Fig. 426.
Corm of Jack-in-the-pulpit.
725. Corm.—A corm is a thick, short, fleshy, underground stem. A good example is found in the jack-in-the-pulpit (Arisæma).
726. Tubers.—These are thickened portions of the subterranean stems. The most generally known example is the potato tuber (“Irish” potato, not the sweet potato, which is a root). The “eyes” of the potato are buds on the stem from which the aerial shoots arise when the potato sprouts. The potato tuber is largely composed of starch which is used for food by the young sprouts.
726a. Phylloclades.—These are trees, shrubs, or herbs in which the leaves are reduced to mere bracts and stems, are not only green and function as leaves, but some or all of the branches are flattened and resemble leaves in form as in Phyllanthus, Ruscus, Semele, Asparagus, etc. The flowers are borne directly on these flattened axes. The prickly-pear cactus (Opuntia) is also a phylloclade. Examples of phylloclades are often to be found in greenhouses.
727. Undifferentiated stems are found in such plants as the duckweed, or duckmeat (Lemna, Wolffia, etc.[ See Chapter III]).
[IV. Annual Growth and Winter Protection of Shoots and Buds.][41]
728. Winter conditions.[42]—While herbs are subjected only to the damp warm atmosphere of summer, woody plants are also exposed during the cold dry winter, and must protect themselves against such conditions. The air is dryer in winter than in summer; while at the same time root absorption is much retarded by the cold soil. Then, too, the osmotic activity of the dormant twig-cells being much reduced, the water-raising forces are at a minimum. It is easy to see, therefore, that a tree in winter is practically under desert conditions. Moreover, it has been found by various investigators, contrary to the general belief, that cold in freezing is only indirectly the cause of death. The real cause is the abstraction of water from the cell by the ice crystals forming in the intercellular spaces. Death ensues because the water content is reduced below the danger-point for that particular cell. It was formerly thought that on freezing, the cells in the tissue were ruptured. This is not so. Ice almost never forms within the cell, but in the spaces between. Freezing then is really a drying process, and dryness, not cold, causes death in winter. To protect themselves in winter, trees provide various waterproof coverings for the exposed surfaces and reduce the activity of the protoplasm so that it will be less easily harmed by the loss of water abstracted by the freezing process.
Fig. 427.
Two-year-old twig of horse-chestnut, showing buds and leaf-scars. (A twig with a terminal bud should have been selected for this figure.)