552 SUMMARY AND CHAP. XII. 



from it. Whether this can be of any service to them 

 is very doubtful, but with seeds germinating on the 

 surface it will slightly aid geotropism in directing 

 the radicles to the ground.* We ascertained in one 

 instance that such sensitiveness resided in the tip, and 

 caused the adjoining parts to bend from the light. 

 The sub-aerial roots observed by Wiesner were all 

 apheliotropic, and this, no doubt, is of use in bringing 

 them into contact with trunks of trees or surfaces of 

 rock, as is their habit. 



We thus see that with seedling plants the tip of the 

 radicle is endowed with diverse kinds of sensitiveness ; 

 and that the tip directs the adjoining growing parts 

 to bend to or from the exciting cause, according to the 

 needs of the plant. The sides of the radicle are also 

 sensitive to contact, but in a, widely different manner. 

 Gravitation, though a less powerful cause of move- 

 ment than the other above specified stimuli, is ever 

 present ; so that it ultimately prevails and determines 

 the downward growth of the root. 



The primary radicle emits secondary ones which 

 project sub-horizontally ; and these were observed in 

 one case to circumnutate. Their tips are also sensitive 

 to contact, and they are thus excited to bend away 

 from any touching object; so that they resemble in 

 these respects, as far as they were observed, the 

 primary radicles. If displaced they resume, as Sachs 

 has shown, their original sub-horizontal position ; and 

 this apparently is due to diageotropism. The secondary 

 radicles emit tertiary ones, but these, in the case of 

 the bean, are not affected by gravitation ; consequently 

 they protrude in all directions. Thus the general 



* Dr. Karl Richter, who has in Wien,' 1879, p. 149), states that 

 especially attended to this subject apheliotropism does not aid ra- 

 (' K. Akad. der Wissenschalten dicles in penetrating the ground. 



