Internal Structure Of Leaves

If a typical leaf is cut transversely and examined with the aid of a microscope, three regions stand out: epidermis, mesophyll, and veins (referred to as vascular bundles in our discussion of roots and stems) (Fig. 7.6). The epidermis is a single layer of cells covering the entire surface of the leaf. The epidermis on the lower surface of the blade can sometimes be distinguished from the upper epidermis by the presence of tiny pores called stomata, which are discussed in the section that follows.

When seen from the top, the wavy, undulating walls of the epidermal cells often resemble pieces of a jigsaw puzzle fitted together. Except for guard cells, the upper epidermal cells for the most part do not contain chloroplasts, their function being primarily protection of the delicate tissues to the interior. A coating of waxy cutin (the cuticle— see Fig. 4.6) is normally present, although it may not be visible with ordinary light microscopes without being specially stained. In addition to the cuticle, many plants produce other waxy substances on their surfaces (Fig. 7.7). In studies of the effects of smog and auto exhaust fumes on plants, it was found that these waxes may be produced in abnormal fashion on beet leaves within as little as 24 hours after exposure to the pollutants. Presumably, the wax affords added protection to the leaves. Beet leaves also respond to aphid damage by producing wax around each tiny puncture.

Leaves 113

Palmately Compound Leaf

Figure 7.4 Types of leaves and leaf arrangements. A. Palmately compound leaf of a buckeye. B. Pinnately compound leaf of a black walnut. C. Alternate, simple but lobed leaves of a tulip tree. D. Opposite, simple leaves of a dogwood. E. Palmately veined leaf of a maple. F. Globe-shaped succulent leaves of string-of-pearls. G. Pinnately veined, lobed leaf of an oak. H. Parallel-veined leaf of a grass. I. Whorled leaves of a bedstraw. J. Linear leaves of a yew. K. Fan-shaped leaf of a Ginkgo tree, showing dichotomous venation.

Figure 7.4 Types of leaves and leaf arrangements. A. Palmately compound leaf of a buckeye. B. Pinnately compound leaf of a black walnut. C. Alternate, simple but lobed leaves of a tulip tree. D. Opposite, simple leaves of a dogwood. E. Palmately veined leaf of a maple. F. Globe-shaped succulent leaves of string-of-pearls. G. Pinnately veined, lobed leaf of an oak. H. Parallel-veined leaf of a grass. I. Whorled leaves of a bedstraw. J. Linear leaves of a yew. K. Fan-shaped leaf of a Ginkgo tree, showing dichotomous venation.

In some plants, waste materials occasionally accumulate and crystallize in epidermal cells. Different types of glands may also be present in the epidermis. Glands occur in the form of depressions, protuberances, or appendages either directly on the leaf surface or on the ends of hairs (see Fig. 4.13A). Glands often secrete sticky substances.

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