THE MOVEMENTS OF PLANTS. 



20 1 



290. Root-cage. — Experiments upon the response of root- 

 lets to the stimulus of gravity upon altering their position 

 may be carried on by means of a root-cage, shown in figure 

 197. It consists essentially of two panes of glass placed close 

 together, between which, in finely sifted soil, the rootlets are 

 grown. By inclining this root- 

 cage at various angles it may be 

 shown that not only the primary 

 root, but its branches, strive to 

 regain their normal angle with 

 the direction of gravity. This 

 is illustrated in figure 198, in 

 which the dark portion of the 

 rootlets represents the growing 

 parts while the cage was in- 

 verted. They then took about 

 the same angle with the horizon 

 as when in normal position. 



Many dorsiventral organs, 



Fig. 198. — Part of the root system of a broad 



bean, grown in a root-cage, first in the 



normal, then in the inverted, and again 



in the normal position. The arrows show 



SUCh aS leaves, are transversely the direction in which gravity acted in 



the different positions. The black por- 



geOtropiC, JUSt as leaves are tion of the roots were the parts growing 



. during inversion. Two thirds natural 



transversely heliotropic. size— After Sachs. 



291. Twining plants. — The movements of twining plants 

 are due to a peculiar reaction to gravity. As the upper inter- 

 nodes of a seedling elongate they soon become too weak to 

 support themselves and bend over, becoming nearly horizon- 

 tal. When this occurs the growth of the right or left flank 

 of the stem near the bend is accelerated (whence the stem is 

 said to be laterally geotropic). The horizontal part is thus 

 swung around, twisting the stem and bringing a new flank 

 under the influence of the stimulus. If in its continued rota- 

 tion the stem comes in contact with a nearly erect support 

 the free part continues to rotate, growing longer at the same 

 time, and encircles the support. The part below the point 



