564 MOTOR SYSTEM 



directly, through the tension or compression induced within them by 

 the stimulating agency. Longitudinal tension, in particular, often 

 induces contraction when it is directly applied to a motor-tissue, even 

 where stimulation is usually effected with the aid of special sense- 

 organs. 



It may be doubted whether all irritable stamens and styles 

 (Arctotis), and stigmatic lobes (Mimulus and other Scrophulaeiaceae, 

 Golclfussid, some Bignoniaceae) conform to the Centaurea type of 

 mechanism. In view of the fact that irritability of floral structure has 

 certainly been evolved independently in a number of different families, 

 it would seem a priori more probable that the motor mechanism 

 should not be the same in all cases. This view is strongly supported 

 by Pfeffer. It cannot therefore be regarded as proved, that contraction 

 of motor-tissues is always brought about as it is in Centaurea, namely, 

 by a decrease of osmotic pressure. Active contraction of protoplasts 

 might equally well produce a fall of turgor. In view, finally, of the 

 very thick-walled character of some active motor-tissues, it is not 

 inconceivable that the walls might themselves be contractile and 

 irritable, while the cells are alive, somewhat after the fashion of 

 smooth muscle-fibres. 



In the case of tendrils, such as occur in a number of Natural Orders 

 {e.g. Cucukbitaceae, Passifloraceae, Papilionaceae, Vitaceae, etc.), 

 the curvature which follows upon contact stimulation, depends upon 

 acceleration of growth on the convex side. The exact proof of this 

 important fact has only recently been furnished by Pitting, although 

 Sachs and De Vries had long previously explained the main features 

 in the behaviour of tendrils. According to Fitting, the concave side 

 of a stimulated tendril undergoes no appreciable contraction. In 

 accordance with their mode of curvature, the above-mentioned tendrils 

 possess no specialised motor-tissue. In all the Sapindaceae that have 

 been examined, on the contrary (Urvillea ferruginea, etc.), the hapto- 

 tropic curvature of the tendrils which are of the watch-spring type 

 depends, according to Ricca, upon a powerful contraction of the concave 

 side. Here the motor mechanism resembles that which occurs in 

 certain irritable stamens ; it is not surprising, therefore, to find that 

 the tendrils of Urvillea ferruginea contain a special motor-tissue, as 

 the author had discovered previous to Ricca's researches. These 

 tendrils are flattened at right angles to the plane in which they are 

 coiled, and are markedly dorsi- ventral in structure (Fig. 230) ; irrita- 

 bility is confined to the abaxial side. The most conspicuous feature in 

 a transverse section is the very excentric position of the stereome 

 ring, which is flattened like the tendril itself, and situated much closer 

 to the adaxial or convex, than to the abaxial or concave side. At the 



