222 TROPIC MOVEMENTS 



tion of growth which accompanies curvature, and this applies not only to 

 radial but also to physiologically-dorsiventral tendrils. In the latter case, 

 therefore, both sides are sensitive to contact, but in different ways, for 

 only stimulation of the concave side is able to produce a curvature. Further 

 research is, however, necessary to elucidate this phenomenon and to deter- 

 mine wherein the difference between the two surfaces lies. 



RHEOTROPISM, TRAUMATROPISM, and HYDROTROPISM have already 

 been shown to be special irritabilities involving distinct powers of perception. 



GEOTROPISM. Gravity and centrifugal force probably act indirectly, 

 the changes of pressure or of the position of the parts in the cell due to 

 their altered direction acting as the stimulus exciting curvature. That the 

 pressures external to the cell are immaterial is shown by the fact that 

 unicellular organisms show geotropic responses, and that a root will curve 

 down into mercury against an upward pressure. It is, however, uncertain 

 whether the pressure of the fluid or of the solid contents of the cell acts 

 as a stimulus, and it does not follow that the relationships are the same in 

 all organisms, or that plants must behave in the same way as certain lower 

 animals whose perception of and orientation in regard to gravity appear 

 to be due to the pressure exercised by solid bodies such as statoliths and 

 otoliths in special ' auditory ' sense-organs. 



By the term geotropism we merely indicate the power of response to 

 a particular tropic stimulus, and hence the same term would still be used if 

 this form of irritability proved to be due to some kind of internal contact 

 stimulation l . In the same way the term magneto-tropism would be used 

 if a tropic response was produced by the action of a magnet upon internal 

 particles of iron or upon the substances of varying magnetic permeability 

 of which the plant-cell is composed 2 . As a matter of fact, plants, like man, 

 seem to be devoid of any direct power of perception of gravitational forces. 



Knight 3 was probably the first to suggest that geotropic curvatures 

 were caused by the distribution of materials of varying specific gravity in 

 the plant, although according to Treviranus the same idea was previously 

 put forward by Astruc 4 . Knight, however, seems to have assumed that the 

 mass-attraction of gravity directly produced a downward plastic curvature 

 of the root, and was unaware that the root will grow downwards in mercury 

 or against considerable resistances. Negative geotropism Knight considered 

 to be the result of the denser nutrient sap collecting on the under side of the 

 horizontally-placed stem, causing this side to grow more rapidly and hence 

 producing an upward curvature of the apex. Hofmeister 5 accepted this 



1 Cf. Verworn, Allgemeine Physiologie, 1901, 3. Aufl., p. 467. 



2 Cf. Ewart, On Protoplasmic Streaming in Plants, Clar. Press, 1903, p. 45. 



3 Knight, Phil. Trans., 1806, PL I, p. 104. 



4 Treviranus, Physiologie, 1838, Bd. II, p. 599. 



5 Hofmeister, Allgemeine Morphologic, 1868, p. 629. 



