560 MOTOR SYSTEM 



mechanism of the curling movement. Duval-Jouve believes that the 

 curling is due to the active contraction of the large epidermal cells at 

 the bases of the furrows, which takes place when their turgor is 

 diminished owing to active transpiration. Tschirch, on the other hand, 

 regards the abaxial fibrous strands as the active components ; he 

 assumes that the internal fibrous layers have a greater power of 

 imbibition (and hence a greater tendency to contract on drying) than 

 the more superficial layers. According to Steinbrinck, finally, the 

 mechanism is a cohesive one, and depends principally upon the action of 

 the large epidermal cells in the furrows and of the green mesophyll, 

 and to a smaller extent, also, to that of the wide internal bast-fibres. 

 In the author's own opinion, Steinbrinck's view is the most plausible. 



There is still much uncertainty, in most cases, as to whether any 

 particular mechanism depends upon imbibition or upon cohesion. The 

 difficulty is due partly to the comparatively recent discovery of the 

 cohesive type of mechanism, and partly to two accessory factors which 

 complicate the main issue. In the first place, the action of a given 

 mechanism may depend upon cohesion up to a certain stage of desicca- 

 tion, but after that may be entirely a matter of differential imbibition; 

 there is no doubt that the dehiscence of certain Fern-sporangia is a 

 composite effect of this sort. Secondly, movements of homologous organs 

 are often performed with the aid of totally different mechanisms in 

 closely related species. Both these circumstances will have to be 

 taken into account in future investigations of this subject. 



C. LIVING MOTOR-TISSUES. 273 



Among the various types of movement in which living tissues play 

 an active part, we may, first of all, mention certain more or less violent 

 movements performed by floral organs or fruits owing to release of 

 tissue-tensions. Explosive movements of this kind are executed, in 

 connection with the dispersal of pollen, by the staminal filaments in 

 Parietaria, Urtica and other Urticaceae, and in Atriplex and Spinacia, 

 by the gynostemium of Stylidium adnatum, by the carina of Indigofcra 

 and by the stalk connecting the pollinia of Catasetum with their common 

 adhesive disc. The different modes of explosive dehiscence of fruits 

 exemplified by Impatiens, Cardamine hirsuta, Cyclanthera explodens, etc., 

 likewise pertain to this first category of physiological movements. The 

 external features of these movements, and the main principles of the 

 underlying mechanism, are sufficiently well understood. The anatomico- 

 physiological aspect of the subject, however, still awaits detailed 

 investigation. 2 ' 4 



The second group of physiological movements comprises all those 

 various forms of curvature which are executed by means of asym- 



