196 Prof. F. Cohn on the Contractile Tissue of Plants. 



chanical forces. Hofmeister's researches have set aside the 

 hypothesis of Dutrochet^ of endosmotic force called into play hy 

 excitants ; but the theory advanced in its place, that the move- 

 ments are not dependent on shortening, but on an augmented 

 expansion or turgescence as the effect of excitation, cannot itself 

 be maintained. In this hypothesis, the pith, as being very full 

 of sap, is assumed to take the most important part in producing 

 the movements — an assumption which Cohn shows, from a priori 

 considerations, to be untenable. Moreover, the positive fact 

 above advanced, of the occurrence of shortening when a part is 

 irritated, stands in direct opposition to this general hypothesis 

 of Hofmeister; for, without doubt, the movements of the young 

 shoots are of a similar nature to those of the stamens exhibited 

 in Centaurea. 



28. Cohn has, from the researches entered into, arrived at 

 the conviction that the accepted dogmas of physiology are erro- 

 neous in ascribing sensation and motion to animals as charac- 

 teristics — a conviction further strengthened by all the newly 

 observed facts relative to the lowest grades of animal life, and 

 the distinctions existing between animals and plants. Sensation 

 in the higher animals is linked with sensory perception or sensi- 

 bility, and with a medium of connexion between the sensorium 

 and surface in the system of nerves; but in the lowest animals 

 this complex apparatus is absent, and the whole tissue responds 

 to impressions from without, these latter operating by the exist- 

 ence of what is called " irritability," or of a degraded sort of 

 sensation. This low form of sensation must be also attributed 

 to plants ; for these organisms exhibit it in a threefold manner : 

 1st, by the property of receiving impressions from without, i. e. 

 by in'itabilitj/ ; 2ndly, by the property of responding to such 

 impressions by internal movements and by changes in form, i. e. 

 by contractility ; and 3rdly, by the power of propagating these 

 impressions from their point of contact to the tissues and parts 

 around, which are themselves, as a result, thrown into action, 

 i. e. by diffusibility of impressions. The action of excitants in 

 developing irritability and calling forth movements is not simply 

 and directly dependent on their mechanical, physical, or chemical 

 characters, but the organized tissue has an organic modifying 

 power upon them, and the irritation gives rise to internal move- 

 ments resulting in change of form, the suspension of elasticity, 

 or the production of heat, &c. This organic power is the vis 

 nervosa of Haller. 



29. When an act of irritation is propagated within the tissue, 

 this again is dependent on an internal motor power, called into 

 activity by the external impression, without which the internal 

 movements necessary to diffusion could not take place. Thus, 



