. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. 118 BOTANY by experiments, external forces act on the organs of a plant so as to change its appearance and often its form and habit. A stem grown in complete darkness is white instead of green. The bleaching of the celery stems by covering them is a familiar example of this. Thus, in nature, forces which we know of as light, gravity, heat, moisture, wind, and many other factors, influence the plant in its growth. Let us now examine some of the examples of modified stems. Stems Modified for
. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. 118 BOTANY by experiments, external forces act on the organs of a plant so as to change its appearance and often its form and habit. A stem grown in complete darkness is white instead of green. The bleaching of the celery stems by covering them is a familiar example of this. Thus, in nature, forces which we know of as light, gravity, heat, moisture, wind, and many other factors, influence the plant in its growth. Let us now examine some of the examples of modified stems. Stems Modified for Water or Food Storage. — Many stems store large quantities of food. The sago palm is an example of such a stem. In most woody stems food is stored during some parts of the year and is used as the plant comes to need it. In other stems the conditions of life are such that the plant has come to store water in the stem. The cactus, which we shall examine more in detail later, is a plant that has developed the stem for the storage of water, and is so adapted to desert conditions as to prevent the evaporation of water from the plant. Underground Stems; the Rootstock. — Other stems not only con- tain stored food but run underground for the protection of the plant. Such a stem is the rootstock of the iris. The root- stock in many respects resembles a root, but can be distinguished from this part of the plant be- cause the leaves come out from definite points or nodes and because true roots leave the under surface. Some under- ground stems do not store food, but grow with considerable rapidity, thus covering ground and starting new outposts of the plant at a distance from the original plants. The pest called quick grass or couch grass, found in almost every lawn, has such a stem. It may be cut in pieces, but each piece may strike root, thus multiplying the plant. The Tuber. — If the underground stem becomes thickened at its end and there forms an enlargement for the storage of fo
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