17.6: Leaves feed the plant.

One of the recurring themes in comparing plants and animals is that plants must acquire energy and nutrients without being able to move. Leaves solve this problem handily. They have one primary purpose—producing food for the plant—and a dazzling array of forms has evolved that allow them to accomplish this purpose efficiently and effectively. Like the solar collectors that can heat houses (but much more efficient), leaves intercept sunlight and, by putting chloroplast-containing photosynthetic cells in the path of that light, convert the potential energy of the sun into the usable chemical energy of sugars. Plants don’t need to move to get energy—they utilize the energy that comes to them.

The arrangement of cells inside the leaf clearly serves the purpose of capturing the energy of sunlight to make sugar and then efficiently transporting that food out of the leaf to the rest of the plant. Starting at the upper surface and moving inward, let’s examine the leaf’s layered structure (FIGURE 17-16).

• 1. A waxy cuticle on the upper surface protects the leaf from damage and reduces the rate at which water evaporates from the leaf in the hot sun.
• 2. The epidermis, the cells forming the upper surface, are non-photosynthetic and protective; they secret the cuticle.
• 3. Below the epidermis are a few layers of photosynthetic cells. The cells in the first layer, called the palisade mesophyll, are closely packed parenchyma cells, rich in chloroplasts. Rates of photosynthesis are highest in this part of the leaf. In the next layer or two, parenchyma cells are irregularly shaped and more loosely arranged, forming numerous air pockets that make the layers somewhat spongy—hence this portion of the leaf is called spongy mesophyll. Surrounded by air pockets, most of the cells in the spongy layer are in contact with the air surrounding the leaf, enhancing the diffusion of gases, including the CO₂ essential to photosynthesis, from the air and into the plant’s photosynthetic cells.
• 4. Veins run through the middle of the leaf. Leaf veins are composed of vascular tissue, xylem and phloem. Much like the veins, arteries, and capillaries in humans, they reach out to all the cells to deliver water and essential minerals and haul away the sugar products of photosynthesis for use elsewhere in the plant.
• 5. Like the top of the leaf, the bottom layer is composed of non-photosynthetic epidermal cells. Interspersed among these cells are stomata (see Section 17-3), the pore-like openings through which the plant takes in CO₂ and releases O₂, one of the by-products of photosynthesis.

Nearly all leaves are thin blades attached to a stem by a small stalk, but they vary tremendously in appearance (FIGURE 17-17). A plant can have a small number of huge leaves. Water lily leaves, for example, can reach almost 7 feet across and inspired Monet’s mural-sized paintings, which are more than 6 feet high and 42 feet wide (FIGURE 17-18). At the other end of the spectrum, plants can have a large number of small leaves. Duckweed is such a plant: 25 duckweed leaves laid end to end cover less than an inch. Both strategies can be effective ways of creating large surface areas for intercepting sunlight. But plants can’t change the laws of physics, and those laws dictate that the energy in sunlight can’t pass through very many layers of cells without being absorbed or reflected. For this reason, even though leaves come in all shapes and sizes, they are all thin. If leaves were more than a few layers thick, the cells near the bottom would not get enough energy to run the sugar-making machinery.

Among the flowering plants, many monocots can be identified by their long, flat leaves that are oriented vertically, as in the grasses, in a pattern that keeps them from shading each other. Eudicot leaves come in two types: simple leaves, each with just a single undivided blade, and compound leaves, having a number of leaflets attached to a central support structure. Each of these two types can vary greatly, from sharp-edged to smooth-edged Gymnosperms—seed plants, such as the conifers, that don’t produce flowers—have a variety of leaf structures, including needle-like leaves, such as those of pine trees. A sampling of the wide range of leaf shapes and styles is shown in Figure 17-17.

Perhaps the most extremely modified plant leaves are the hard, dry spines of most cacti. The spines don’t contribute to photosynthesis—that occurs in the stems, which we see as the prominent green parts of the plant. Instead, the spines help adapt cacti for desert living. They slow the speed of warm winds as they pass over the plant, reducing the water lost to evaporation. The spines also help moisture from the cooler night air condense, enabling the plant to capture a bit of the most limited desert resource. Spines go a long way toward discouraging birds and mammals from eating the plant, as well. In Chapter 19, we explore some of the chemicals that plants sequester in their leaves that also help protect them from being eaten.