200 THE MECHANISM OF GASEOUS EXCHANGE 



a negative one. Hence, when a cut is made in the stem of an uninjured, 

 submerged, green aquatic plant which is well illuminated, bubbles of gas ooze 

 out and continue to stream from the cut surface in slow or rapid succession 

 so long as light reaches the plant. A similar positive pressure is exhibited 

 when terrestrial plants are immersed in water, although so long as they are 

 in air, and their stomata are open, no perceptible difference of pressure can 

 be developed. This positive pressure is mainly due to the decomposition 

 of the carbon dioxide in the green illuminated parts, and to the excretion 

 of part of the liberated oxygen into the intercellular spaces, whence it 

 diosmoses outwards less rapidly than the more soluble carbon dioxide does. 

 Hence the pressure falls in darkness, for then the production of oxygen ceases ; 

 indeed, a slight negative pressure is usually produced, provided no disturbing 

 influences intervene, owing to the carbon dioxide formed by respiration 

 diffusing more rapidly than oxygen. It is mainly in this manner that 

 differences are produced in the enclosed air of submerged aquatic plants, 

 according to whether they are illuminated or not. During the day the per- 

 centage of oxygen on the whole increases, the enclosed gas containing more 

 oxygen than the air does. In darkness the reverse takes place, and the 

 minimal percentage of carbon dioxide present during the day increases. The 

 composition of the gas present in the chlorophyll-containing organs of land 

 plants changes in a similar manner, while owing to the presence of an 

 impermeable cuticle, more marked differences than are found in submerged 

 aquatic plants may arise if the stomata are closed. The intercellular air 

 always contains more or less nitrogen, for the gaseous exchange is perfectly 

 general, and hence under normal conditions pure oxygen or pure carbon 

 dioxide is never present within the plant. 



A negative pressure is created without the aid of any special vital 

 activity when the water which living or dead tissue-elements contain is 

 withdrawn from them by transpiration. How perfect a vacuum will be 

 produced, and how long the internal negative pressure will be maintained, 

 will depend upon the rapidity with which air penetrates the enclosing 

 membranes 1 . The shrivelling of thin- walled tissues when dried is partly the 

 result of the negative pressure created within the cells, which is frequently 

 very marked. Thus in vessels and tracheides the internal pressure may 

 often fall to one-third or one-fourth of that of the atmosphere. It is not yet, 

 however, quite certain whether the negative pressure in the tracheae, &c. 

 is due simply to the loss of water by transpiration, or whether in addition 

 to this, other factors aid in exhausting the air within the vessels. 



The existence of internal negative pressure in the tracheae plays a most 



1 [Kamerling finds that the cells of certain moss-leaves, Selaginella Jepidophylla, &c., are 

 impermeable to air when dry, and hence within the cell a vacuum may be maintained for an 

 indefinite length of time (Bot. Centralbl., 1897, Bd. LXXll, p. 53).] 



