IRRITABILITY. 493 



Pisum sativum; tip of radicle sensitive to attached objects, to caustic, to 



lateral slicing. 

 Phaseolus multiflorusj tip of radicle slightly sensitive to attached objects, 



to caustic, to lateral slicing. 



Tropaeolum ; tip highly sensitive to attached objects. 

 Gossypiitm herbaceum ; tip certainly sensitive to caustic. 

 Cucurbita oviferaj tip moderately sensitive to attached objects, highly 



so to caustic. 



Raphanus sativus j a doubtful case. 



Aesculus ; tips indifferent to attached objects, sensitive to caustic. 

 Quercus Robur and Zea Mais ; tip highly sensitive to attached objects, 



and in the latter plant, to caustic. 



The curvature of the radicle sometimes occurred within 

 6 8 hours after the tip had been irritated, and almost always 

 within 24 hours. The curvature often amounted to a rect- 

 angle, that is, the lower end of the radicle bent upwards 

 until the tip projected almost horizontally ; occasionally the 

 tip, from the continued irritation of the attached object, con- 

 tinued to bend up until it formed a hook with the apex 

 pointing straight upwards, or a loop, or even a spire. After 

 a time the radicle apparently becomes accustomed to the 

 irritation, as is the case also with tendrils, for it again grows 

 downwards, although the bit of card or other object may 

 remain attached to the tip. In some of the experiments the 

 radicles were placed horizontally, and the lower sides were 

 stimulated : under these circumstances the upward curvature 

 did not take place, as it did when the radicles were vertical, 

 in consequence of the stronger opposing influence of geo- 

 tropism. 



The length of the apex which is sensitive is i 1*5 mm., 

 and the length of the curving portion of the radicle, which 

 lies immediately behind the sensitive apex, is 6 12 mm. 

 The curvature which results from stimulation is generally 

 symmetrical. The part which bends most, is apparently the 

 part which is growing the most rapidly: the tip and the basal 

 part grow very slowly and they bend very little. 



With respect to the degree of sensitiveness, Darwin found 

 that a very minute square of writing-paper, attached by 

 shellac, sufficed to cause the radicle of Vicia Faba to curve : 



