189 



A Natural Proof That the Root Tip Alone is Sensitive to 

 the Gravitation Stimulus. 



By Frank Marion Andrews. 



Pfeffer* and Czapek* have demonstrated that it is the root tip only 

 that is sensitive to the stimulus of gravitation. In order to accomplish 

 this end they resorted to the following method. Glass tubes of such a 

 diameter as would just fit over the end of the root tip were made by draw- 

 ing out thick-walled glass tubes. The tubes thus obtained were bent into 

 an L shape, had a total length of about three (3) mm., and weighed about 

 thirty (30) milligrams. They were closed at one end and left open at the 

 other: each limb of the L-shaped tube made in this way had a length of 

 1.5 mm. The inside diameter of the tube depended on the size of the root 

 for which it was intended. It was necessary in all cases to have this 

 glass tube fit The root loosely. It was connected to a piece of cork and 

 the germinated seedling also fastened to the same cork in such a way 

 that the root tip projected into the glass cap about to the bend. The 

 whole being rotated in a klinostat for some hours the root, freed of the 

 stimulus of gravity, grew into the above mentioned glass cap and finally 

 assumed its L-shaped form. When removed from the klinostat and placed 

 with the curved tip of the root vertical and the rest of the root horizontal 

 no geotropic curvature took place, which shows that since the tip of the 

 root was constrained from bending, the sensitiveness of geotropic stimulus 

 must be located there, else it would have bent at a point outside the glass 

 tube. I have accidentally found a natural proof of this admirable and con- 

 clusive discovery of Pfeffer and Czapek. In some germinating corn 1 

 observed one instance in which the root of the embryo had not freed 

 itself in the usual way. but instead, the scutellum was broken about mid- 

 way and carried down by the root on its tip as a mass of tissue. The 

 outer coats were not broken, and these adhering about the scutellum on 

 one side in the usual way made the mass so strong that the root could not 

 grow out of it — at least it did not do so. This mass of tissue when re- 

 moved weighed fifteen (15) milligrams. The root had turned and grown 



' Jahr f. wiss. Bot., 1895, Bd. 27, p. 243, and 1900, Bd. 35. p. 313. 



