ABSORPTION AND MOVEMENT OF WATER 153 



of sugar passes by osmosis more rapidly into the body and 

 into the cells of the plant than the liberated oxygen passes 

 out. The latter, therefore, accumulates in the large and well 

 developed intercellular spaces and passages, escaping only 

 slowly from these by osmosis, escaping rapidly only when 

 the plant is wounded. 



The reverse of this result is attained during the night, 

 when the plant is photosynthetically inactive but is steadily 

 respiring, taking in oxygen as fast as it needs it, and giving 

 out the more rapidly diosmosing and diffusing carbon-di- 

 oxide. But since respiration is never so active as photo- 

 synthesis, the negative pressure is never so high as the 

 positive. 



Since plants are subjected to inconstant but frequent 

 movement by the winds, by passing animals, by the rise and 

 fall of the tide, by waves, etc., the gases contained in their 

 bodies are subjected to varying pressures, are forced out 

 and drawn in, are moved from part to part. These me- 

 chanical influences brought to bear upon plants play a very 

 important r61e in contributing to the movements of enclosed 

 gases and vapors. Besides these, temperature-changes and, 

 as we have already noted in connection with transpiration 

 (p. 139), the temperature of the plant-body as compared 

 with the temperature of the air outside, also affect the 

 movements of gases in the plant. The effects of these in- 

 fluences, however, are mainly upon the gases enclosed in the 

 intercellular spaces while the other influences which we have 

 considered affect more directly the gases within the cells. 

 But all of these influences contribute to the perfect aeration 

 and ventilation of the plant-body just as the elaborate 

 musculature of the higher animal automatically maintains 

 the movements of the gases needed by the cells of its body. 

 In both animals and plants, osmosis and diffusion underlie 

 all gas movements in the body, but both are controlled by 

 the vital activities, that is, by the amounts of the gases 

 consumed and liberated by the living cells. 



From the foregoing it is evident that, in most plants, and 

 under ordinary conditions, the composition of the enclosed 

 gases can differ only slightly and temporarily from that of. 



