CHAP. IX. TRANSMITTED EFFECTS OF LIGHT. 483 



generally occurred, 41 radicles, which had germinated in damp 

 sawdust, were immersed in water and exposed to a lateral light ; 

 and they all, with two doubtful exceptions, became curved from 

 the light. At the same time the tips of 54 other radicles, 

 similarly exposed, were just touched with nitrate of silver. 

 They were blackened for a length of from *05 to '07 mm., and 

 probably killed ; but it should be observed that this did not 

 check materially, if at all, the growth of the upper part; for 

 several, which were measured, increased in the course of only 

 8-9 h. by 5 to 7 rnm. in length. Of the 54 cauterised radicles 

 one case was doubtful, 25 curved themselves from the light in 

 the normal manner, and 28, or more than half, were not in the 

 least apheliotropic. There was a considerable difference, which 

 we cannot account for, in the results of the experiments tried 

 towards the end of April and in the middle of September. 

 Fifteen radicles (part of the above 54) were cauterised at the 

 former period and were exposed to sunshine, of which 12 failed 

 to be apheliotropic, 2 were still apheliotropic, and 1 was doubt- 

 ful. In September, 39 cauterised radicles were exposed to a 

 northern light, being kept at a proper temperature ; and now 

 23 continued to be apheliotropic in the normal manner, and 

 only 16 failed to bend from the light. Looking at the aggregate 

 results at. both periods, there can be no doubt that the de- 

 struction of the tip for less than a millimeter in length destroyed 

 in more than half the cases their power of moving from the 

 light. It is probable that if the tips had been cauterised for 

 the length of a whole millimeter, all signs of apheliotropism 

 would have disappeared. It may be suggested that although 

 the application of caustic does not stop growth, yet enough may 

 be absorbed to destroy the power of movement in the upper 

 part; but this suggestion must be rejected, for we have seen 

 and shall again see, that cauterising one side of the tip of various 

 kinds of radicles actually excites movement. The conclusion 

 seems inevitable that sensitiveness to light resides in the tip 

 of the radicle of Sinapis alba; and that the tip when thus 

 stimulated transmits some influence to the upper part, causing 

 it to bend. The case in this respect is parallel with that of 

 the radicles of several plants, the tips of which are sensitive to 

 contact and to other irritants, and, as will be shown in the 

 eleventh chapter, to gravitation. 



