128 THE VEGETATIVE FUNCTIONS OF PLANTS 



rection of the "pull" (centrifugal tendency), while the 

 shoot will curve in the opposite direction (Fig. 85). 



The causal relationship between gravity and the direc- 

 tion of growth of roots and shoots was first established 

 by the English botanist, Thomas Andrew Knight, who 

 devised the experiment illustrated in Fig. 78. 



128. Geotropism. — Careful thought about these results 

 will make it clear that the horizontally placed root, in 

 the first experiment, does not merely bend down because 

 of its weight. If this were so, we would expect the shoot 

 to bend down also. The curving is the response of the 

 organs to the stimulus of the pull. The property of an 

 organ by virtue of which it may detect the direction of 

 the pull of gravity is geotropism. It is thus seen that 

 geotropism is a particular kind of irritability. Organs 

 which respond by a curvature in the direction of the pull 

 are positively geotropic; those which respond by a curvature 

 in the opposite direction are negatively geotropic. 



129. Zone of Ctirvature. — The following simple experi- 

 ment shows that the geotropic curvature always takes 

 place in a definite region. A germinating bean or other 

 seed, with the sprout (hypocotyl) about 15 to 20 milli- 

 meters long, is pinned to a strip of cork, fastened to the 

 bottom of a Petri dish (Fig. 86). The sprout is marked 

 with fine lines of India ink 2 millimeters apart, beginning 

 2 millimeters back from the tip, as in the study of growth 

 (page 1 18) . Up to this point in the operations care must 

 be exercised to keep the sprout as nearly parallel with the 

 plumb-line as possible. By rotating the cork, or the 

 entire Petri dish, the sprout is now fixed at right angles 

 to the plumb-line, and the Petri dish covered and set in 



