RELATION OF PLANTS TO GRAVITATION 



action is shown, although it is supposed that this displacement 

 may form the method of perception of the stimulus in sen- 

 sory zones. Gravity is found to exert 

 a slight influence upon the rate of growth 

 of complex organs, as a mechanical 

 consequence of weight, which may alsc^ 

 set up stresses with their own proper 

 results, in compression and stretching re- 

 actions. 



As will be shown below, gravity exerts 

 some influence upon the disposition, origin 

 and form of organs. 1 



110. Nature of the Stimulating Influence 

 of the Force of Gravity. No actual demon- 

 stration of the manner in which gravity acts 

 upon an organ as a stimulus has yet been 

 made. The two principal theories suggest 

 that the gross weight of the plastic organ 

 acts as a stimulus, by its change of form 

 when thrown out of equilibrium with 

 gravity, or that bodies like centrosomes, 2 

 the heavier organs of the cell or the 



FIG. 30. Portion of a 

 longitudinal section through 

 a node of Tradescantia Vir- 

 ginica, the long axis of 



which forms an angle of granules in the starch sheath act in the 

 same manner as the heavy bodies in a com- 

 pound statolith. In the last named method 

 the granules would fall to the lower side 

 of the cell and their contact with the ec- 

 toplasmic layer would give the actual stim- 

 ulus. Neither of these methods would 



account for all of the features of geotropic reaction however. 3 



1 Mottier, D. The effect of centrifugal force on the cell. Annals of Botany, 

 13 : 325. 1899- 



2 Czapek, F. Weitere Beitrage zur Kenntniss der geotropischen Reizbewegungen, 

 Jahrb. Wiss. Bot. 32 : 175. 1898. 



3 Noll, F. Ueber heterogene Induktion. 1892. 



Noll, F. Ueber Geotropismus. Jahrb. Wiss. Bot. 34 : 457. 1900. 



13 with the vertical. The 

 arrow marks the direction 

 of the action of gravity 

 which throws the starth 

 grains to the lower left-hand 

 angles of the cells in the 

 sheath. After Haberlandt. 



