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The leaf is the site of two major processes: gas exchange and light capture, which lead to photosynthesis.
If you’ve ever eaten a piece of lettuce, cabbage, celery or onion, you’ve eaten a leaf or at least part of it. Celery is a petiole, which is the part of the leaf that connects the blade to the stem. Now that you’re acquainted with what leaves look and taste like, let’s take a closer look at what’s on the inside of leaves.
The leaf is arranged like a layered cake. It has a spongy layer, a palisade layer that consists of parenchyma cells and is where photosynthesis happens and an outer layer (the epidermis) with little holes (stomata) that could hold birthday candles. The only problem with our lovely analogy is that the candles would have to go on the bottom of the cake, since most of the stomata are on the bottom of the leaf. We’ll come back to that.
The inside cake layers are made up of parenchyma cells. This tissue is called the mesophyll, meaning "middle leaf," and comes in two flavors: the palisade mesophyll (sometimes called palisade parenchyma) and the spongy mesophyll. Most photosynthesis takes place in the palisade mesophyll, which is conveniently located at the top of the leaf just under the epidermis. Palisade parenchyma cells are long, bunched close together, and look like sausages hanging from the ceiling in a butcher’s shop.
The inside of the leaf looks like this:
Spongy mesophyll cells are not packed so tightly together, which allows carbon dioxide and oxygen to reach the palisade cells where they are needed in photosynthesis. Spongy mesophyll cells and guard cells (see below) also get some photosynthetic action.
Carbon dioxide and oxygen can’t just diffuse across the epidermis to get into the leaf. They have to be let in through special doors called stomata. Stomata can be on both sides of the leaf, but are usually concentrated on the bottom of the leaf to limit water loss due to evaporation.
If you collect leaves from many different plants, you’ll notice that they don’t all look the same. They have different shapes, different venation (the way the veins are arranged in the leaf), and are even attached to the stem differently. Often the shape of a leaf is advantageous in its environment, possibly because it doesn’t break off in strong winds or because it can limit water loss.
There are lots of kinds of leaf shapes, but one distinction is the simple vs. compound leaf. A simple leaf is, as its name suggests, pretty simple. It’s just a blade connected to a stem by a petiole. Oaks and maples have simple leaves, as do beeches:
Compound leaves look like a bunch of leaves that all come from the same stem. However, this stem is really a petiole, and the individual blades connected to it are called leaflets ("little leaves").
An ash leaf is an example of a compound leaf:
In addition to simple and compound leaves, there are some obvious differences in leaf shape. Maple leaves don’t look the same as oak leaves or aspen leaves. Some leaves are lobed, like an oak leaf (on the left below); others are serrated with little teeth. Even compound leaves can take on different shapes. Palmate leaves look like the palm of a hand with fingers spread out, and pinnate leaves have their leaflets arranged on opposite sides of the stem.
Leaves on the same plant can have different shapes. This occurs if some of the leaves were produced by the juvenile plant and later leaves were produced when the plant matures. Leaf thickness and size also varies within a plant, depending on whether the leaf is in the sun most of the day (a "sun leaf") or in the shade most of the day (a "shade leaf"). Sun leaves are smaller and thicker because they pack a bunch of chloroplasts into a small space. Since shade leaves don’t get as much light, they tend to be bigger but thinner, with a wider surface area to capture what light they do receive.
The pattern of veins on a leaf is usually determined by whether the plant is a monocot or eudicot. Monocots have a few parallel veins running down their leaves, which is called parallel venation. Eudicots typically have networks of veins, which is called net venation.
The way leaves are arranged on a plant isn’t something you probably think about everyday, or even at all. However, just as pastry chefs pay attention to where every frosted detail goes when the Food Network has a cake challenge, plants care about where their leaves go. Sometimes leaves are very neatly arranged in an alternating pattern. Other plants try to optimize the light their leaves receive by spreading their leaves out around the stem, in an effort to limit the amount of shade they receive from their own leaves. The arrangement of a plant’s leaves on the stem is called phyllotaxy.
Though most leaves perform photosynthesis, some plants have modified their leaves to do other things. Cacti are a great example of this: those prickly spines are actually leaves. The green part of a cactus is actually a photosynthetic stem. By producing leaves that are thin and pointy, cacti reduce water loss, which could be quite substantial in a desert, and discourage animals from eating their precious leaves or stems. Onions are also modified leaves, but they store food for the plant. Other plants, including many desert succulents (e.g. Aloe vera) use their leaves as storage organs, keeping water in them. Pea plants have tendrils, which are vine-like extensions of leaves. Tendrils can cling to other plants or structures to support the pea plant. Other plants, such as poinsettas, have colored leaves to help attract pollinators.
Many people are familiar with the vibrant autumn leaf displays of the northern United States. Red, orange and yellow leaves fill up the woods and later, people’s lawns. Why do leaves get all dressed up in pretty colors just before they fall from the tree?
The autumn leaves you see are senescing—getting old. Most angiosperms in cold climates can’t keep their leaves through the winter; there just isn’t enough sunshine or water to keep up their metabolism in the winter, so they salvage what nutrients they can before the leaves drop off the tree. The bright colors you see are the pigments that are left after the tree has reabsorbed what it can from its leaves.
Colored compounds called anthocyanins make reds, and other pigments called carotenoids appear yellow or orange. These pigments have been there all along, but are only visible once the plant stops photosynthesizing and stops making chlorophyll. In other climates where leaves don’t change color all at once, they still senescence and drop off the tree. Often old leaves turn yellow, but an individual plant may only have a few yellow leaves at any given time.
Just like curly-haired people sometimes straighten their hair and straight-haired people sometimes curl their hair, plants like to change their appearances too. Some plants can change the way they hold their leaves throughout the day to maximize the sunlight they receive. Lupines always have their leaves fanned out to receive direct sunlight. Other plants, particularly those that live near the equator and get more intense sunlight, change the orientation of their leaves to minimize the solar radiation they get. These plants line their leaves up so that the sun hits the leaf edge rather than the top part of the blade.
Stone plants are plants whose leaves are mostly underground. Only the tops of the leaves are visible, and the leaves look like rocks:
Image from here.