390 NATURAL SCIENCE [June 



Spalding shows that injury to the aerial roots of Antlmtrimn, etc., 

 although they are destitute of root-cap, still induces strong mani- 

 festation of trauniatropic curvatures. Moreover, roots in which tlie 

 injury is to the root-cap alone, without at the same time affecting 

 the apical meristem or actual growing-point, do not carry out 

 trauniatropic bendings. The whole evidence, in fact, points to the 

 growing-point as the only region sensitive to cuts, burns, or other 

 injuries, and suggests that these agencies act as true stimuli which 

 are conveyed to the elongating region lying behind. 



These results show a close correspondence with those already 

 mentioned in the case of geotropism, and in yet another respect do 

 we find a similarity between the two phenomena. The latent period, 

 viz., the time between the perception of the stimulus and the result- 

 ing reaction, can be greatly extended both in geotropism (as we have 

 seen) and in trauraatropism. A seedling of Lupmus alius, for 

 example, had its root-tip branded with a heated glass rod. The 

 rootlet was immediately afterwards confined in a plaster cast, so that 

 its growth, and therefore also its reaction, were prevented. After 

 eight days the cast was removed, and the root allowed to resume its 

 growth. It at once executed a trauniatropic curvature, which could 

 only have been in response to the injury received from the hot glass 

 more than a week before. This shows that in the case of stimula- 

 tion by uni-lateral injuries the latent period can be even more 

 protracted than in geotropic actions. 



Quite a different aspect of root-physiology has been touched upon 

 by Czapek (4) in his paper " Zur Lehre von den Wurzelausscheidun- 

 gen." In this paper the substances which are excreted by the root 

 into the surrounding medium are subjected to a careful and search- 

 ing examination. In the present article it is impossible to enter 

 at any length into the numerous important facts and conclusions 

 which Czapek has set down in the memoir quoted, we can only 

 pick out one or two significant points, and shortly deal with these. 



The older teachings of Liebig and others, that, in the excretions 

 of the root, substances are present which alone are capable of laying 

 open to the plant the nutritive store of the soil, has, with the course 

 of time, been modified gradually, and taken with ever increasing 

 reserve. The fact that roots excreted acids, capable of permanently 

 reddening litmus-paper and of corroding a smooth surface of marble, 

 had, however, been long believed in by botanists. The researches of 

 •Czapek have now thrown considerable doubt upon this point. In 

 no instance could he satisfy himself that any of the stronger acids 

 passed out from the root in the free state. Acid salts and carbonic 

 acid could alone be detected, and all the phenomena upon which the 

 idea of free acids in the excreta depend, can certainly (as Czapek 

 points out) be explained by the presence of these two substances. 



