GENERAL 



177 



bodies, only in the fact that the absorbed or evolved gas is first brought 

 into solution or liberated on the outer surface of the cell-wall. Hence, 

 the gaseous exchanges in an air-space surrounded on all sides by turgid 

 cells take place entirely diosmotically, as is the case also for aeriferous 

 cells and vessels, to enter which the gaseous particles must pass through 

 an absorbent cellulose membrane. 



Diosmotic absorption is essential in all turgid cells, and the aeriferous 

 system is simply for the purpose of rapidly distributing gaseous products 

 throughout the interior of the plant, so that the slower diosmotic trans- 

 ference from cell to cell may be only necessary over short distances. 

 Similarly, certain well- arranged channels serve to convey water to the 

 immediate neighbourhood of the transpiring cells and tissues. In cor- 

 respondence with the purpose which it subserves, we find, as a general 

 rule, that the formation of a more or less marked aeriferous system of 

 intercellular spaces accompanies all marked tissue differentiation. Even 

 in submerged aquatic plants a well-developed but closed aeriferous system 

 is present, although the final exit and entry of gases can take place 

 only by diosmotic means. Stomata and lenticels are usually present in 

 terrestrial plants, and through these open channels gases may pass directly 

 into the interior of the plant. 



The existence of such open channels becomes more and more important 

 as the impermeability to gases of the cuticle and cork increases. Moreover, 

 by means of the system of communicating intercellular spaces, gases are 

 brought directly into contact with very large areas of readily permeable 

 cell-walls. Usually, as the impermeability of the cuticle and cork to water 

 increases, they become also less permeable to gases and dissolved sub- 

 stances, and hence when a thick and relatively impermeable cuticle is 

 developed for protection against transpiration, gaseous exchange is rendered 

 more or less difficult, though not perhaps to the same relative extent. It is 

 generally of the utmost importance that sufficient oxygen for the continu- 

 ance of respiration and vital activity may still reach the living cells, when, 

 in order to lower the rate of transpiration, the stomata close and are no 

 longer available for the entry or exit of gases. The closing of the stomata 

 also causes the production of organic substance to be markedly diminished, 

 for the traces of carbon dioxide present in the air are unable to diffuse 

 with sufficient rapidity through the cuticle to the chloroplastids lying 

 beneath it. For the general good of the plant it is, however, unavoidable 

 that particular functions should sometimes be unfavourably affected, and 

 thus a regulatory protection against excessive transpiration cannot be exer- 

 cised without at the same time causing the rate of gaseous exchange to be 

 diminished (Sect. 27). Even in those mosses which can withstand drougnt, 

 their vital activity gradually diminishes, and ultimately ceases when the 

 percentage of water falls below a certain limit. 



PFEFFER N 



