THE MOVEMENTS OF PLANTS. I75 



Support an onion, roots down, in a vessel of water so that it is half im- 

 mersed, until the leaves are about lo cm. long. Then turn it so that 

 leaves are horizontal and observe where curvature occurs. 



246. Transverse geotropism. — Not all stems, however, 

 are negatively geotropic, nor all roots positively geotropic. 

 The central axis of both root and stem in the majority of 

 plants is so, but lateral branches of both place themselves at 

 an angle to the action of gravity, sometimes at a right angle, 

 at other times at a highly obtuse or acute angle. That is, 

 they are more or less transversely geotropic. Whatever the 

 normal position of any organ, it will be regained by the 

 growing parts as rapidly as possible when the plant is forcibly 

 displaced. This can only be brought about by the curva- 

 tures produced by unequal growth of the younger parts. 



If a potted plant be laid upon its side for a short time and 

 then erected before any response to the stimulus occurs its 

 growing parts still curve to one side, although not so far as if 

 they had been allowed to remain in the horizontal position. 



247. Grasses. — In only a few cases do the maturer parts 

 of plants regain their power of growth under the stimulus of 



Fig. T2g.— Part of a wheat-stalk, showing strong geotropic curvature. The shoot was 

 placed horizontal, and the growth of the basal part of the intemode with the leaf-sheath 

 connected witli it was stimulated on the under side, the upper remaining short. No 

 curvature occurs in the older part of the intemode. About two thirds natural size. 

 —After Pfefter. 



gravity. The basal portion of the internodes of grasses, 

 however, remain for a long time capable of growth ; hence, 

 when grasses are blown down or trampled their stems erect 

 themselves by the geotropism of this basal growing zone 

 and of the leaf-sheath (fig. 129). 



