60 MOVEMENT OF MATERIALS IN PLANTS 



They absorb O^ continuously, for it is involved in those 

 metabolic changes of various foods which release the 

 energy stored in them during their manufacture. The 

 absorption of COo and O- by cells from the intercellular 

 spaces in the mesophyll of leaves takes place in the same 

 manner as the absorption of water and mineral matter 

 by root-hairs from the soil. The air of the intercellular 

 spaces is continuous, through the open stomata, with 

 the unconfined air outside. Throughout this continuous 

 mass diffusion will tend to maintain uniform concentra- 

 tion of the constituents. Through the wet membranes 

 bounding cells containing chlorophyll and absorbing light 

 CO;, will enter, dissolving in the water of the membranes, 

 and will be combined in photosynthesis to form sugar. 

 Thus during the hours of daylight COo will always tend 

 to be combined in chlorophyll-containing cells which, 

 therefore, will continue to absorb it from adjacent inter- 

 cellular spaces. Similarly, Oo will be taken in as rapidly 

 as it is used. 



Leaves as Organs of Gas Exchange. — Correspond- 

 ing to the intake of CO2 and O^ by the living cells of 

 leaves is the discharge of O2 and CO2 from living cells 

 into the intercellular spaces whenever the pressures of 

 these dissolved gases within the cells exceed their pres- 

 sures in air which fills the intercellular spaces. Thus 

 during the hours of photosynthetic activity the volume 

 of O2 given off from green leaves is equal to the volume 

 of CO2 absorbed. But during the hours when photo- 

 synthesis is not taking place the living cells absorb O2, 

 using it in destructive metabolism (see Chap. VIII), 

 and give off COo. Hence leaves are called organs of 

 gas exchange. 



Evaporation will take place inevitably, and its rate 

 will depend upon the dryness of the air. The loss of 

 water from the body of the plant has unfortunately been 

 called trail spiral ion, but there is no difference between 

 transpiration and evaporation in plants except that plants 



