Fig. 112.—Cowpea. Describe the leaves. For what is the plant used?
CHAPTER XII
LEAVES—STRUCTURE OR ANATOMY
Besides the framework, or system of veins found in blades of all leaves, there is a soft cellular tissue called mesophyll, or leaf parenchyma, and an epidermis or skin that covers the entire outside part.
Fig. 113.—Section of a Leaf, showing the air-spaces.
Breathing-pore or stoma at a. The palisade cells which chiefly contain the chlorophyll are at b. Epidermal cells at c.
Mesophyll.—The mesophyll is not all alike or homogeneous. The upper layer is composed of elongated cells placed perpendicular to the surface of the leaf. These are called palisade cells. These cells are usually filled with green bodies called chlorophyll grains. The grain contains a great number of chlorophyll drops imbedded in the protoplasm. Below the palisade cells is the spongy parenchyma, composed of cells more or less spherical in shape, irregularly arranged, and provided with many intercellular air cavities (Fig. [113]). In leaves of some plants exposed to strong light there may be more than one layer of palisade cells, as in the India-rubber plant and the oleander. Ivy when grown in bright light will develop two such layers of cells, but in shaded places it may be found with only one. Such plants as iris and compass plant, which have both surfaces of the leaf equally exposed to sunlight, usually have a palisade layer beneath each epidermis.
Epidermis.—The outer or epidermal cells of leaves do not bear chlorophyll, but are usually so transparent that the green mesophyll can be seen through them. They often become very thick-walled, and are in most plants devoid of all protoplasm except a thin layer lining the walls, the cavities being filled with cell sap. This sap is sometimes coloured, as in the under epidermis of begonia leaves. It is not common to find more than one layer of epidermal cells forming each surface of a leaf. The epidermis serves to retain moisture in the leaf and as a general protective covering. In desert plants the epidermis, as a rule, is very thick and has a dense cuticle, thereby preventing loss of water.
There are various outgrowths of the epidermis. Hairs are the chief of these. They may be (1) simple, as on primula, geranium, nægelia; (2) once branched, as on wall-flower; (3) compound, as on verbascum or mullein; (4) disk-like, as on shepherdia; (5) stellate, or star-shaped, as in certain crucifers. In some cases the hairs are glandular, as in Chinese primrose of the greenhouses (Primula Sinensis) and certain hairs of pumpkin flowers. The hairs often protect the breathing-pores, or stomates, from dust and water.
Stomates (sometimes called breathing-pores) are small openings or pores in the epidermis of leaves and soft stems that allow the passage of air and other gases and vapours (stomate or stoma, singular; stomates or stomata, plural). They are placed near the large intercellular spaces of the mesophyll, usually in positions least affected by direct sunlight. Fig. [114] shows the structure. There are two guard-cells at the mouth of each stomate, which may in most cases open or close the passage as the conditions of the atmosphere may require. The guard-cells contain chlorophyll. In Fig. [115] is shown a case in which there are compound guard-cells, that of ivy. On the margins of certain leaves, as of fuchsia, impatiens, cabbage, are openings known as water-pores.