Fig. 52.—Creeping underground stem of Solomon’s Seal. a, bud of next year’s aërial growth;
b, scar of this year’s growth; c, d, e, scars of aërial growth of previous years; w, roots. (× ¾.)
Underground stems.—Although the stem is usually that part of the axis of a plant which is above ground, there are many exceptions. The bracken fern, daisy, coltsfoot, Solomon’s seal ([Fig. 52]), and many other plants have stems which are ordinarily buried in the ground, giving off leaves above and roots below. In some cases such underground stems become much swollen with stored food-material—manufactured by the leaves in excess of immediate requirements. In the potato, for example, certain underground branches of the stem store up starch to such an extent that their ends become fleshy, ovoid masses some inches in thickness ([Fig. 53]). The true nature of these tubers is revealed by the buds or “eyes” which spring upon them. The buds are arranged spirally—as may be seen by sticking a pin into each and joining up the pins with thread—and when the tubers are kept in a warm, moist place, each bud grows out into a new leafy shoot.
Fig. 53.—Part of a Potato plant, showing the
old tuber (dark) and several new ones. (× ⅙.)
The structure of a bulb is easily made out in the onion, tulip ([Fig. 54]), hyacinth, or daffodil. When such a bulb is cut down the middle, it is seen to be mainly composed of leaves or leaf-bases, swollen with stored food. Inside these are the young leaves and the flower bud, which would have expanded next season; and on the outside are a few thin scale leaves. All these leaves spring from a fleshy button at the base, which gives off roots below. The button is the flattened stem. When a plant produces a bulb it will generally be found that it flowers either very early or very late in the season; that is, at a period which would not be very favourable for the work of the leaves. The flower ([Fig. 55]) is produced at the expense of the stored food in the bulb—made in excess of the requirements of the previous season. After the plant has flowered, the new leaves work until they have made enough food for next season’s flower, and then they also die.
