to restrain the Evaporation of the Cell-Sap. 37 



In the figures of this table lies undoubtedly the proof, that in 

 a dead plant evaporation goes on more actively than in a living 

 one, and that this is the more active the thicker their leaves are. 

 If I am asked what power limits the production of vapour in the 

 living plant, I openly confess that I am unable to answer this 

 question. Those even who believe in the existence of a peculiar 

 vital force, will be little inclined to assume that this force can 

 act in direct opposition to the physical production of vapour; 

 there are indeed only two possible ways of explaining the pheno- 

 menon. Either it must be assumed, that in consequence of the 

 death an alteration takes place in the solid parts of the plant, 

 in the cell-membranes, which makes them less dense, more 

 readily penetrable by water or aqueous vapour, than they are in 

 the living plant; or we must assume that chemical changes 

 occur in the cell-contents of the dead plant ; that compounds, 

 which by reason of their hygroscopic peculiarity retain water 

 with a certain power in the living plant, are decomposed, or are 

 separated from the cell- fluid and rendered inactive. Our present 

 knowledge of the structure and of the nature of the chemical 

 conditions of plants scarcely place us in the position to decide 

 whether one or other, or both, of these circumstances occur. 

 An alteration in the membrane of the elementary organs, which 

 indeed many may be inclined at first to reject, does not appear 

 to me to be so totally improbable, since in a dead plant the ten- 

 sion which the parts of a living plant exhibit is immediately lost 

 in so great a degree, as to render it impossible to ascribe this to 

 the slight loss of water occurring in the earliest period, and the 

 mere mechanical collapsing of the cells arising from this loss of 

 water, and one is compelled to think of the removal of a tension 

 connected with life. That the loss of this tension renders the 

 cell-membrane more readily permeable by water and aqueous 

 vapour is at least conceivable, and to me at least so much the 

 more probable that I believe that I have often observed foreign 

 substances, such as iodine, penetrate the membrane of a cell 

 which though dead was still full of water, much faster than that 

 of a living cell. I am well-aware that this view will meet with 

 little sympathy at a time when the universal endeavour is to 

 refer the functions of living plants to purely physical and che- 

 mical processes, — when in absorption and excretion of fluids the 

 phenomena of endosmose are singly and solely regarded : I must 

 be content, yet entreat a consideration of how little service are 

 these purely physical explanations in reference to the study of 

 the absorption or excretion of sap through the cells of plants ; 



fifteenth day, rai?es a suspicion in my mind that I have made a mistake in 

 writing down the weight. This of course I cannot now ascertain j but, at 

 the same time, it does not essentially prejudice the general result. 



