THE MOVEMENTS OF PLANTS. 



199 



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 perfectly transversely geotropic. What- 



Fig. 195. — Part of centrifuge, a, the axle, rotated at a high speed by water or electric 

 motor, to which is attached the circular metal plate, r, r, carrying a disk of cork, k. 

 To the latter are attached two seedling beans, A , B, by means of pins ; st, the primary 

 stem; h, the primary root. Over the seedlings the cover, g, is placed to keep them 

 moist. After a few hours the lateral roots have turned into the direction of the cen- 

 trifugal force, which was sufficiently powerful to overcome that of gravity except near 

 axis of rotation, x. One half natural size. — After Sachs. 



ever 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 

 curvatures 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. 



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

 of plants regain their power of growth under the stimulus of 

 gravity. The basal portion of the internodes of grasses, 

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



