180 SENSITIVENESS OF THE APEX CHAP. ITT. 



seed, but not laterally from the primary one. Ten of 

 these secondary radicles, which were directed obliquely 

 downwards, were experimented on with very small 

 squares of card attached with shellac to the lower 

 sides of their tips. If therefore the squares acted, the 

 radicles would bend upwards in opposition to gravity. 

 The jar stood (protected from light) on a sand-bath, 

 which varied between 76 and 82 F. After only 

 5 h. one appeared to be a little deflected from the 

 square, and after 20 h. formed a loop. Four others 

 were considerably curved from the squares a f ter 20 h., 

 and three of them became hooked, with their tips 

 pointing to the zenith, one after 29 h. and the 

 two others after 44 h. By this latter time a sixth 

 radicle had become bent at a right angle from the side 

 bearing the square. Thus altogether six out of the 

 ten secondary radicles were acted on, four not being 

 affected. There can, therefore, be no doubt that the 

 tips of these secondary radicles are sensitive to slight 

 contact, and that when thus excited they cause the 

 upper part to bend from the touching object; but 

 generally, as it appears, not in so short a time as in 

 the case of the first-formed radicle. 



SENSITIVENESS OF THE TIP OF THE RADICLE TO 

 MOIST AIE. 



Sachs made the interesting discovery, a few years 

 ago, that the radicles of many seedling plants bend 

 towards an adjoining damp surface.* We shall here 

 endeavour to show that this peculiar form of sensitive- 

 ness resides in their tips. The movement is directly 

 the reverse of that excited by the irritants hitherto 

 considered, which cause the growing part of the 



* ' Arbeiten des Bot. Institut., in Wiirzburg,' vol. i. 1872, p. 209. 



