Leaves are the primary sites of photosynthesis

In gymnosperms and most flowering plants, the leaves are responsible for most of the plant's photosynthesis, producing energy-rich organic molecules and releasing oxygen gas.

Tuber (modified stem) Branches

Tuber (modified stem) Branches

Modified Stems Figure

In certain plants, the leaves are highly modified for more specialized functions, as we will see below.

As photosynthetic organs, leaves are marvelously adapted for gathering light. Typically, the blade of a leaf is a thin, flat structure attached to the stem by a stalk called a petiole. During the daytime, the leaf blade is held by its petiole at an angle almost perpendicular to the rays of the sun. This orientation, with the leaf surface facing the sun, maximizes the amount of light available for photosynthesis. Some leaves track the sun, moving so that they constantly face it.

The leaves at different sites on a single plant may have quite different shapes. These shapes result from a combination of genetic, environmental, and developmental influences. Most species, however, bear similar, if not identical, leaves of a particular broadly defined type. A leaf may be simple, consisting of a single blade, or compound, with blades, or leaflets, arranged along an axis or radiating from a central point (Figure 35.5). In a simple leaf, or in a leaflet of a compound leaf, the veins may be parallel to one another, as in monocots, or in a netlike arrangement, as in eudicots.

The general development of a specific leaf pattern is programmed in the plant's genes and is expressed by differential growth of the leaf veins and of the tissue between the veins. As a result, plant taxonomists have often found leaf forms (outlines, margins, tips, bases, and patterns of arrangement) to be reliable characters for classification and identification. At least some of the forms in Figure 35.5 probably look familiar to you.

During development in some plant species, leaves are highly modified for special functions. For example, modified leaves serve as storage depots for energy-rich molecules, as in the bulbs of onions. In other species, the leaves store water, as in succulents. The spines of cacti are modified leaves (see Figure 35.4c). Many plants, such as peas, have modified portions of leaves called tendrils that support the plant by wrapping around other structures or plants.

Leaves, like all other plant organs, are composed of cells, tissues, and tis-

35.4 Modified Stems (a) A potato is a modified stem called a tuber; the sprouts that grow from its eyes are shoots, not roots. (b) The runners of this beach strawberry are horizontal stems that produce roots at intervals. Runners provide a local water supply and allow rooted portions of the plant to live independently if the runner is cut. (c) The stem of this barrel cactus is enlarged to store water. Its thorny spines are modified leaves.

Types

Shapes

Types

Shapes

Margins

Types Leaf Divisions

35.5 The Diversity of Leaf Forms Simple leaves are those with a single blade. Some compound leaves consist of leaflets arranged along a central axis. Further division of leaflets results in a doubly compound leaf. Other characters of leaf form can also be used to identify a plant's species.

Margins

35.5 The Diversity of Leaf Forms Simple leaves are those with a single blade. Some compound leaves consist of leaflets arranged along a central axis. Further division of leaflets results in a doubly compound leaf. Other characters of leaf form can also be used to identify a plant's species.

sue systems. Let's now consider plant cells—the basic structural and functional units of plant organs.

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  • gundahar
    Is the blade the primary structure for photosynthesis?
    2 years ago

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