198 Prof. F. Cohn on the Contractile Tissue of Planfi. 



as the result of irritation are seldom observed in the vegetable 

 kingdom, but that the anatomical structure of irritable tissues 

 presents no appreciable characteristic peculiarities not seen in 

 other vegetable tissue, and that the suceptibility to the excita- 

 tion of light, as well as to that of mechanical and probably of 

 electrical impulse, is possessed by all young vigorous tissues ; 

 and further, on comparing these phenomena with those of ani- 

 mal irritability, the conclusion forces itself upon us, that the 

 faculty of responding to external irritation by internal move- 

 ments and changes of form belongs to cells as such, and holds 

 good as well in the vegetable as in the animal kingdom. To be 

 irritable, to change its normal form as a result of excitation, and 

 to revert to it after a while by its inherent elasticity, are charac- 

 teristics of the living cell. In plants, these properties are met 

 with only when the vital processes are in full activity, and there- 

 fore are particularly noticed during the period of flowering, 

 when those processes are at their maximum. And here it may 

 be remarked that the stamens, in which irritability is more fre- 

 quently noticed, are the only organs in which an elevation of 

 temperature, measureable by the thermometer, occurs, although 

 doubtless a certain degree of heat is generated in all plant-cells 

 by the chemical processes going on within them. So soon as 

 the processes of life in an organ begin to fail in power, so soon 

 also does their irritability decline, so far, at least, as its external 

 manifestations are concerned. 



These circumstances suggest a reason for the rarity of the 

 phenomena of irritability and contractility existing in any con- 

 siderable degree in plant-cells ; but they furnish no ground for 

 concluding that irritable tissues possess properties not to be 

 found in other vegetable tissues ; on the contrary, it is to be 

 supposed that similar properties belong to all, but exist in an 

 intensified degree, and for a certain epoch, in those parts where 

 their results arrest observation. There is a difiiculty in believing 

 that, in possessing the faculties of sensation and motion, the 

 animal kingdom, including its lowest and most simple repre- 

 sentatives, partakes of vital endowments entirely denied to 

 plants for the sole reason of their being plants. 



There is a physiological differentiation in the organs and cells 

 of the higher animals, which progressively declines as we descend 

 the scale of animal life ; and we find in the lower grades of ani- 

 mals the same tissue, and this, too, in a less elaborated form, 

 carrying on functions which, in the higher, are shared in by two 

 or more highly organized special structures. The same holds 

 good in a higher degree with respect to plants, in which, as a 

 rule, one and the same cell performs all the functions necessary 

 to life, though in some cases certain cells are constituted into 



