Prof. P. Cohn on the Contractile Tissue of Plants. 195 



opposed to the natural elasticity of its tissue. On the contrary, 

 in the filaments of the plant, elasticity seems to act as the 

 shortening agent, and to represent the passive condition, whilst 

 extension or elongation appears to be a state of activity. A 

 diflFerence such as this implies in the active causes in operation 

 in the contractile parts of plants and animals is, however, not 

 probable. Indeed, presuming the physiology of muscular action, 

 as generally taught, to be correct, we may still assume that the 

 contraction of the filaments, like that in muscle, is a sequel to 

 the operation of an active force — of contractility — which has 

 been suspended in consequence of irritation, and also associated, 

 as is true of muscle at the moment of its activity, with a change 

 in the elasticity of the tissues. 



25. We should be in a better position towards understanding 

 the true relations between the contractility of plant- and of ani- 

 mal tissue did we rightly comprehend the remarkable contrac- 

 tion which the withered filaments undergo. So far as concerns 

 the fact itself, it appears not to be without analogies in the 

 animal kingdom, among the lowest classes endowed with con- 

 tractile parenchyma. For instance, in Amcfibese and Foramini- 

 fera, the contractile processes are retracted on the application of 

 an excitant, and also on the approach of death, and the whole 

 animal shrinks into a smaller compass. So it is in Vorticella 

 and in Stentor, and also in Hydra. Such analogy is more ob- 

 scure in the muscles of the higher animals ; yet Cohn believes 

 that the rigidity after death is a fact of the same class. 



26. It is improbable that contractility as exhibited in the 

 stamens of Centaurea should be an isolated phenomenon in the 

 vegetable kingdom. On the contrary, a very large number of 

 facts are on record respecting many plants, parallel in kind to 

 those detailed. The peculiar phenomena attributed to vegetable 

 " irritability" are of this order : such are the movements of the 

 stamens of Berberis, Cactus, Cistus, &c., of the anthers of Ges- 

 neraceae and of the Stylidese, of the labella of some Orchidese, 

 of the leaves of many Leguminosse, Oxalidacese, Droseraceae, &c., 

 of the climbing stems and tendrils of many climbing plants and 

 creepers — all more or less afiected by external excitants, electri- 

 cal, chemical, and mechanical. To the same categoiy belong 

 also those phenomena described as the sleep of plants (regulated 

 by and dependent upon the intensity of light), the movements 

 of all younger parts of plants towards the light, and those 

 changes in form, lately remarked by Hofmeister, in all young 

 shoots and leaves, which become curved by mechanical shaking. 



27. It has been generally assumed that these phenomena of ir- 

 ritability in plants have nothing in common with those witnessed 

 in animals, but are to be explained by the action of some me- 



