Page:First course in biology (IA firstcourseinbio00bailrich).pdf/77

 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.

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.