Fig. 48.—The Marking of the Stem and Root.
The Root-cap.—Note the form of the root-cap shown in the microscopic section drawn in Fig. [47]. Growing cells, and especially those which are forming tissue by subdividing, are very delicate and are easily injured. The cells forming the root-cap are older and tougher and are suited for pushing aside the soil that the root may penetrate it.
Region of most Rapid Growth.—The roots of a seedling bean may be marked at equal distances by waterproof ink or by bits of black thread tied moderately tight. The seedling is then replanted and left undisturbed for two days. When it is dug up, the region of most rapid growth in the root can be determined. Give a reason why a root cannot elongate throughout its length,—whether there is anything to prevent a young root from doing so.
Fig. 49.—The Result.
In Fig. [48] is shown a germinating scarlet runner bean with a short root upon which are marks made with waterproof ink; and the same root (Fig. [49]) is shown after it has grown longer. Which part of it did not lengthen at all? Which part lengthened slightly? Where is the region of most rapid growth?
Geotropism.—Roots turn toward the earth, even if the seed is planted with the micropyle up. This phenomenon is called positive geotropism. Stems grow away from the earth. This is negative geotropism.
Suggestions (Chaps. VII and VIII).—25. Tests for food. Examine a number of roots, including several fleshy roots, for the presence of food material, making the tests used on seeds. 26. Study of root-hairs. Carefully germinate radish, turnip, cabbage, or other seed, so that no delicate parts of the root will be injured. For this purpose, place a few seeds in packing-moss or in the folds of thick cloth or of blotting paper, being careful to keep them moist and warm. In a few days the seed has germinated, and the root has grown an inch or two long. Notice that, except at a distance of about a quarter of an inch behind the tip, the root is covered with minute hairs (Fig. [44]). They are actually hairs; that is, root-hairs. Touch them and they collapse, they are so delicate. Dip one of the plants in water, and when removed the hairs are not to be seen. The water mats them together along the root and they are no longer evident. Root-hairs are usually destroyed when a plant is pulled out of the soil, be it done ever so carefully. They cling to the minute particles of soil (Fig. [46]). The hairs show best against a dark background. 27. On some of the blotting papers, sprinkle sand; observe how the root-hairs cling to the grains. Observe how they are flattened when they come in contact with grains of sand.
Fig. 50.—The Grasp of a Plant on the Particles of Earth. A grass plant pulled in a garden.