562 Comparative Physiology 



may spongioles possess a power of rejecting also : it is not 

 proved, however, that leaves reject or cannot absorb nitrogen, 

 nor does it follow that spongioles are similar though it were. 

 It may not appear proved that roots have the power of absorb- 

 ing gaseous substances as such, unless dissolved in water ; at least 

 it is said in experiments on record that they do so, and some 

 assert they cannot. Thomson says Saussure has proved that 

 roots absorb oxygen as well as leaves ; and Liebig talks of it as 

 certain. If oxygen is absorbed by the roots in this way, so may 

 atmospheric air and nitrogen. 



" The quantity of fluid absorbed, and the force with which it 

 is propelled upwards in the stem, vary in different species and 

 individuals, also at different periods of the year and day. It 

 seems intimately connected with the activity of the other pro- 

 cesses of vegetation, and especially with the quantity of vapour 

 transpired from the leaves. Hales found that, when the sap of 

 the vine was rising rapidly, a column of mercury, 26 in. high, 

 nearly equal to 31 ft. of water, might be supported by the pro- 

 pellent force of the absorbent organs : the power diminished, 

 and after a time ceased altogether, when the upper part of the 

 plant was cut off. The mere act of absorption, there is much 

 reason to believe, is due to the physical propeily of the mem- 

 brane to produce endosmose ; the difference of the density of 

 the fluids necessary for the commencement and continuance of 

 this action being supplied, in the first instance, by the store of 

 nutritious matter obtained by the embryo from its parent, and 

 latterly by the mixture of a portion of the dense elaborated 

 descending sap with the crude ascending fluid. Professor 

 Henslow likens the carrying off of the sap as imbibed by vital 

 action, producing further demand, to the combustion of oil in a 

 lamp. DeCandolle's axiom (that when a particular function is 

 not sufficiently carried into effect by the organs ordinarily ap- 

 propriated to it, it is performed wholly or in part by another) 

 is the result of the general principle, before laid down, that the 

 general surface of a plant can perform, in a considerable degree, 

 the functions of all the rest. When the roots are absent or 

 imperfect, as in Orchidese, absorption is performed through the 

 leaves ; and when these are absent, as in Cacti, through the 

 stem. Bonnet experimented on plants of M&ccxvc\alis, by im- 

 mersing the roots only of some plants in water, while the leaves 

 only of other plants were allowed to touch the fluid; and he 

 found, after five or six weeks, that those which imbibed by their 

 leaves only were nearly as vigorous as those that had absorbed 

 by the roots. It is by the under surface of the leaf, where the 

 cuticle and cellular tissue are least compactly arranged, that 

 absorption is performed with the greatest rapidity. The downy 

 hairs so plentiful on some plants seem to contribute to this func- 



