244 TROPIC MOVEMENTS 



heliotropic response was due solely to a tendency to curvature of the concave 

 side, and the same objection applies to Kohl's view that geotropic curvature 

 is due to active contraction of the concave side. 



SECTION 54. The Internal Causes of Movement. 



Neither the mode in which the changes of turgor responsible for varia- 

 tion movements, nor that in which the altered growth of nutation curvatures 

 is produced, is precisely known. It is, however, certain that the changed 

 rates of growth are not due to alterations of turgor, as de Vries l supposed, 

 for, apart from the fact that no curvature could be produced in this manner 

 in unicellular organs, plasmolytic researches have shown that no rise of 

 turgor takes place during geotropic curvature 2 . Beit's unfounded supposi- 

 tion that positive heliotropism is the result of the decomposition of sugar 

 on the illuminated side requires no discussion. In the case of rapidly 

 curving organs, a slight fall of turgor may actually take place in the cells 

 of the concave side, which apparently results from the rapid increase of 

 volume, water being absorbed in greater amount than the self-regulatory 

 production of osmotic materials is able to compensate for immediately. 



Even when a general or unilateral rise of turgor accompanies a tropic 

 reaction, its relationship to the induced irregularity of growth is accessory 

 and not causal. Kohl's observations 3 do not prove that a rise of turgor 

 takes place in the cells of the concave side during geotropic response, and 

 Noll 4 has shown that a rise of turgor will not cause the cells of the concave 

 side to shorten. The turgor of the nodal parenchyma of Hordeum vulgare 

 rises by the equivalent of about i to 2 per cent, of potassium nitrate when 

 the stem is fixed in a horizontal position 5 , but this is not in itself sufficient 

 to directly cause the growth of the cells, while no such rise is shown by the 

 nodal cells of Triticum vulgare and T. sfielta, which are capable of as ready 

 and rapid geotropic response as those of Hordeum vulgare. In the same 

 way a rise of turgor is shown by some plants, but not by all, when working 

 against external resistance ; and although such rises act as an aid to growth 

 they do not directly induce it. 



Although the mechanism of growth need not always be the same, 

 the required expansion is usually produced by a plastic stretching of the 

 cell-wall. Evidence of this is afforded by the fact that during curvature 



1 De Vries, Landw. Jahrb., 1880, Bd. IX, p. 502. 



* Wortmann, Ber. d. hot. Ges., 1887, p. 961 ; Bot. Ztg., 1889, p. 456 ; Noll, Arb. d. hot. Inst. 

 in Wiirzburg, 1888, Bd. Ill, p. 511 ; Flora, 1895, Ergzbd., p. 36. 

 s Kohl, Mechanik der Reizkriimmnngen, 1894, p. 59. 



4 Noll, Flora, 1895, Ergzbd., pp. 48, 54. 



5 Pfeffer, Druck- und Arbeitsleistungen, 1893, pp. 399, 405. 



