surprising amount of food in your own yard. Ask me how!