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. 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.
Also, different plants have different nutrient needs. Simply dumping a box of 10-10-10 around the garden isn’t a good idea and it’s a waste of money. Those chemicals can pollute groundwater and damage beneficial soil microorganisms. It can also make plants grow faster than they can maintain over the long haul, leaving them weak and vulnerable later in life. [For a hysterical read about the effects of too much fertilizer, check out Don Mitchell’s Moving/Living/Growing Up Country series.]
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. Synergistic elements help each other to be absorbed by plants. 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 use 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
Personally, 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. Technically, there is no chemical difference between nitrogen from compost and nitrogen formulated in a lab. Nitrogen is nitrogen. The difference lies in everything else. What are the fillers? What else is in the compost that plants need? Honestly, there’s a lot we don’t yet understand about how living things interact. I prefer to err on the natural side, just in case.
Finally, keep in mind that a 10-pound bag of fertilizer does not contain 10-pounds of plant food. The numbers next to the letters tell you the percentage of total weight. This means that a 10-pound bag of 10-20-10 fertilizer contains 1 pound nitrogen, 2 pounds phosphorus, 1 pound potassium, and 6 pounds of filler. Yes, filler. If all your plants need is nitrogen, blood meal is a far better choice.
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.
Pulses are the grain seeds of plants in the legume family.
Legumes are a great high protein, high fiber food that tends to be pretty easy to grow. Popular legumes include beans, peas, lentils, chickpeas, cowpeas, and fava beans, just to name a few. Soybeans, carob, peanuts, tamarind, and alfalfa are also legumes, but not all legumes produce pulses.
Legume fruits, or pulses, are simple dry fruits that are low in fat. They develop from a single carpel and are normally dehiscent, which means they unzip along one edge. These fruits are often called pods, but that isn’t exactly inaccurate. Pulses are only one type of pod. A radish silique and a vanilla capsule are also pods. While peanuts and soybeans are both legumes, they both have a high fat content, they are not considered pulses.
No green pulses
If you harvest peas or beans while they are green, they are not called pulses. They are simply vegetable crops (even though they are fruits). The same is true for legumes harvested specifically for their oil. This is a rule put out by the United Nations’ Food and Agriculture Organization (FAO).
Differences between pulses and cereal grains
Cereal grains, such as rice, wheat, barley, corn, and sorghum certainly deserve garden space for their seed crops, they are not the same thing as pulses. Pulses may seem like just another bunch of seeds, but there are fundamental differences that make them stand alone:
The only true pulses are the seeds from dry beans, dry peas, chickpeas, and lentils. These plants provide one of the best bangs for your gardening buck, providing excellent nutrition and soil health improvement.
Hesperidium is the name given to certain types of fruits.
Hesperidia are berries with a tough, leathery skin that tends to be bitter.
If you cut a hesperidium open, you will see separate compartments, called carpels. Within these carpels, you will see hundreds of tiny, fluid-filled vessels that are made out of specialized hair cells. These vessels are called vesicles.
If you haven’t already guessed, all citrus fruits are that special type of berry, known as hesperidia.
Bud scar may sound like a great punk band name, but knowing how to recognize this tiny bit of plant anatomy can come in handy.
At the tip of most twigs is an area of meristem tissue. This plant tissue can turn into several different types of plant cells. When the tissue grows upward, to continue the trunk of a tree, or a branch stem, it is called apical meristem, or a terminal bud. In this sense, terminal does not mean lying on its death bed. Rather, it refers to the bud at the end of the branch.
As these terminal buds burst forth with new growth, the protective scale normally falls away, leaving a bud scar. Bud scars look like rings around stems and branches of trees and other woody plants. Bud scars are from the terminal bud on a stem. These marks are different from leaf scars. Leaf scars occur at the point of attachment for a leaf, after the leaf has fallen off. Just above a leaf scar, there is usually a lateral bud that can grow into a twig or flower.
Ultimately, the growth of the tree or branch will grow over these scars, but that can take a long time. Until then, you can use the number of bud scars to determine the age of a branch, since each terminal bud indicates one year’s growth.
As a child, I would eat around the center core of my carrots, leaving the darker, sweeter core for last. I didn’t know it then, but that inner core is called the stele.
Vascular plants have both root and stem steles, but they didn't start out that way. Primitive steles were nothing more than a strand of xylem, surrounded by phloem. [Remember, water and minerals ‘rise up the xylem’ from the roots, and manufactures sugars ‘flow down the phloem’ from the leaves. In case you forgot.]
More modern steles may consist of vascular tissue, pith, and pericycle. Pith is the spongy material seen in the center of stems, and the pericycle is a thin layer of tissue between the xylem and the endodermis. There are two major types of stele: protostele and siphonostele.
Protostele describes the more primitive stele, which consists of a strand of xylem, surrounded by phloem. Protosteles may or may not have an endodermis that controls the flow of water. There are three different types of protostele:
Siphonosteles are a little more complex than protosteles. Siphonosteles may have gaps in their vascular tissue in places where leaves are born. These spaces are called leaf gaps. You can think of these leaf gaps as sections cut from a hula hoop and pulled a little apart, making room for leaf tissue to grow through. Siphonosteles also contain pith. If the xylem is found only outside of the pith, it is called ectophloic. If the xylem can be found both within and outside of the pith, it is called amphiphloic. Members of the nightshade family, such as tomatoes and peppers, are amphiphloic. There are three types of amphiphloic steles:
Diseases of the stele include phytophthora root rot, verticillium wilt, black root rot, and crown rot. In each case, prolonged exposure to wet soil creates the conditions needed for pathogens to infect your plants. Maintaining good drainage and soil structure can help prevent these diseases.
So, why would you care what sort of stele your plants have? Besides sounding really smart, being able to look up information about what’s inside a plant stem can help you identify unknown plants.
What's inside your stems?
Every rose has its thorns, right? Well, no. They don’t.
Roses do not have thorns. Roses have prickles. Citrus trees have thorns.
Thorns, prickles, and other spiky bits
Thorns are a type of spinose structure made out of a modified leaf, stem, root, or bud. Many people use the terms bristles, prickles, spines, and thorns interchangeably. Botanically, these terms mean very different things:
So, where bristles are stiff hairs and prickles are hard, spiked skin (neither of which contain plant veins), spines, being modified leaves, and thorns, modified stems, do contain plant veins.
Plants use thorns as a mechanical defense against herbivores (and gardeners). Cacti are far less likely to be eaten when they are covered with hard thorns. And the pollinators who specialize in pollinating these particular types of plants seem to be unaffected by the presence of thorns. In some cases, thorns are also used to shade certain plant varieties, or to provide a layer of insulation.
Home, sweet thorn
Some thorns are hollow. These tiny chambers are called domatia. Plants, such as certain acacia species, produce domatium to provide shelter for beneficial arthropods (insects, spiders, and crustaceans). Similar to galls, which are produced by the resident, rather than the landlord, domatium are the plant’s side of a mutually beneficial relationship, most commonly with ants or mites. Occasionally, thrips may also move into these tiny apartments, but they are generally unhelpful to the plant. The plants that create these thorny thresholds are called myrmecophytes.
While I do not expect any of you to stop calling rose prickles thorns, why not impress your friends with your new-found knowledge?
Wax is made by honey bees to build the comb used to store honey and to protect larvae.
Did you know that plants also make wax?
Nearly all vascular plants manufacture wax. This wax is used as part of the cuticle, or outer layer of the epidermis, of leaves, stems, and even some fruits.
Having a waxy outer layer reduces evaporation, making it easier for plants to hang on to the water they need. It also reduces the chance of abrasion, when plant parts rub against each other. Finally, wax makes it more difficult for pests to attack.
Wax is actually a class of fatty compounds that are insoluble in water and tend to be relatively soft at room temperature. When honey bees are between 12 and 20 days old, they develop a special gland on their belly that converts the sugars in honey into waxy flakes. The flakes are collected by other bees and chewed up before being used to make new comb. [I thought you’d want to know about that.] Plants, however, have neither the organ nor the chewing ability. Instead, plants synthesize wax out of hydrocarbons, made up of fatty acids and long chain alcohols, along with aromatics, ketones, and other chemicals. The chemical make up of a plant’s wax varies by species and geographic location.
Plant wax candles
Carnauba wax, of shiny car and confectionary fame, is a wax made by the Brazilian palm Copernicia prunifera. A lighter colored substitute, ouricury wax, comes from the Brazilian feather palm Syagrus coronata. Several species of native bayberry (Myrica cerifera), also known as wax myrtle, and the succulent stems of candelilla (Euphorbia antisyphilitica), produce so much wax that they were used by Native Americans to make candles. In the case of bayberry, the berries are boiled until the wax separates from the plant material. After it hardens, it is removed from the soup. These candles are still made today, due to the pleasant smell as they burn. Candelilla plants are now endangered and collecting them is forbidden. Other plant waxes include castor wax, rice bran wax, and tallow tree wax.
The next time you look at a leaf or stem, take a closer look and see if wax is part of that plant’s defense system.
Heliotropism refers to a plant’s ability to track the sun’s movement.
For many centuries, it was believed that a plant’s tendency to follow the sun as it crossed the sky was a passive action caused by water loss on the side of the plant exposed to sunlight. Now, we know that there is far more to it than that.
Growing toward sunlight
Instead of passively shrinking to one side as the sun’s harsh rays boil away a plant’s bodily fluids, we now know that plants actively grow toward (or away from) sunlight. [When a plant grows away from sunlight, it is called skototropism.]
Experiments conducted in the 1800’s demonstrated that plants will respond to any type of light: street lights, grow lights, or sunlight. When plants are attracted to this light, it is called phototropism. Phototropism is a function of the hypocotyl, or individual cells found in the same region. Hypocotyls are the embryonic stem found below the seed leaves (cotyledons) and directly above the root. You can easily see examples of phototropism when seedlings first emerge and they don’t get enough sunlight - they become leggy and lean toward whatever light they can. This is phototropism.
In heliotropism, not any old light source will do. It is only radiation from the sun that causes the reaction. And the mechanical causes of these two types of movements are very different.
Mechanics of plant movements
When plants move in response to the position of an external stimulus, it is called a tropic [TRO-pic] movement. If a plant’s movement is independent of the stimuli’s position, it is called a nastic movement. In phototropism, plant hormones (auxins), found in the meristem tissue of leaf and stem tips, photoreceptors, and multiple signaling pathways are used to direct a plant to grow more rapidly toward sunlight. In heliotropism, a structure called the pulvinus is used to direct movement.
The power of pulvinus
The pulvinus is an amazing, fluid-controlled joint found at the base of a plant leaf stem (petiole) or just below a flower.
The pulvinus causes movement by altering fluid pressure in the surrounding plant tissue. These changes in fluid pressure start when sucrose is moved from the phloem into the apoplast. The apoplast is the conjoined spaces between plant cells. As sugar is pumped into the apoplast, potassium ions are pushed out, followed by water molecules. This changes the pressure within the affected cells, causing movement. This is called turgor-mediated heliotropism. But not all heliotropic flowers have a pulvinus. Those that do not are still able to move by permanently expanding individual cells. This is called growth-mediated heliotropism. Pulvini are also used in response to nyctinastic and thigmonastic movements.
Heliotropic flowers face the sun from dawn to dusk. Slowly tracking the sun’s path across the sky, these flowers are believed to use heliotropism as a way to improve pollination, fertilization, and seed development. Heliotropic flowers often have five times as many beneficial insects present, due to the added warmth. [Many tropical flowers exhibit a modified form of heliotropism in which flowers maintain an indirect tracking of the the sun. This is believed to reduce the chance of potential overheating.] Beans, alfalfa, sunflowers, and many other species turn their blooms to follow the sun’s path across the sky each day. But sunflowers only use heliotropism in their early development, in the bud stage. Once a sunflower head emerges, it may track the sun for a short time, as an expression of phototropism, until the flower head reaches full size. The majority of sunflowers found in the northern hemisphere nearly always end up facing east.
Like floral heliotropism, leaf heliotropism is the method by which plants focus their leaves perpendicularly to the sun’s morning rays (diaheliotropism), or parallel to midday sun (paraheliotropism). Diaheliotropism allows leaves to capture the maximum amount of energy from the sun, while diaheliotropism protects plants from overheating and dehydrating.
How do your plants move during the day?
What bottle of wine would be complete without its cork? The same is true of most trees.
Everyone knows that trees and woody shrubs are made of wood, surrounded by bark. But there’s a lot more going on in those outer layers than meets the eye.
The bark you see protecting the living wood of a tree is made up of dead plant cells. This layer is called the rhytidome. The reason these cells are dead is because the cork layer cuts them off from the tree’s resources.
Components of bark
Bark is made up of three basic layers. The inner layer, or phloem, is a living part of a tree’s vascular system. Manufactured sugars ‘flow’ down the phloem to feed the rest of the plant. The middle tissue, or cortex, is made up of porous tissue that stores and transports carbohydrates, tannins, resins, and latex. The outermost layer of bark is called its periderm.
The periderm is also made up of three layers: the cork, cork cambium, and phelloderm. Cork (phellem) is produced by a specialized layer of cambium tissue, known as the cork cambium, or phellogen. This cork cambium layer is only one cell thick and the cells divide in parallel (or periclinally) toward the outside of the tree. In some trees, the cork cambium layer also produces cells towards the inside of the tree. These inner cells are the phelloderm layer.
Function of cork
Cork keeps wine safe from the elements because it is impermeable to gases and water. Because of the cork, your wine stays where it is and (as long as the cork remains intact) will only grow better with time. The cork of a tree also blocks air and water. Cork is able to keep trees and wine safe from the elements, along with insects, bacteria, and fungal disease because it contains suberin. Surberin is a waxy material that creates a protective barrier. This barrier also blocks water and gas exchanges between the outermost layers of the tree killing the epidermis, cortex, and secondary phloem. This is the bark you see.
Trees and shrubs also use cork to cut off an unwanted body part (leaf, diseased twigs, mature fruit) from the rest of the plant. This is called abscission.
Most fruits hang in their own singularity: apples, oranges, and apricots are common examples. Other fruits, such as grapes, form clusters. Still other fruits are formed when a group of flowers merge to create a fruit. Soroses are that type of fruit.
What is fruit?
Fruit is the fertilized ovary of a flowering plant (angiosperm). After pollination and fertilization occur, two new structures are produced: seeds (fertilized ovules) and pericarp (thickened ovary walls). In the case of apples and oranges, one flower produces one fruit. Sometimes, multiple flowers can fuse together to create a fruit. There are three different ways that this can happen:
In nearly every piece of literature you see, pineapples are listed as a common example of sorosis, but this is incorrect. I don’t know why they do this.
How a sorosis fruit develops
If you look at a mulberry flower cluster, you will see several flower buds held tightly together. Each of these individual flowers open up, awaiting pollination.
If you look closely, you can see tiny fruits at the base of each flower. Each of these fertilized fruits will develop around the stem that they emerged from in the first place. This is unlike pineapples, which include the receptacles and flower parts in their fruit development.
Berries vs. soroses
While mulberries may appear to have the same structure as blackberries and raspberries, botanically, they are quite different. Raspberries and other members of Rubus are made up of several drupes (a type of fruit) that are clustered around and attached to a dry thalamus. All of the drupes in a single fruit are made from a single flower. In mulberries, and other soroses, each rounded bit is its own fruit, formed from its own flower.
It won’t make any difference, as you enjoy a fig, some pineapple, or a mulberry, but now you can impress your friends with this fascinating word!
Some garden words are fun to say. Schizocarp [ˈskitsōˌkärp] certainly qualifies.
A schizocarp is a type of dry fruit that splits into single-seeded parts, called mericarps, when ripe. Each mericarp is made from its own carpel. [A carpel is the female reproductive parts of a flower, including an ovary, stigma, and usually a style.] Mericarps can be dehiscent, which means they split open when ripe, or indehiscent, which means they stay closed.
The seeds of carrots, celery, coriander, anise, dill, parsnip, and other umbellifers are all indehiscent schizocarps. Hibiscus (Malvaceae), mallows and cheeseweeds (Malva), false mallows (Malvastrum), and wireweed (Sida acuta) fall in the same category.
Members of the Geranium genus produce dehiscent schizocarps. [These are not the garden variety geraniums, which are another genus altogether (Pelargonium). I know, I know, it gets confusing.] True Geranium species include the cranesbill, horns’ bill and filaree plants that produce needle-shaped schizocarps that twist and gyrate into the soil (and were fun to play with, when we were children).
Maple trees produce winged schizocarps, called samaras.
Unlike the juicy fruits we enjoy each summer, or the dried caryopsis of cereal grains, plants that produce schizocarps have found that procreation works best when each flower produces a number of independent seeds protected by a dried fruit coating.
Now you know.
Plants cannot be green without magnesium, but too much magnesium in the soil can turn plants yellow. How can this be?
Magnesium is essential for plant health. Magnesium stabilizes cell membranes, making plants better able to withstand drought and sunburn. Magnesium is found in enzymes that plants use to metabolize carbohydrates. Most important, magnesium is contained in the chlorophyll molecules that convert the sun’s energy into food. This process, the Calvin Cycle, is what makes photosynthesis possible. Clearly, magnesium is important to plant health. But too much magnesium can interfere with the absorption of other plant nutrients.
Plants use 13 dissolved minerals as food. There are three primary macronutrients (nitrogen, potassium, and phosphorus) and three secondary macronutrients (calcium, sulfur, and magnesium). Plants use large amounts of these macronutrients to grow, thrive, and produce. Seven other nutrients, used in smaller amounts, are called micronutrients. Fertilizers claim to provide all the food your plants need, but it’s not that simple. [Is it ever?]
The chemical interplay, taking place in the soil, that allows plants to absorb nutrients, is a delicate balancing act. Too much, or not enough, of one nutrient can create a domino effect that causes starvation for plants that are surrounded by a banquet of nutrients.
What is magnesium?
Magnesium is an elemental metal. Pure magnesium (Mg) is too stable of a molecule for plants to absorb. The less stable, cation form of magnesium (Mg2+) is a dissolved salt that plants use for food. To be able to attract and hold those positively charged molecules, plants also need negatively charged molecules (anions), such as nitrogen, phosphorus, and sulfur. The ability of soil to perform this balancing act is called its Cation Exchange Capacity (CEC). Without a soil test from a reputable, local lab, you cannot know your soil’s CEC or nutrient levels.
For example: My first soil test found magnesium levels of 798 parts per million (ppm). The ideal range is 50 to 120ppm. Clearly, before I moved in, someone was applying an awful lot of fertilizer. The problem they were probably trying to correct was not insufficient nutrients, but a nutrient imbalance. Without a soil test from a local, reputable lab, you simply do not have enough information.
Base saturation and magnesium
Soil test results also include base saturation figures for potassium, calcium, and magnesium. Base saturation is the percentage of available connections being used. [Think of it as how many grocery bags you can carry in from your car.] The optimal range for magnesium base saturation is 10 to 30%. This means that soil particles, because of their electrical charge, will ideally hold on to 10 to 30% of the magnesium in the soil. It takes the right absorption percentage and the right volume of magnesium in the soil for plants to be healthy.
My soil’s magnesium base saturation was 32%. That sounds close enough to the 10 to 30% optimal range, right? The problem is, with seven times the amount of magnesium needed in the soil, my plants were getting 32% of too much.
Too much magnesium in the soil makes it difficult for plants to absorb calcium and other anion nutrients, which can lead to blossom end rot, bronzing, and many other problems. This is a common problem in areas with alkaline soil. The opposite is true in areas with acidic soil. Insufficient magnesium symptoms look very much like potassium toxicity symptoms: older leaves, at the bottom of the plant, start turning brown, between and alongside the leaf veins, working upward through the plant. Magnesium deficiencies in stone fruits often start out as slightly brown areas along leaf edges (margins) that expand inward, causing cracking, necrosis, and leaf loss. Magnesium deficiency in California is extremely rare.
Stabilizing magnesium levels
Reaching and maintaining ideal mineral levels in soil for healthy plant growth is both science and art - mostly science. To start, get a soil test from a local, reputable lab. Unfortunately, over-the-counter soil tests are not yet accurate enough to be useful. Once you have your results, you can take these other factors into consideration:
Finally, schedule regular soil tests for your garden and landscape. Look at these tests as an annual physical for the living skin of your property. The information in these tests will help you make informed decisions about the magnesium in your soil.
You can grow a surprising amount of food in your own yard. Ask me how!