510 



THE POPULAR SCIENCE MONTHLY. 



through radiation. The movements of various organs to the light, 

 which ai*e so general throughout the vegetable kingdom, and occa- 

 sionally from the light, or transversely with respect to it, are all modi- 

 fied forms of circumnutation, as again are the equally prevalent 

 movements of stems, etc., toward the zenith, and of roots toward the 

 center of the earth. In accordance with these conclusions, a consider- 

 able difficulty in the way of evolution is in part removed, for it might 

 be asked. How did all their diversified movements for the most differ- 

 ent purposes first arise ? As the case stands, we know that there is 

 always movement in progress, and its amplitude or direction, or both, 

 have only to be modified for the good of the plant, in relation with 

 internal or external stimuli." 



The discovery of the sensitiveness of the apex of the radicle was 

 made by Darwin when he was looking for something else. Wishing 

 to know how the radicles of seedlings passed over obstacles in the 

 ground, he placed germinating beans in such a way that the tips of 



A. 



B. 



C. 



Fig. 14. Vicia paba : A, radicle beginning to bend from the attached little square of card ; B, 

 bent at a rectangle ; C, bent into a circle or loop, with the tip beginning to bend downward 

 ihrough the action of geotropism. 



the radicles came into contact with opposing surfaces at a high angle. 

 "When the root-cap touched such an obstacle it was at first a little flat- 

 tened, but this flattening soon disappeared, and the apex took a direc- 

 tion at right angles to its former course. Straight lines had been 

 painted along the growing terminal part of some of these radicles before 

 they met the opposing objects, and the lines became sensibly curved 

 in two hours after the apex had come into contact with them. The 

 explanation of this curvature of the growing part could not be me- 



