SECT. II 



PHYSIOLOGY 315 



comes about by the revolving movement together with negative 

 geotropism. The support plays a part in that it prevents the other- 

 wise inevitable straightening. It must stand more or less vertically, 

 because otherwise it would not be continually grasped by the overhanging 

 tip of the shoot ; this is so since the support does not influence the 

 shoot as is the case where tendrils are concerned (p. 318). The 

 twining is further assisted by the shoots of the twining plant having 

 to begin with elongated internodes while the leaves remain small. 

 In this respect these shoots resemble those of etiolated plants ; the 

 delayed unfolding of the leaves allows of the regular circling of the 

 tip which might otherwise be interfered with by the leaves encounter- 

 ing the support. The firm hold on the support is frequently increased 

 by the roughness of the surface of the stem owing to hairs, 

 prickles, ridges, etc. Torsions also, the causes of which cannot be 

 entered into here, have a similar effect. 



A detailed analysis of the process of twining is unfortunately still wanting, and 

 all investigators do not agree that twining is due only to revolving movement 

 and negative geotropism. Thus Schwendener (1881) holds that in addition to these 

 factors a "grasping movement" is necessary. The point of the twining shoot will 

 from time to time come in contact with the support, and by the tensions which will 

 thus result the curvatures become " permanent " coils. 



Geotaxis. — In some lower organisms an influence of gravity on changes of 

 position has been shown to exist. This fact can only be mentioned here. 



(e) Chemical Stimuli 



In the same way as light and gravity, heat and electricity, when 

 their action is one-sided, may bring about directive movements of the 

 plant. Since, however, these movements play no great part in nature 

 they need not be further considered. Those directive movements 

 which are brought about by the unequal distribution of dissolved or 

 gaseous substances in the neighbourhood of the plant are of much 

 greater importance ; these movements are termed chemotactic and 

 chemotropic. 



Chemoteopism {^^^). In the case of Fungi and of pollen-tubes 

 chemotropic movements have been demonstrated which bring the 

 organism into a certain concentration of particular substances ; this 

 concentration is the optimal one. With the same organism and the 

 same stimulating substance these movements are sometimes positive 

 and sometimes negative ; positive when they lead towards a higher 

 concentration of the substance, and negative in the converse case. In 

 the case of pollen-tubes sugar is the chief substance that acts as a 

 stimulus; in fungi, in addition to sugar, peptone, asparagin, compounds 

 of ammonia and phosphates. There are also substances such as free 

 acids which always liave a re}'ellent influence even in extremely ^veak 

 concentration. Chemotropic irritability has also been demonstrated 



