^2 



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 also 

 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 

 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. 

 Jalirb. Wiss. Bot. 32 : 175. 1898. 



3 Noll, F. Ueber heterogene Induktion. 1892. 



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



Fig. 30. Portion of a 

 longitudinal section through 

 a node of Tradescantia Vir- 

 ginica, the long axis of 

 which forms an angle of 

 13 with the vertical. The 

 arrow marks the direction 

 of the action of gravity 

 which throws the starch 

 grains to the lower left-hand 

 angles of the cells in the 

 sheath. After Haberlandt. 



