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Show Tbe Direction of Growth in Roots. While it is the rule for primary roots, or those first developed, to grow downward, the secondary branches usually tend to assume a direction almost al-most at right angles to the vertical, and so grow outward and a little downward, as if they were but slightly slight-ly susceptible to the action of gravity; while tertiary brandies, and the farther brandies to which those give rise, grow in all directions quite independent of gravity. It is plain that as a result of these peculiarities the active parts of the root are distributed in such a manner man-ner as to search tlio surrounding eartli more thoroughly than would otherwise be possible. In case a stone or othur obstruction is encountered by any of the brauehes. the tip is turaed aile and follows the coutour closely until the edge is reached, when it soon assumes as-sumes its proper direction. Not infrequently in-frequently it must happen that some root-enting nuimal will destroy the end of a young primary rout, and so endanger the prope1 development if the whole system, but experiment ha sliowu that in the event of such injury one of the younger secondary brauehes changes its" direction of growth, so as to point directly downward, and thus assume the function of the primary root to promote the search for food in the deeper regions. Popular Science MoiUIUy. |