534 SENSITIVENESS TO GRAVITATION. Chap. XL 



takes place; and that when the tip of the root is cauterised it is 

 unable to originate the stimulus necessary to produce geotropic 

 curvature. 



As vru had observed that grease was highly injurious to some 

 plants, we determined to try its effects on radicles. When the 

 cotyledons of Phalaris and Avena were covered with grease 

 along one side, the growth of this side was quite stopped or 

 greatly checked, and as the opposite side continued to grow, the 

 cotyledons thus treated became bowed towards the greased side, 

 This same matter quickly killed the delicate hypocotyls and 

 young leaves of certain plants. The grease which we employed 

 was made by mixing lamp-black and olive oil to such a con- 

 sistence that it could be laid on in a thick layer. The tips ol 

 five radicles of the bean were coated with it for a length of 

 3 mm., and to our surprise this part increased in length in 23 h. 

 to 7 ■ 1 mm. ; the thick layer of grease being curiously drawn 

 .■)ut. It thus could not have checked much, if at all, the growth 

 of the terminal part of the radicle. With respect to geotropism, 

 the tips of seven horizontally extendtd radicles were coated for 

 a length of 2 mm., and after 24 h. no clear difference could be 

 perceived between their downward curvature and that of an 

 equal number of control specimens. The tips of 33 other radicles 

 were coated on different occasions for a length of 8 mm. ; and 

 they were compared with the controls after 8h., 24 h., and 48 h. 

 On one occasion, after 24 h., there was very little difference in 

 curvature between the greased and control specimens; but 

 generally the difference was unmistakable, those with greased 

 tips being considerably less curved downwards. The whole 

 growing part (the greased tips included) of six of these radicles 

 was measured and was found to have increased in 23 h. from 

 10 mm. to a mean length of 17 ■ 7 mm. ; whilst the corresponding 

 part of the controls had increased to 20'8 mm. It appears there- 

 fore, that although the tip itself, when greased, continues to 

 grow, yet the growth of the whole radicle is somewhat checked, 

 and that the geotropic curvature of the upper part, which was 

 free from grease, was in most cases considerably lessfflied. 



Pisum sativum. — Five radicles, extended horizontally over 

 water, had their tips lightly touched two or three times with dry 

 caustic. These tips were measured in two cases, and found to 

 be blackened for a length of only half a millimeter. Five other 

 radicles were left as controls. The part which is most bowed 

 through geotropism lies at a distance of several milUmeters from 



