76 VEGETABLE ORGANOGRAPHY. 



in coloured leaves, and in some petals which do not 

 exhale oxygen. 



Theodore de Sanssure has made known the manner 

 in which plants absorb oxygen during the night; and 

 he seems to think that this absorption is performed by 

 the stomatd, because succulent and marshy plants, which 

 have few, absorb less than others. But, 1st, Herbaceous 

 plants, which have many, are not those which absorb 

 most gas. 2d, This function is performed during the 

 night, at which time the stomata appear closed. We 

 are at present acquainted with the nocturnal absorption 

 of air in too limited a number of plants, to be able to 

 form an opinion upon the function of the stomata in this 

 respect ; we must be informed, in particular, if fleshy 

 fruits, petals, and cellular plants, which are devoid of 

 stomata, absorb air. 



We can with less uncertainty appreciate the action of 

 the stomata with regard to their perspiration of water. 

 They exist in all the foliaceous parts, where we know 

 that this function is performed ; they are in gi'eater 

 abundance in plants with membranous leaves, which 

 perspire much, than in those with fleshy ones, which 

 perspire little ; they are wanting in aquatic leaves, etio- 

 lated surfaces, fleshy fruits, petals, and roots, which do 

 not appear to perspire, at least not in a manner analo- 

 gous to leaves. They are closed during darkness, that 

 is to say, when the perspiration ceases ; and are open to 

 the sun, that is, when they perform this function more. 

 Finally, they are absent in cellular plants, where perspi- 

 ration does not take place as in other plants. We 

 must distinguish simple evaporation, which takes place 

 through the tissue, in all the organs, in graduated and 

 moderate proportion, from the perspiration, which is 

 caused by the eflect of solar light in great quantity only 

 in the organs furnished with stomata, and which, I think. 



