INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 



2Q7 



Nemec 1 has advanced the idea that starch grains act as such "statoliths" in 

 plant cells. The forv,e of gravitation is thus supposed to act upon the starch 

 grains, which are of higher specific gravity than the liquid about them, so that 

 they always lie in that part of the cell nearest to the center of the earth (Fig. 

 143). The pressure exerted by these grains, upon the protoplasm of the cell, 

 is supposed to inaugurate the series of protoplasmic changes which finally result 

 in visible bending. 



To this attempt at a physical interpretation, Czapek 2 has opposed a chemical 

 one.' This writer was able to demonstrate certain chemical changes in 

 tissues affected by geotropic as well as in those affected by phototropic stimuli. 

 In this connection the observations of 0. Richter 3 may be important, to the 

 effect that negative geotropism disappears in plants under the influence of the 

 more or less poisonous air of the laboratory (see also page 261). 



Chemical investigation of growth phenomena is the only method of ap- 

 proach that promises to furnish a fundamental explanation of geotropic and 



Fig. 142. — Normal flower of Epilobrium angustifolii 



Fig. 143. — Tip of cotyledon 



(at the left), and actimorphic flower (at the right), the latter of Panicum mileaceum, showing 



produced by rotation of the plant about a horizontal axis. 

 (After Vöchling.) 



starch grains lying on the phys- 

 ically lower side of each cell. 

 (After Nemec.) 



phototropic reactions. For the present it can be said simply that under the 

 influence of gravitation the primary shoot grows upward and the primary root 

 downward. 



1 Nemec, В., Die Perception des Schwerkraftreizes bei den Pflanzen. Ber. Deutsch. Bot. Ges. 20 : 

 339-3S4- 1902. 



- Czapek, F., Stoffwechselprocesse in der geotropisch gereizten Wurzelspitze und in phototropische 

 sensiblen Organen. Ber. Deutsch. Bot. Ges. 20: 464-470. 1902. Czapek, F., and Rudolf, Bertel, 

 Oxydative Stoffwechselvorgänge bei pflanzlichen Reizreaktionen. (I. Abhandlung.) Jahrb. wiss. Bot. 43: 

 361-467. 1906. Gräfe, V., and Linsbauer, K., Zur Kenntnis der Stoffwechselvorgänge bei geotropischer 

 Reizung. Sitzungsber. (math.-naturw. Kl.) K. Akad. Wiss. Wien. 119': 827-852. 1910. 



3 Richter, Oswald, Die horizontale Nutation. Sitzungsber (math.-naturw. Kl.) K. Akad. wiss. Wien 

 H9 7 : 1051-1084. 1910. 



z To the editor there seems to be no opposition between these two views. The suggestions 

 of Nemec and Haberlandt attempt to explain only how the attraction of gravitation may 

 become converted into a pressure of some cell-components upon others, and it is self-evident 

 that this can represent only the first link in the chain of cause and effect that finally terminates 

 in an alteration of growth rate in certain cells of the bending region of the plant. Between the 

 pressure postulated by the physical theory and the bending itself, there must occur, as the 

 author has already suggested, an unknown series of chemical and physical reactions, and 

 Czapek's studies seem to deal with some of these. — Ed. 



