236 Dv Daubcny 07i tlic Writings and 



neva, during the last century, have devoted especial atten- 

 tion. 



Bonnet, it will be recollected, had observed the evolution 

 of air from leaves whilst exposed to the sun's light, long before 

 Priestley had been led by observations of a similar kind to 

 his beautiful theory as to the influence of plants in purifying 

 the atmosphere. 



Senebier had examined, in much detail, the circumstances 

 under which this evolution of gas took place, and the causes 

 by which it was determined, to which Decandolle himself has 

 made some additions, by shewing, that artificial light may, to 

 a certain extent, supply the place of solar radiation, — a fact, 

 however, which had been first pointed out by Humboldt, so 

 early as 1793, in the aphorisms appended to his Flora Subter- 

 ranea of Freyberg.* 



But the younger De Saussure had distinguished himself 

 above the rest, by investigating the influence at once of water, 

 of air, and of the soil, on the processes of vegetation, and by 

 introducing into this department of inquiry, a degree of ac- 

 curacy, which renders his labours, even after a lapse of nearly 

 forty years, still the best authority Ave can appeal to on many 

 of the questions under discussion. 



Into the consideration of questions Avhich had exercised the 

 genius of some of the most distinguished of his countrymen, 

 Decandolle entered, as might be expected, with more than 



* I have been reminded by the perusal of M. Flourens' Eloge, of an 

 omission in the former part of this memoir relative to the subject here 

 alluded to, namely, of the fact, that these researches on artificial light were 

 made known to the world so early as the year 1800. (See Memoircs des 

 Savants Etrangers de I'Institut, vol. i.) He there established, that the 

 period of the sleep of plants may be gradually reversed, by keeping tliem in 

 the dark during day, and exposing them to artificial li^ht at night ; and that 

 we might communicate a green colour to etiolated leaves by the same action, 

 although we could not produce from them a sensible evolution of oxygen. 

 Thus concludes Flourens — 



"La vie des plantes est un phenomene bicn plus compliqut', bien plus 

 rapproche de la vie des animaux qu' on ne I'avoit souptonne encore: elles 

 ont leur action, leur repos, leur sommeil, leur veille, leurs habitudes (car 

 ce n'est pas tout de suite, ce n'est qu'au bout d'un certain temps qu'elles 

 perdent leurs heures ordinaires pour en prendre d'autres) et lorsque Uelille, 

 s'empressant de celebrer ces resultats en beaux vers, va jusqu'a dire: 



" Dc la crcdule floui- le calice est troiupe." 



Ce language metaphorique de la poesie ne parait prcsque plus metaphorique. 



