3 6 THE POPULAR SCIENCE MONTHLY. 



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" in- 

 duces 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 out- 

 ward and a little downward, as if they were but slightly suscep- 

 tible 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 en- 

 danger the proper development of the whole system, but experi- 

 ment 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 sen- 

 sitive to gravity, and from this part the stimulus is transmitted 

 to the adjoining region of growth, which bends downward in con- 

 sequence. 



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 saw- 

 dust, contained in a sieve-like framework, and this suspended ob- 

 liquely as shown in Fig. 5. The young roots grow directly down- 



* Abwaitskriiramung der Wurzel. Inaugural Dissertation. Breslan, 1871. 



