190 



Causes and Course of Organic Evolution 



three roots studied. While each one of tliose in the soil will 

 grow directly downward as above indicated, that away from 

 the sponge will take the course indicated in (a) while that adja- 



bp 







.»,.».,.^;;.>a.^»^4 



c 



Fig, 3. — Young seedlings of pea. a, radicle when placed transversely has 

 responded directly at its tip to gravity by bending downward; h, oblique posi- 

 tion of tip due to hydrotropic action of moist sponge, sp, that has slightly 

 counteracted geotropic action; c, downward growth of tip, as due to geotropism, 

 and hydrotropism from moisture in dish below, both slightly counteracted by 

 apoheliotropic effect of light from slit s in belljar covered with black paper, hp. 



cent to the sponge will grow as in (b). The third grows nearly 

 downward in virtue of gravity acting powerfully in a straight 

 line upward, and equally of moisture that rises upward. But 

 the lumic stimulus is sufficiently powerful to cause the root to 

 bend away from the light, and to a slight degree therefore to 

 bend away from the downwardly vertical, that indicates the 

 response to gravic and hydric stimulus. The second takes the 

 course sho^\^l, because the hydrotactic stimulus of the wet 

 sponge plus the negative lumic stimulus greatly overcome the 

 gravic and the weaker diffuse hydric stimulus. 



Here then in the above three cases the same type of radicle, 

 when acted on by the same stimuli, applied at different angles, 

 starts an intrinsic molecular machinery in order to plan out 

 a pathway that is the most satisfying for eacli to pursue, but 

 that yet differs in all three, and that is the compounded resultant 



