MOVEMENTS AND PRESSURE OF THE INTERNAL GASES 199 



SECTION 32. The Movements and Pressure of the Internal Gases. 



In the living plant various causes induce a continual disturbance of equi- 

 librium, and not only respiration, but also the assimilation of carbon dioxide 

 acts in this manner, as indeed does every metabolic process involving con- 

 struction or consumption. It is, moreover, the disappearance of water from 

 tissue-elements which leads to the formation of air-spaces in them, and the 

 pressure and composition of the enclosed air depends on the one hand upon 

 the rapidity with which the water is removed, and on the other upon the 

 rate at which the gaseous exchanges proceed, while the maintenance of any 

 constant difference of pressure or composition necessitates the existence of 

 a definite relationship between the interchanges which restore equilibrium. 



Differences in percentage composition may cause differences of pressure 

 to arise, and vice versa, for just as a bladder filled with carbon dioxide 

 collapses when in contact with air, so a negative pressure may be 

 caused in an enclosed intercellular space in which a large quantity of 

 rapidly diosmosing carbon dioxide is present. When, however, gaseous 

 diosmosis is induced by negative pressure, oxygen penetrates in greater 

 abundance from the air than nitrogen does, and indeed all the facts observed 

 coincide with the general principles already given (Sect. 30). Thus, when 

 the aeriferous system is well developed and in direct connexion with the 

 external world, the pressure and composition of the enclosed gas differs but 

 little from that of the surrounding air. In terrestrial plants with a strongly 

 developed cuticle, we find, however, greater differences in the composition 

 of the enclosed air than in submerged plants with more permeable 

 epidermal layers, and obviously more or less marked differences may exist 

 between the intercellular air of roots, stems, and leaves. The air of the 

 tracheae may be under pronounced negative pressure, as is always possible 

 in air-spaces which are not in direct communication with one another or with 

 the external world, whereas at the same time the air of the intercellular 

 spaces may be slightly compressed. 



The external conditions not only influence the pressure and movements 

 of the enclosed air indirectly by the effect they produce on respiration, 

 the assimilation of carbon dioxide, transpiration, the width of the stomatal 

 pores, &c., but their effect is felt directly in various ways which are purely 

 physical. Such are the bending and shaking of branches due to the wind, 

 &c., also the variations of barometric pressure and of temperature. All 

 movements of diffusion, however induced, are of the highest importance in 

 accelerating gaseous exchange, and a current of gas in any given direction will 

 continue so long as the difference of potential which induces it is maintained. 



Of the normal metabolic processes, the assimilation of carbon dioxide 

 usually tends to induce a positive pressure in the intercellular air, respiration 



