INVESTIGATORS IN PHOTOSYNTHESIS 65 



which air bubbles rise continually to the top of the jar, if 

 exposed to sunshine, and that this air is fine dephlogisti- 

 cated air." 



Ingen-Housz then goes on to give the results of his 

 own experiments and observations. These results are 

 summarised in the preface to his volume, but as the 

 statement is diffuse and couched in old-fashioned ter- 

 minology it may be convenient to condense and translate 

 it into rather more modern phraseology, using the terms 

 with which we are now more famihar. " I observed 

 that plants were able to purify bad air in a few hours if 

 subjected to sunHght ; that they could transform air 

 absorbed from the exterior into oxygen ; that this 

 oxygen is exhaled into the atmosphere, thus rendering it 

 more fit for animal life ; that the exhalation of oxygen 

 by plants begins after sunrise, is the more active the 

 brighter the day and the more the plants are exposed to 

 solar radiation, and the less active the more they are 

 shaded by buildings or by other plants, when, so far 

 from purifying the air, they contaminate it as animals do ; 

 that they cease to exhale , oxygen as. darkness comes on ; 

 that only leaves and petioles carry out this function ; that 

 the exhalation of oxygen in sunlight is independent of 

 the poisonous or other quahty of the plant ; that the 

 oxygen is given off chiefly by the under surfaces of leaves, 

 and that mature leaves, caeteris paribus, give off more 

 oxygen than young leaves ; that some plants, especially 

 •aquatics, give off more oxygen than others; that, on 

 the other hand, all plants in darkness render the air 

 impure, especially such parts as flowers, fruits, and roots, 

 no matter how economically useful they may be ; that 

 the sun alone has no power to purify the air — it may 

 indeed tend to render it less pure — unless when acting in 

 conjunction with green plants; and finally that the degree 

 of purity of the air given off by green plants depends 

 upon many factors such as the intensity of the light 

 falling on them, the extent of exposure of the leaves to 



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