Fig. 742.—Cells of epidermis (leaf).
Sometimes we find a milky juice in plants. This is called latex; and caoutchouc is always present in it. This juice is contained in tiny tubular vessels, which have their origin in the new cellular tissue of the lactiferous plants.
Fig. 743.—Stomata.
The tissue of the cuticle, or epidermis, which externally covers all parts of the plant while they remain green, is of a peculiar nature, and demands special consideration. It is formed of flat tubular cells, very much compressed, and in close contact, with the exception of some parts where the stomata, or mouths, are placed. In fig. 742 a section of the leaf is represented, the large transparent empty cells of the epidermis, and above these the parenchymatous cells of the leaf filled with greenish-coloured granules. In four places (fig. 743) stomata (s s s s) are seen, which have their openings surrounded by parenchymatous cells disposed in semilunar forms. Under each stoma (mouth) there is a hollow space which is connected with the intercellular passages of the leaf. These stomata, represented in fig. 743, are so numerous on the under side of the leaf, that hundreds have been counted in the space of a square line. Through these minute organs an intimate connection exists between the interior of the plant and the external air.
The epidermal cells not unfrequently exhibit very abnormal formations. When much extended in length they appear as hairs which are frequently branched, and in many plants they contain an irritating sap (in the nettle, for example). Bristles, prickles, glands, warts, and especially the substance which forms the well-known cork, are all due to the metamorphosis of this exterior integument.
Flowering Plants.
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