88 BIOLOGY. [Book ii. 



not fix more than for a moment the carbon derived from the 

 carbonic acid. As this carbon does not accumulate in the 

 chloi-ophyllian tissues, as the composition of chlorophyll is always 

 perceptibly the same, the molecules of carbon assimilated by it 

 must be instantly surrendered to the sap to which they bring the 

 supply needful to the formation of complex substances, ternary 

 and quaternary. Whence, besides, could the tissues take the 

 carbon which constitutes the half of their weight, if they had 

 not, to provision themselves, this perpetual supply ? We shall 

 have some words to say on this living chemistry. Let us occupy 

 ourselves for the moment with the vegetal function, comparable 

 in everything with what is called respiration in animals, that is 

 to say, with the absorption of oxygen. 



3. Absorption of Oxygen or Vegetal Respiration. 



A green phanerogamous plant is asphyxiated in a medium 

 of hydrogen, of azote, and even of carbonic acid, and if its 

 sojourn in this artificial atmosphere is too prolonged it loses 

 for ever the chlorophyllian property.' 



Besides, M. de Saussure had already remarked, when extract- 

 ing by the aid of a pneumatic machine the air impregnating the 

 tissues of plants, that this air contained notably less oxygen 

 than the ambient atmosphere. The proportion is very variable ; 

 that found by Saussure was 85 of azote and 15 of oxygen.? 

 Moreover it has long been known that during the day when 

 in darkness, and consequently during the night, plants disengage 

 carbonic acid. Ingenhouz had already observed that this dis- 

 engagement of carbonic gas was constantly operated by flowers 

 and roots. Finally, in our own day, Boussingault, Garreau, 

 Sachs have been able to demonstrate that this exhalation is a 

 permanent and general fact, that it is effected even by the leaves 

 exposed to the sun. In truth we have here to deal with an act 

 indispensable to everything which lives. Without this continual 



• Boussingault, A/nnoOes de Chimie et de Physique. TV' s^rie, 1868, t. XIII. 

 ' Th. de Saussure, Eeeherehes Chimiqiies mr la FigUation. 1804. 



