538 SENSITIA'ENESS TO GKAVITATION. Chap. XJ. 



all -nere strongly geotropic, most of them pointiiig perpen- 

 dicularly downwards. Ten other radicles, similarly placed, had 

 their tips touched with caustic on the lower side; after 8 h. 

 three were slightly geotropic, but not nearly so much- so as the 

 least geotropic of the foregoing specimens ; four remained hori- 

 zontal; and three were curved upwards in opposition to geo- 

 tropism. After 19 h. the three which were slightly geotropic 

 had become strongly so. Of the four horizontal radicles, one 

 alone showed a trace of geotropism; of the three up-curved 

 radicles, one jretained this curvature, and the other two had 

 become horizontal. 



The radicles of this plant, as already remarked, do not succeed 

 well in damp air, but the result of one trial may be briefly 

 given. Nine young radicles between '3 and '5 inch in length, 

 with their tips cauterised and blackened for a length never 

 exceeding i mm., together with eight control specimens, were 

 extended horizontally in damp air. After an interval of only 



4 h. 10 m. all the controls were slightly geotropic, whilst not 

 one of the cauterised specimens exhibited a trace of this action. 

 After 8 h. 35 m., there was the same difference between the 

 two sets, but rather more strongly marked. By this time both 

 sets had increased greatly in length. The controls, however, 

 never became much more curved downwards ; and after 24 h. 

 there was no great difference between the two sets in their 

 degree of curvature. 



Eight young radicles of nearly equal length (average -36 inch) 

 were placed beneath and on peat-earth, and were exposed to a 

 temp, of 75°-76° F. Their tips had been touched transversely 

 with caustic, and five of them were blackened for a length of 

 about 0'5 mm., whilst the other three were only just visibly dis- 

 coloured. In the same box there were 15 control radicles, mostly 

 about '36 inch in length, but some rather longer and older, and 

 therefore less sensitive. After 5 h., the 15 control radicles were 

 all more or less geotropic : after 9 h., eight of them were bent 

 down beneath the horizon at various angles between 45° and 90°, 

 the remaining seven being only slightly geotropic : after 25 h. all 

 were rectangularly geotropic. The state of the eight cauterised 

 radicles after the same intervals of time was as follows: after 



5 h. one alone was slightly geotropic, and this was one with 

 the tip only a very little discoloured: after 9 h. the one just 

 mentioned was rectangularly geotropic, and two others were 

 slightly so, and these were the three wliich had been scarcely 



