192 STJMMAEY OF CHAPTER. Cdap. Ill 



fact that they would not haye been sensitive to some- 

 what greater continued pressure, if this could have 

 been applied. 



The peculiar form of sensitiveness which we are 

 here considering, is confined to the tip of the radicle 

 for a length of from 1 mm. to 1 ■ 5 mm. When this 

 part is irritated by contact with any object, by caustic, 

 or by a thin slice being cut off, the upper adjoining 

 part of the radicle, for a length of from 6 or 7 to 

 even 12 mm., is excited to bend away from the side 

 which has been irritated. Some influence must there- 

 fore be transmitted from the tip along the radicle for 

 this length. The curvature thus caused is generally 

 symmetrical. The part which bends most apparently 

 coincides with that of the most rapid growth. The 

 tip and the basal part grow very slowly and they 

 bend very little. 



Considering the widely separated position in the 

 vegetable series of the several above-named genera, 

 we may conclude that the tips of the radicles of all, or 

 almost all, plants are similarly sensitive, and transmit 

 an influence causing the upper part to bend. With 

 respect to the tips of the secondary radicles, those of 

 Vicia faba, Pisum sativum, and Zea mays were alone 

 observed, and they were found similarly sensitive. 



In order that these movements should be properly 

 displayed, it appears necessary that the radicles 

 should grow at their normal rate. If subjected to a 

 high temperature and made to grow rapidly, the 

 tips seem either to lose their sensitiveness, or the 

 upper part to lose the power of bending. So it 

 appears to be if they grow very slowly from not being 

 vigorous, or from being kept at too low a temperature , 

 also when they are forced to germinate in the middle 

 of the winter. 



