Leaf

Did you know that thorns are modified leaves?

Leaves are food producing factories for most plants. This is where photosynthesis takes place. The only exception is succulents. The leaves of succulents are modified stems, and the thorns on cacti are modified leaves. Weird, right?

So let’s learn about the amazing leaf.

In addition to photosynthesis, leaves breathe and sweat - sort of. On the underside of a leaf, there are small openings called stomata. The stomata control moisture and gas exchanges that regulate temperature and water content within the plant.

Inside a leaf

The inside of a leaf is surprisingly complex. It has an outer layer called the epidermis, which is covered with a waxy cuticle. The cuticle changes thickness depending on how much light is present. The epidermis also has hairs that provide protection.

Inside, under the epidermis, there are columns (parenchyma) called the palisade layer. The parenchyma contain chloroplasts for photosynthesis. Chloroplasts hold the chlorophyll that harvests the sun’s energy and begins turning that energy into sugar.

Under the palisade layer is a thickened area called the spongy mesophyll. The spongy mesophyll is a collection of loosely packed parenchyma tissues that allow oxygen, water vapor, and carbon dioxide to move around within the leaf. The xylem and phloem make up the vascular bundle, which is also held in this spongy mass.

On either side of the stomata are guard cells. When there isn’t enough water, the guard cells become flaccid (through sugar and ion osmosis) and the stoma close, preserving whatever water is already in the plant. 

Leaf interior (Maksim) Public Domain

Leaf structure

Understanding the vocabulary related to parts of a leaf can go a long way toward plant identification. These are the basic parts:

• Bud - unopened leaf
• Node - where the leaf emerges from a stem
• Blade (or lamina) - the largest flattened part of a leaf
• Petiole - the tiny stalk that attaches a leaf to a twig at the bud
• Base - where the blade meets the petiole
• Glands - different colored hard bumps found at the base and on the margin
• Stipules - tiny leaves at the base of a leaf blade (not always present)
• Tip (or apex) - the furthest point of a leaf away from the stem
• Midrib - the vein in the middle of the leaf
• Lateral veins - veins that move out from the midrib
• Leaf Margin - the outer edge of a leaf
• Pulvinus - spade-like base of the petiole that attaches to the stem 
• Auxiliary bud (or lateral bud) - from which a flower or branch may emerge

Drawing of leaf parts (Kate Russell)

Leaf arrangement

Another helpful tool in plant identification is leaf arrangement. This describes the way leaves are positioned along a stem. Leaf arrangement can be alternate (taking turns on either side), opposite (in pairs), whorled (several leaves at one position), or rosette (a spiral cluster at the base of the plant, like a dandelion).

Leaf arrangement also refers to the way a leaflet is positioned on the petiole. A simple leaf is a single blade. Compound leaves have many leaflet at the same petiole. Palmately compound leaves look like a hand, with leaflets radiating from a central point, while the leaflets on a pinnately compound leaf share a common leaf stalk (rachis). Double pinnately compound leaves have a double set of compound leaflets.

​Leaf shape

The shape of the blade is probably the most important when it comes to plant identification. When considering leaf shape, we look at the edge, or margin, and the apex, or tip. 

Leaf shape by apex (Wikipedia)

Leaf shape by margin (Wikipedia)

Leaf damage

Not only do leaves help with plant identification, but they can also help narrow down pests and diseases. Leaf damage falls under these categories:

• Discoloration/Deformation - sucking insects (aphids, psyllids, thrips, mites, scale, whitefly, mealybugs )
• Holes/Ragged Edges/Skeletonization - chewing/rasping insects (caterpillars, beetles, sawflies, slugs and snails)
• Galls (bumps) - plant’s defensive reaction (wasps, psyllids, aphids, mites)
• Patch/Serpentine Mines - larval feeding (flies, beetles, leaf miners)
• Folding - nesting (crickets, spiders, caterpillars)

Natural beauty

Leaves from the garden can make beautiful additions to home decor and welcome gifts. Leaves can be used for many crafts and the variety of shapes is practically limitless!

Challenge

Go outside and find a leaf. Any leaf will do. Bring it inside and draw it as well as you can. Then label all its parts. Try different species to see how they are similar and how they are different. You might be amazed and it’s a lot of fun.