CHAP. III. OF THE KADICLE TO MOIST AIR. 185 



of the air may have caused the difference in the results at the 

 two periods. 



Finally, the facts just given with respect to Phaseolus 

 multiflorus, Vicia faba, and Avena sativa show, as it 

 seems to us, that a layer of grease spread for a length 

 of 1J to 2 mm. over the tip of the radicle, or the 

 destruction of the tip by caustic, greatly lessens or 

 quite annuls in the upper and exposed part the power 

 of bending towards a neighbouring source of moisture. 

 We should bear in mind that the part which bends 

 most, lies at some little distance above the greased or 

 cauterised tip ; and that the rapid growth of this part, 

 proves that it has not been injured by the tips having 

 been thus treated. In those cases in which the radicles 

 with greased tips became curved, it is possible that the 

 layer of grease was not sufficiently thick wholly to ex- 

 clude moisture, or that a sufficient length was not thus 

 protected, or, in the case of the caustic, not destroyed. 

 When radicles with greased tips are left to grow for 

 several days in damp air, the grease is drawn out into 

 the finest reticulated threads and dots, with narrow 

 portions of the surface left clean. Such portions 

 would, it is probable, be able to absorb moisture, and 

 thus we can account for several of the radicles with 

 greased tips having become curved towards the sieve 

 after an interval of one or two days. On the whole, 

 we may infer that sensitiveness to a difference in the 

 amount of moisture in the air on the two sides of a 

 radicle resides in the tip, which transmits some influ- 

 ence to the upper part, causing it to bend towards the 

 source of moisture. Consequently, the movement is 

 the reverse of that caused by objects attached to one 

 side of the tip, or by a thin slice being cut off, or by 

 being slightly cauterised. In a future chapter it 

 will be shown that sensitiveness to the attraction of 



