202 PLANT PHYSIOLOGY 



toplasm or by means outside of it, the relations of the parts 

 and of the protoplasm to gravity are also changed. The 

 force of gravity acts constantly. Therefore, whether the 

 ponderable parts of a cell are changed or are constant in 

 position, they and the protoplasm are constantly influenced 

 by gravity. Only when there is a change in the relation of 

 the parts to gravity is the protoplasm called upon to 

 change its response. The action of gravity in a cell and 

 upon its parts must be the same as upon any ponderable 

 substance. The heavier (or lighter) the substance, the 

 greater (or less) its specific weight as compared with the 

 protoplasm in which it is imbedded, the more promptly 

 will it exert a different pressure upon the protoplasm 

 and change in position when the position of the cell 

 is altered. In every living cell there are solid particles of 

 greater or less size, living or lifeless, presumably differing 

 among themselves and from the protoplasm in weight. In 

 certain plants, e.g. some Desmids, in rhizoids of Chara, * in 

 the starch-sheath of stems and in the root-cap, etc., of higher 

 plants, f solid particles are conspicuous in size or arrange- 

 ment. The force of gravity will, however, act similarly, 

 though less evidently, upon the solid contents of all cells. 



The reaction of the living protoplasm to the pull of 

 gravity is very different from that of lifeless material. The 

 reaction may show Itself in a variety of ways, chemical and 

 physical. CzapekJ has shown that a chemical change takes 

 place in root cells stimulated geotropically. In the periblem 

 cells of sensitive root-tips there is present, in larger quantity 



* Giesenhagen, K. Uber inneie Vorgange bei der geotropischen Kriim- 

 mung der Wurzeln von Chara. Ber. d. Deutsch. Bot. Gesellsch., XIX., 1901. 



f Haberlandt, G. Uber die Perception des geotropischen Reizes. Ber. d. 

 Deutsch. Bot. Gesellsch.. Bd. XVIII., 1900. Uber die Statolithenfunction 

 der Starkekorner. Ibid., Bd. XX., 1902. Nemec, B. Uber die Art der 

 Wahrnehmung des Schwerkraftreizes bei den Pflanzen. Ibid., Bd. XVIII., 

 1900. Uber die Wahrnehmung des Schwerkraftreizes bei den Pflanzen. 

 Jahrb. f. wiss. Bot., Bd. XXXVI.. 1901. Noll, F. Uber Geotropismus. 

 Ibid., Bd. XXXIV.. 1900. Zur Controverse tiber den Geotropismus. Ber. 

 d. Deutsch. Bot. Gesellsch., Bd. XX., 1902. 



f Czapek, F. Ein mikroskopischer Befund an geotropisch gereizten Wur- 

 zeln. Ber. d. Deutsch. Bot. Gesellsch., XV., 1898. Also Jahrb. f. wiss. Bot. 

 Bd. 32, 1898. 



