398 EFFECT OF GRAVITY UPON GROWTH [Ch. XV 



restrained for two hours in a horizontal position than if re- 

 strained in any oblique position, whether the tip be directed up 

 or down. 



Concerning the cause of the curvature there is little to add 

 to what was said under "Roots." The remote cause is appar- 

 ently the dissimilar action of gravity on the upper and the 

 under sides of the stem ; the immediate cause is the difference 

 in growth on the two sides of the stem. In the special case of 

 stems with knots, the knots show themselves especially respon- 

 sive to the geotropic stimulus. 



c. RJiizoma. — These horizontally running, root-like, subter- 

 ranean stems are strikingly responsive to gravity, as Elfvikg 

 ('80), especially, has shown. He has reared various rushes in 

 a glass box with their axes making various angles with the 

 vertical. In their subsequent growth all the rhizomes of the 

 plants extended in a strictly horizontal direction. In this case 

 any component of gravity, however small, running in the direc- 

 tion of the axis of the rhizome seems to irritate. The curving 

 into a horizontal plane may be called transverse geotropism 

 (diageotropism, FkAKk). 



d. Cryptogams. — We have already seen (p. 391) that the 

 sporangiferous hyphee of Phycomyces nitens are negatively 

 geotropic. The same is true of certain algre. Thus Richter 

 ('94) has found that when the stem of Chara is inverted the 

 youngest two or three internodes curve upwards in their 

 further growth so that the apex of the stem is now directed 

 zenithwards. On the other hand the rhizoids of this species 

 are positively geotropic. Rotation experiments show that in 

 the absence of the directive pressure of gravity there is no 

 definite orientation. Finallj^, some mosses are slightly geo- 

 tropic. Tlius, Bastit ('91) reared Polytrichum juniperinum 

 in the dark in both air and water, some plants being placed right 

 side up, others inverted. In both cases new branches budded 

 from the roots and, although etiolated, grew irregularly up- 

 wards — there was a feeble negative geotropism. 



e. Animals. — Since only sessile organisms can be expected 

 to show marked geotropism, this phenomenon among animals 

 must be confined to rather few groups. It has been studied 

 hitherto exclusively in the group of hydi'oids. Many repre- 



