530 SENSITIVENESS TO GKAVITATION. Chap XL 



Bome preliminary remarks must first be made. It may be ob- 

 jected that the caustic would injure the radicles and prevent them 

 from bending; but ample evidence was given in Chapter III. 

 that touching the tips of vertically suspended radicles with 

 caustic on one side, does not stop their bending; on the 

 contrary, it causes them to bend from the touched side. We 

 also tried touching both the upper and the lower sides of the 

 tips of some radicles of the bean, extended horizontally in damp 

 friable earth. The tips of three were touched with caustic on 

 their upper sides, and this would aid their geotropic bending ; 

 the tips of three were touched on their lower sides, which 

 would tend to counteract the bending downwards ; and three 

 were left as controls. After 24 h. an independent observer was 

 asked to pick out of the nine radicles, the two which were most 

 and the two which were least bent ; he selected as the latter 

 two of those which had been touched on their lower sides, and 

 as the most bent, two of those which had been touched on the 

 upper side. Hereafter analogous and more striking experiments 

 with Pisum mtivum and Cucurhita ovifera will be given. We 

 may therefore safely conclude that the mere application of 

 caustic to the tip does not prevent the radicles from bending. 



In the following experiments, tne tips of young horizontally 

 extended radicles were just touched with a stick of dry caustic ; 

 and this was held transversely, so that the tip might be cau- 

 terised all round as symmetrically as possible. The radicles 

 were then suspended in a closed vessel over water, kept rather 

 cool, viz., 55°-59° F. This was done because we had found 

 that the tips were more sensitive to contact under a low than 

 under a high temperature ; and we thought that the same rule 

 might apply to geotropism. In one exceptional trial, nine 

 radicles (which were rather too old, for they had grown to a 

 length of from 3 to 5 cm.), were extended horizontally in damp 

 friable earth, after their tips had been cauterised, and were 

 kept at too high a temperature, viz., of 68^ P., or 20° G. The 

 result in consequence was not so striking as in the subsequent 

 cases; for although when after 9 h. 40 m. six of them were 

 examined, these did not exhibit any geoti'opio bending, yet after 

 24 h., when all nine were examined, only two remained hori- 

 zontal, two exhibited a trace of geotropism, and five were 

 slightly or moderately geotropic, yet not comparable in degree 

 with the control specimens. Marks had been made on seven oi 

 these cauterised radicles at 10 mm. from the tips, which includes 



