Prof. F. Cohn on the Contractile Tissue of Plants. 193 



the entire parenchyma possesses the properties of extensibility 

 and contractility, together with those of contractility and elastic 

 city ; but he would not deny that probably the different layers 

 of cells partake of these properties in various degrees, 



19. Another question is, supposing contractility to reside in 

 the cells, whether the cell-membranes or their contents are the 

 active agents in its manifestation. Dutrochet^s hypothesis of 

 endosmose as the cause of plant-movements has given place to 

 the hypothesis of Hofmeister ; and Cohn is disposed to believe 

 that the primordial layer or the proteine contents are endowed 

 with contractility, and that the enclosing cellulose membrane 

 gives the required elasticity to cells. 



20. It is, again, necessary to determine whether the changes 

 in form of contractile cells are exclusively effected by the shorten- 

 ing of the long diameter which may be actually recognized. 

 Cohn can give no decisive opinion on this matter, but presumes 

 that the decrease in length must be followed by an increase in 

 the width of the cells. But, even after the solution of these 

 questions, the problem would remain unsolved. On what histo- 

 logical qualities and relations does the circumstance depend, 

 that cells should by irritation undergo a change of form, and, 

 whilst contracting in one dimension, expand in another ? But 

 the same difficulty prevails with regard to animal contractile 

 tissue on these physiological points. 



21. On comparing together the several observations adduced, 

 two interpretations are possible. In the filament of Cein.taurea 

 two properties exist in a state of antagonism — viz. elasticity, a 

 physical property independent of vitality, and seated in the cell- 

 wall, and an expansive power, associated with living action, and 

 probably referable to the cell-contents (the primordial lamina). 

 So long as the living filament retains its irritability, the property 

 of expansion predominates, and the filaments are consequently 

 extended and curved, and most so when they are exempt from 

 irritation, though still in a considerable degree after having been 

 temporarily shortened in consequence of irritation. Again, as 

 the expansive energy declines with the vitality of the filaments, 

 the elasticity comes more and more into play, and causes a pro- 

 gressive shortening of those organs. Irritation acts in a certain 

 measure as a momentary and partial death, and paralyzes the 

 expansive power ; and when the vitality of the stamens actually 

 vanishes, elasticity assumes the entire sway, and gives rise to 

 a maximum degree of shortening. Thus, according to this in- 

 terpretation, the shortening of the cells after an external irrita- 

 tion is peculiarly a passive phenomenon ; and the active power 

 is displayed in their extension during life in general, and par- 

 ticularly during the period of elongation. 



