152 LIGHT, VEGETATION AND CHLOROPHYLL 



Some precise ideas began to be formed, and Malpighi 

 already suspected the importance of the air for respiration, 

 but it did not occur to anyone that any substance whatever 

 could penetrate into the plant otherwise than through the 

 roots. 



Gaseous Exchanges 



Nevertheless, at this period research started on an entirely 

 new Une. In 1769, Bonnet, a Swiss, plunged a twig of vine in 

 water, in full sunHght, and saw that bubbles escaped from the 

 leaves. Did they come from the leaves or from the water? 

 Bonnet boiled the water, then plunged the leaves in again; no 

 more bubbles appeared and he concluded that they had come 

 from the water and not from the leaves. Moreover, even dry 

 leaves produced bubbles in well-aerated water. 



Such experiments would have been only an obstacle to 

 progress if Priestley, an Enghshman, had not become interested 

 in these bubbles and observed that they consisted of very 

 pure air, perfectly suitable for the respiration of animals. 

 From this he concluded that the respiration of a plant was 

 diametrically different from that of an animal, the one using 

 what the other rejected. Vitiated air that an animal could no 

 longer breathe was purified by the presence of a plant. And 

 did not Priestley hint at all future discoveries on this subject 

 when he said, in terms that had not then been explained 

 by Lavoisier, that the plant dephlogisticates the air?^ "It 

 follows that the phlogiston of the air is retained in the 

 interior of the plant and is there used for the work of 

 nutrition." 



A Dutchman, Ingen-Houss, affirmed that this purification 

 of the air was accomplished only in sunUght and by the green 

 parts of the plant and that it consisted in a fixation of the 

 carbon of carbon dioxide. In darkness, particularly during the 



'As phlogiston was the principle of inflammability, dephlogisticated 

 air became incapable of burning. (Today we say that a substance which 

 has been burnt is saturated with oxygen and has not suffered any loss.) 

 These old investigators thought that the plant built itself up by taking 

 phlogiston from the air and accumulating it. 



