66 . LECTURE IV. 



has therefore received much attention from physiologists. 

 Hales, Bonnet, Duchartre, Heiden, and others, investigated 

 it without, however, arriving at any very definite results. 

 Among the more recent and more satisfactory researches 

 may be mentioned those of Detmer and of Boussingault. 

 Detmer found that if leaves be immersed in water they 

 distinctly increase in weight in so short a time as a quarter 

 of an hour, and Boussingault obtained similar results by 

 means of experiments of much the same kind. It appears 

 then that leaves can absorb water, but it is by no means 

 proved that they do so under ordinary circumstances. The 

 objection to these experimental results is, as Eder has pointed 

 out, that long-continued immersion in water produces changes 

 in the walls of the epidermal cells of such a nature that they 

 become permeable to water. Leaves are not exposed to this 

 under ordinary circumstances ; as a rule their epidermis 

 cannot be moistened by water, so that the rain or dew which 

 falls upon them forms drops which either roll off by their 

 own weight or are shaken off by the wind. The differences 

 in internal organisation as well as in external form which 

 exist between submerged and subaerial leaves go far to prove 

 that since the former are known to absorb water, the latter 

 are not adapted for this purpose. This view is further sup- 

 ported by the fact that these differences do not only exist 

 between the leaves of a submerged plant on the one hand 

 and of a subaerial plant on the other, but that they are very 

 evident also between the submerged and subaerial leaves of 

 plants which, like Salvinia, are only partially immersed. It 

 is true that plants which are flaccid very soon become turgid 

 and assume a fresh appearance when rain falls, but it must 

 be admitted that the most important causes of this are the 

 absorption of water by the roots and the diminution of the 

 transpiration of the plant in consequence of the increased 

 moisture of the air. In comparison with these a possible 

 direct absorption of water by the leaves may be neglected. 



With regard to the absorption of watery vapour by leaves, 

 it appears, from the observations of Detmer and of Boussin- 

 gault, that it may take place to a slight extent when the 



