ABSORPTION OF FLTITD BY THE LKAVF.S. 177 



tained in J05 ounces, from the same vino, at tlio conclusion of 

 the summer. Even these minute quantities are sufficient to 

 communicate a perceptible taint, when separated by their decay, 

 into the gases of which they are composed, and of which the 

 bulk is very much greater. 



X Absorption of Fluid by the Leaves. 



270. Although the leaves of the higher plants are, without 

 doubt, the special organs of exhalation, they also have the power, 

 like the leafy surfaces of the lower tribes of plants, of supplying 

 fluid nourishment, under peculiar circumstances, when the sys- 

 tem requires it. It has been already shown, that it is only as 

 we ascend the scale, that we begin to meet with distinct roots 

 for the purpose of absorption ; the same general surface an- 

 swering both purposes in the simplest tribes. And even where 

 distinct root-fibres are developed, they are often closely connected 

 with the leafy surface ; these fibres being sent off from the under 

 side of the leafy expansion of the Marchantia, and often from a 

 ? similar part in the separate and delicate leaflets of Mosses. 



271. It is not to be wondered at, then, that the leaves of 

 higher plants should be capable of supplying, in some degree, 

 the functions of the roots ; when these are absent or imperfect, or 

 are unable, from the nature of the soil which surrounds them, to 

 obtain a sufficient supply of fluid nourishment. Not unfrequently 

 the roots, where they exist, serve merely to fix the plant, finding 

 their way into the crevices of the hard dry soil, or of the barren 

 rock, on which it grows ; and then it must be altogether de- 

 pendent upon the moisture it imbibes through the general sur- 

 face. In such cases the function of exhalation is but feebly 

 performed ; and all the processes of growth are proportionably 

 slow. Plants of this description flourish best near the sea-shore, 

 where there is always a certain amount of moisture in the atmo- 

 sphere : and especially, too, in tropical regions, where the con- 

 stant high temperature increases the evaporation from the surface 

 of the water, and at the same time enables the atmosphere to 

 dissolve a greater quantity of watery vapour. It is wonderful 

 to see the precipitous faces of rocks apparently the most barren, 



