CIIAP. III. SENSITIVENESS OF RADICLES. 131 



tiicir curvature ; and Sachs has shown * that the 

 growing part is more rigid than the part immediately 

 above which has ceased to grow, so that the latter 

 might have been expected to yield and become curved 

 as soon as the apex encountered an unyielding object ; 

 whereas it was the stiff growing part which became 

 curved. Moreover, an object which yields with the 

 greatest ease will deflect a radicle : thus, as we have 

 seen, when the apex of the radicle of the bean 

 encountered the polished surface of extremely thin 

 tin-foil laid on soft sand, no impression was left on it 

 yet the radicle became deflected at right angles. A 

 second explanation occurred to us, namely, that even 

 the gentlest pressure might check the growth of the 

 apex, and in this case growth could continue only on 

 one side, and thus the radicle would assume a rectan- 

 gular form ; but this view leaves wholly unexplained 

 the curvature of the upper part, extending for a length 

 of 8-10 mm. 



We were therefore led to suspect that the apex 

 was sensitive to contact, and that an effect was trans- 

 mitted from it to the upper part of the radicle, which 

 was thus excited to bend away from the touching object. 

 As a little loop of fine thread hung on a tendril or 

 on the petiole of a leaf-climbing plant, causes it to 

 bend, we thought that any small hard object affixed 

 to the tip of a radicle, freely suspended and growing 

 in damp air, might cause it to bend, if it were sensitive, 

 and yet would not offer any mechanical resistance to 

 its growth. Full details will be given of the experi- 

 ments which were tried, as the result proved remark- 

 able. The fact of the apex of a radicle being sensitive 

 to contact has never been observed, though, as we shall 



Arbeiten Bot. Inst. Wiir/burg,' Heft i'ii. 1873, p. 398. 



