258 GASEOUS EXCHANGE [CH. 



stance, a remarkable bubbling of gas from the leaves of an intact 

 plant may sometimes be observed 1 , but there seems little agree- 

 ment among different observers as to the reasons for this curious 

 phenomenon, or even as to the actual facts of its occurrence 2 . 

 The whole subject needs to be reinvestigated by a botanist who 

 is also a competent physicist. The only point about which there 

 is some degree of certainty, seems to be that, at least, while 

 assimilation is actually proceeding, high gas pressures occur in 

 the air passages. This can be demonstrated by various direct 

 means, for instance by cutting into the plant beneath the water- 

 surface, when a stream of bubbles arises from the wound. A 

 curious piece of indirect evidence, bearing on the same point, 

 is perhaps worth recalling. It has been shown that, when a 

 Waterlily petiole suffers from a wound which involves any of the 

 air-canals, the cells bounding these cavities grow out in the form 

 of hairs, until they choke the channel 3 . The suggestion has been 

 made that this growth is induced by the temporary diminution 

 of the high pressure in the air-canals, due to their sudden 

 connexion with the external atmosphere 4 . 



The cause of the high pressure in the canals during assimi- 

 lation is doubtless to be sought in the continual production 

 of oxygen, which accumulates in these intercellular spaces. In 

 the dark, when respiration is the only form of gaseous exchange 

 that persists, the high pressure is often replaced by a negative 

 pressure, since the relatively small quantity of carbon dioxide, 

 produced partly at the expense of the oxygen in the internal 

 atmosphere, diffuses away with considerable rapidity, in contrast 

 to the oxygen, which diffuses slowly. The high pressure of the 

 oxygen, in the lacunae adjoining the assimilating cells, may have 

 an effect in inducing movement towards regions of lower 

 pressure, such as the roots and rhizomes, where oxygen is 

 presumably in great request. Differences of temperature, be- 

 tween the sun-warmed upper parts of the plant and those in 

 the relatively cold lower layers of the water, may also have their 

 effect in causing currents in the internal atmosphere. 



1 Raffeneau-Delile,A.(i84i),Ohno,N.(i9io). 2 Ursprung, A. (1912). 

 3 Mellink, J. F. A. (i 886). 4 Schrenk, J. (i ! 



