CHAP. III. OF THE KADICLE TO MOIST AIR. 181 



radicle to bend away from the source of irritation. 

 In our experiments we followed Sachs' plan, and sieves 

 with seeds germinating in damp sawdust were sus- 

 pended so that the bottom was generally inclined at 

 40 with the horizon. If the radicles had been acted 

 on solely by geotropism, they would have grown out 

 of the bottom of the sieve perpendicularly down- 

 wards; but as they were attracted by the adjoining 

 damp surface they bent towards it and were deflected 

 50 from the perpendicular. For the sake of ascertain- 

 ing whether the tip or the whole growing part of the 

 radicle was sensitive to the moist air, a length of from 

 1 to 2 mm. was coated in a certain number of cases 

 with a mixture of olive-oil and lamp-black. This 

 mixture was made in order to give consistence to the 

 oil, so that a thick layer could be applied, which 

 would exclude, at least to a large extent, the moist air, 

 and would be easily visible. A greater number of 

 experiments than those which were actually tried 

 would have been necessary, had not it been clearly 

 established that the tip of the radicle is the part which 

 is sensitive to various other irritants. 



Phaseolus muttiflorus. Twenty-nine radicles, to which no- 

 thing had been done, growing out of a sieve, were observed 

 at the same time with those which had their tips greased, 

 and for an equal length of time. Of the 29, 24 curved them- 

 selves so as to come into close contact with the bottom of the 

 sieve. The place of chief curvature was generally at a distance 

 of 5 or 6 mm. from the apex. Eight radicles had their tips 

 greased for a length of 2 mm., and two others for a length of 

 U mm. ; they were kept at a temperature of 15-16 C. After 

 intervals of from 19 h. to 24 h. all were still vertically or 

 almost vertically dependent, for some of them had moved 

 towards the adjoining damp surface by about 10. They had 

 therefore not been acted on, or only slightly acted on, by the 

 damper air on one side, although the whole upper part was 

 freely exposed. After 48 h. three of these radicles became 



