to restrain the Evaporation of the Cell-Sap, 35 



which is not capable of increasing the production of vapour 

 from a dead substance permeable by water), gives off a greater 

 quantity of watery vapour to the atmosphere than it does in 

 the dark. The fundamental cause of this phenomenon, the alte- 

 rations which are brought about in the plant by light, and 

 which condition this increased separation of water, are unknown 

 to us*. The said phenomenon however leads, and I believe in 

 strict correctness, to the assumption, that the evolution of watery 

 vapour from plants is to be referred to two causes ; in the first 

 place, to the universal physical law of vaporization, in obedience 

 to which every moist substance gives off water until the at- 

 mosphere around is perfectly saturated; and secondly, to a pro- 

 cess the more recondite conditions of which are as yet altogether 

 unknown to us, dependent on the vital action of the plant. If I 

 have correctly gathered the views which are advanced in physio~ 

 logical works, the generally received opinion is this : — 1st, that 

 the said physical production of watery vapour is regarded as to a 

 certain extent understood ; and it is assumed that this is only 

 more or less interfered with by the more or less perfect condition 

 of the cuticle, which is not readily permeable by water or watery 

 vapour ; 2ndly, that the said second cause is considered to account 

 for a more abundant separation of water than the physical cause 

 alone would be capable of producing. 



A series of facts now appear to me to stand in opposition to 

 this view; I will only mention the well-known phenomenon, 

 that those plants which are most difficult and tedious to dry, as 

 for instance the bulbous plants, the genus Sedum, &c, dry very 

 quickly if previously killed by immersion in boiling water. 

 Moreover, it is well known how quickly plants dry which have 

 been killed by poison, frost, &c. From these circumstances it 

 undoubtedly follows that a dead plant, in what way soever it 

 may have been killed, dries quicker than a living plant of the 

 same species, notwithstanding that the evaporation, occurring 

 peculiarly in the living plant under the influence of light, is 

 wanting, and only a true physical separation of water takes place. 



I did not remember to have found special researches into this 

 circumstance brought forward in physiological writings ; it ap- 



* As far as my knowledge goes, no positive observations have been 

 brought forward to show that this separation of water takes place in sub- 

 merged plants, which would prove the nature of the fact to be not an exha- 

 lation of vapour, but a secretion of drops of watery fluid ; yet this is in the 

 highest degree probable, since it would be inexplicable how water-plants 

 could accumulate in their interior large quantities of such substances as are 

 contained in so small proportion in water, as for instance the iodine com- 

 pounds in the plants of sea-water, if they did not in the course of time ab- 

 sorb a considerable quantity of water, and again give it off after depriving 

 it of particular salts. 



3* 



