THE INTERNAL CAUSES OF MOVEMENT 247 



but this does not necessarily afford positive proof of an increase of extensibility in the 

 cell-walls of the convex side. In addition, it has still to be shown why a plasmo- 

 lysing solution often produces at first a slight increase of curvature \ and subsequently 

 a decrease. Possibly this result may be due to the continuance of the tropic induc- 

 tion, but it might also be due to the more rapid penetration on the concave side or to 

 other factors. 



The special Metabolism connected with tropic reactions has been investigated by 

 Kraus 2 . Kraus observed a rise in the percentage of reducing sugar and a diminution 

 of acidity, especially in the under-surface of an ageotropic shoot when placed horir 

 zontally, even before the upward curvature had begun. During the curvature the 

 total amount of sugar, and often also of free acid, decreased on the convex side. Thus 

 in an etiolated bean-shoot, two hours after being placed horizontally, the upper half 

 contained 0-2358 of a gram, the lower 0-2404 of a gram of reducing sugar, an 

 excess of 0-0046 of a gram. Three-quarters of an hour later, other similarly-treated 

 shoots of the same plant contained 0-2095 of a gram of reducing sugar in the upper 

 half of the stem, and 0-2074 of a gram in the lower half, a deficiency of 0-0021. 

 These changes are not produced in the absence of oxygen. 



During and before the commencement of the geotropic curvature the percentage 

 of water in the lower half of the shoot increases, so that the density of the expressed 

 sap decreases. Kraus 3 found, for instance, that in a stem of Anthriscus sylvestris, 

 which had been kept in a horizontal position for twenty-four hours, but had only 

 slightly curved, the specific gravity of the expressed sap from the upper side was 

 1-0240, and that from the lower 1-0226, a difference of 0-0014. This coincides with 

 the fact that during curvature the turgor of the cells on the convex side decreases, as 

 measured by plasmolysis. At the same time, we have an interesting instance of the 

 fact that the distribution of the denser nutrient sap in the tissues is not directly 

 determined by gravity. % 



According to Kraus, the above changes begin before the commencement of cur- 

 vature, and are also shown in stems which are no longer capable of curvature. Hence 

 they do not appear to result from the performance of the bending, and, like the latter, 

 represent reactions due to the stimulating action of gravity. Whether any causal 

 relationship exists is, however, as uncertain as in the case of the increase of silver- 

 reducing substances due to the inductive action of gravity, and taking place before the 

 commencement of curvature. It is also uncertain whether changes in the respiratory 

 activity accompany tropic curvature, and whether the phenomena observed are asso- 

 ciated with all forms of tropic curvature, although Kraus * found similar differences 



} Noll, Arb. d. hot. Inst. in Wurzburg, 1888, p. 517 ; 1895, p. 84. Cf. also Pfeffer, Studien zur 

 Energetik, 1893, p. 247. 



3 G. Kraus, Ueber die Wasservertheilung in d. Pflanze, Bd. II, 1880, p. 38, and Bd. I, 1879, p. 23 

 (reprint from Abhandl. d. Naturforsch. Ges. in Halle). See also Bot. Ztg., 1877, p. 596 ; Ueber die 

 Wasservertheilung, Bd. IV, 1884, p. 59. [The reducing sugars were estimated in the expressed sap 

 by Fehling's method, which gives, of course, merely the total percentage of reducing substances. 

 The differences observed are small and almost within the limit of experimental error.] 



3 Kraus, 1. c., Bd. II, p. 42 ; Wiesner (Die heliotropischen Erscheinungen, 1878, p. 65) and Thate 

 (Jahrb. f. wiss. Bot, 1882, Bd. xni, p. 718) were, however, unable to detect any such differences, 

 possibly owing to less 'exact experimentation. 



* Kraus, 1. c., Bd. II, p. 41. 



