THE GENERAL CHEMISTRY AND PHYSICS OF PLANT-LIFE. 283 



root-tip. Cutting off 2 mm. from the root-tip does not prevent 

 the root from continuing its growth. Accordingly Darwin con- 

 cludes that the root-ti]3 receives a stimulus from the force of 

 gravity and transmits it to the zone of maximum growth. Of 

 course it is not implied that the root possesses a " conscious " 

 power to transmit this stimulus. Sachs maintains, moreover, 

 that shoots behave differently ; they curve geotropically even 

 when the entire apex is removed. (According to Dar\nt:n, root- 

 tips undergo circumnutation ; this is denied by Sachs.) 



In regard to the utility of irritable movements due to shock 

 and contact we will state the following : In the case of tendrils 

 this utility is very evident, for by such irritable movements 

 they are enabled to function as organs of adhesion and support. 

 The irritable movements of the stamens of Cynarece are de- 

 scribed and explained as follows. A visiting insect causes the 

 stimulus by coming in contact with the anther, wdiich thereby 

 suddenly contracts the anther-tube, while the hair-like bristles 

 of the style which is not irritable " brushes " out some of the 

 pollen, which is carried away by the insect to fertilize another 

 plant. In regard to Mimosa, Sachs made the observation that 

 this plant is able to withstand hail much better than more robust 

 plants, because the very first contact suffices to place it in the 

 irritated position ; that is, the leaflets become folded and the 

 petiole sinks. Pfeffer supposes that such movements also 

 serve as a protection by frightening away animals. Among 

 " insectivorous " plants the irritable organs serve to catch in- 

 sects, as has been explained in the case of Drosera. 



E. The Physiology of Twining. 



One of the most complicated phenomena in the plant-creation 

 is the tivining of plants. It is not an irritable movement in the 

 narrower sense, produced by shock and contact. Here we find 

 combined with circumnutation the effect of negative geotropism 

 and the influence of apparent and real torsion. 



It is well known that the stem of a climbing plant (as hops, 

 beans, Calystegia, Ipomcea, etc.) winds about a support in the form 

 of a spiral. Ths mechanics of this process is essentially dif- 

 ferent from that of the winding of tendrils. To understand it 

 well it is necessary for the investigator to have special prepara- 



