THE LIVING WORLD
Unit Seven. Plant Life
Leaves are usually the most prominent shoot organs and are structurally diverse (figure 33.8). As outgrowths of the shoot apex, leaves are the major light-capturing organs of most plants. Most of the chloroplast-containing cells of a plant are within its leaves, and it is there where the bulk of photosynthesis occurs (see chapter 6). Exceptions to this are found in some plants, such as cacti, whose green stems have largely taken over the function of photosynthesis for the plant. Photosynthesis is conducted mainly by the “greener” parts of plants because they contain more chlorophyll, the most efficient photosynthetic pigment. In some plants, other pigments may also be present, giving the leaves a color other than green. Recall in chapter 6, we described accessory pigments that absorb light of other wavelengths. Thus, although coleus plants and red maple trees have leaves that are reddish in color, these leaves still contain chlorophyll and are the primary sites of photosynthetic activity in the plant.
The apical meristems of stems and roots are capable of growing indefinitely under appropriate conditions. Leaves, in contrast, grow by means of marginal meristems, which flank their thick central portions. These marginal meristems grow outward and ultimately form the blade (flattened portion) of the leaf, while the central portion becomes the midrib. Once a leaf is fully expanded, its marginal meristems cease to grow.
In addition to the flattened blade, most leaves have a slender stalk, the petiole. Two leaflike organs, the stipules, may flank the base of the petiole where it joins the stem. Veins, consisting of both xylem and phloem, run through the leaves. As mentioned in chapter 32, in most dicots the pattern is net, or reticulate, venation—as you can see in figure 33.9a. A In many monocots, the veins are parallel, like the parallel veins that pass vertically up through the monocot leaf in figure 33.9b.
Figure 33.8. Leaves.
Leaves are stunningly variable. (a) Simple leaves from a gray birch, in which there is a single blade. (b) A simple leaf, its margin lobed, from the vine maple. (c) A pinnately compound leaf of a black walnut tree, where leaflets occur in pairs along the central axis of the main vein. (d) Palmately compound leaves of a horse chestnut tree, in which the leaflets radiate out from a single point. (e) The leaves of pine trees are tough and needlelike. (f) Many unusual types of modified leaves occur in different kinds of plants. For example, some plants produce floral leaves or bracts; the most conspicuous parts of this poinsettia flower are the red bracts, which are modified leaves that surround the small yellowish true flowers in the center.
Leaf blades come in a variety of forms from oval to deeply lobed to having separate leaflets (the blade being divided but attached to a single petiole like the black walnut leaf in figure 33.8c). In simple leaves (see figure 33.8a,b), such as those of birch or maple trees, there is a single blade, undivided, but some simple leaves may have teeth, indentations, or lobes, such as the leaves of maples and oaks. In compound leaves, such as those of ashes, box elders, and walnuts, the blade is divided into leaflets. If the leaflets are arranged in pairs along a common axis—the equivalent of the main central vein, or midrib, in simple leaves—the leaf is pinnately compound, such as in the black walnut (see figure 33.8c). If, however, the leaflets radiate out from a common point at the blade end of the petiole, the leaf is palmately compound, such as in horse chestnuts (figure 33.8d) and Virginia creepers.
The positioning of leaves can also vary. Leaves may be alternately arranged (alternate leaves usually spiral around a shoot, like the ivy shown below) or they may be in opposite pairs (like the periwinkle). Less often, three or more leaves may be in a whorl, a circle of leaves at the same level at a node (like the sweet woodruff).
Figure 33.9. Dicot and monocot leaves.
(a) The leaves of dicots have netted, or reticulate, veins; (b) those of monocots have parallel veins.
A typical leaf contains masses of parenchyma, called mesophyll (“middle leaf’), through which the vascular bundles, or veins, run. Beneath the upper epidermis of a leaf are one or more layers of closely packed, columnlike parenchyma cells called palisade mesophyll (the red-stained cells in the photo in figure 33.10). These cells contain more chloroplasts than other cells in the leaf and so are more capable of carrying out photosynthesis. This makes sense when you consider that the cells on the surface receive more sun. The rest of the leaf interior, except for the veins, consists of a tissue called spongy mesophyll. Between the spongy mesophyll cells are large intercellular spaces that function in gas exchange and particularly in the passage of carbon dioxide from the atmosphere to the mesophyll cells. You can see the spongy mesophyll in the photo of figure 33.10, but the air spaces that are the basis of this tissue’s function might be easier to see in the drawing. These intercellular spaces are connected, directly or indirectly, with the stomata in the lower epidermis.
Figure 33.10. A leaf in cross section.
Cross section of a leaf, showing the arrangement of palisade and spongy mesophyll, a vascular bundle or vein, and the epidermis, with paired guard cells flanking the stoma.
Key Learning Outcome 33.5. Leaves, the photosynthetic organs of the plant body, are varied in shape and arrangement.