CHAP, n.] VEGETAL NUTRITION. 103 



all the nutritive mechanism of the plant. Besides their special 

 chlorophyllian function, and perhaps in consequence of that 

 function, the leaves exercise on the sap a double propulsion : 

 they aspire it from below to above, and they drive it back, after 

 elaboration, from above to below. This is doubtless a kind of 

 circulation, but rudimentary, without regularity, effected little by 

 little, sometimes by one route, sometimes by another. The cells 

 of the leaves, whose contents are dense and albuminoidal, are well 

 arranged for absorbing by endosmosis the ascending lymphatic 

 sap, to determine there, when it has penetrated into their cavity, 

 synthetic chemical phenomena, to carbonise it, that is to say, to 

 enrich it with atoms of carbon, thanks to their special property, 

 then finally to expulse it by exosmosis. At the same time they 

 concentrate it by depriving it of a notable portion of its aqueous 

 vehicle, which is exosmosed, and escapes by evaporation. 



Experience has demonstrated that this exhalation is an active 

 phenomenon, the result of a vital act, probably chlorophyllian, 

 and not a passive evaporation. The confirmatory facts are 

 numerous. Hales was the first to discover that light augments 

 the aqueous exhalation on the surface of the leaves. The 

 phenomenon depends on the solar light and very little on heat : 

 for, simple light diffused, slightly calorific, suffices, whilst in the 

 shade a heat equal or even very inferior does not act much, and 

 at night the aqueous exhalation stops. 



If of two simple plants one is exposed to the light, and the 

 other is kept in darkness, the first absorbs much more water than 

 the second. 



Sennebier has shown that leafy boughs dipped in water at 

 their lower extremity aspire much more water in light than in 

 darkness. He observed also that heat in darkness has little 

 influence on this sort of suction, and that the results vary 

 according to the species of plants. In general the plants which 

 in the light resist the most the desiccating action of the 

 atmosphere are those which in darkness resist it the least, and 

 reciprocally. The intense contact of the air with the cells of 



