were here acting at right angles to each other upon the seedlings, the oblique direction of their axes shows that they were affected by the resultant of the two forces concerned, in just the manner called for by Knight's supposition.
Although gravity is thus seen to be the influence which" induces a downward tendency in roots, it of course does not follow that all the younger parts of a root-system are equally affected. 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 at right angles to the vertical, and so grow outward and a little downward, as if they were but slightly susceptible to the action of gravity; while tertiary branches, and the farther branches to which these 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 as to search the surrounding earth more thoroughly than would otherwise be possible.
In case a stone or other obstruction is encountered by any of the branches, the tip is turned aside and follows the contour closely until the edge is reached, when it soon assumes its proper direction. Not infrequently it must happen that some root-eating animal will destroy the end of a young primary root, and so endanger the proper development of the whole system, but experiment has shown that in the event of such injury one of the younger secondary branches 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.
At first sight it would seem that surely gravity must affect all parts of the growing region of a rootlet in the same manner, since all parts are equally exposed to its influence. In 1871, however, Ciesielski announced that rootlets from which the tip had been carefully removed with a razor lost all sensitiveness to gravity until a new tip had grown, when the behavior became normal. Other investigators failed to obtain the same results; but some years later Darwin repeated Ciesielski's experiments successfully, and confirmed his conclusion that it is the tip alone which is sensitive to gravity, and from this part the stimulus is transmitted to the adjoining region of growth, which bends downward in consequence.
Another influence to which roots are very sensitive is that of moisture. This is strikingly exhibited in an experiment devised by Sachs. Seeds are made to germinate in a layer of moist sawdust, contained in a sieve-like framework, and this suspended obliquely as shown in Fig. 5. The young roots grow directly down-
- Abwärtskrümmung der Wurzel. Inaugural Dissertation. Breslau, 1871.