90 INTERCHANGES OF GAS AND VAPOUR [CH. 



entirely those of atmospheric air, viz. carbon-dioxide, 

 oxygen, and water-vapour in various proportions according 

 to very complex and varying conditions, among the 

 principal of which are the hygrometric and barometric 

 condition of the atmosphere, the life-activities of the cells, 

 the amount of water in the plant, the temperature, and 

 the illumination to which the leaf is exposed, together 

 with certain controlling effects to be discussed later in 

 connection with the epidermis. 



The point to be here emphasized is that the mesophyll- 

 cells are continually, during the whole life of the leaf, 

 giving off into, and taking up from the intercellular 

 spaces water- vapour, oxygen, and carbon-dioxide in all 

 sorts of proportions according to circumstances ; for each 

 living mesophyll-cell must have oxygen to enable its 

 protoplasm to respire, and it gets this oxygen from the 

 air which has diffused into the intercellular spaces from 

 the outside atmosphere ; again, each living mesophyll- 

 cell must get rid of most of the water passed into it from 

 the vessels, as otherwise it cannot make room for more 

 water, and would then be deficient in the necessary salts 

 of potassium, calcium, &c., brought to it in such minute 

 quantities dissolved in the water ; finally, each living 

 mesophyll-cell must have supplied to it quantities of 

 gaseous carbon-dioxide in order that the materials for 

 the work of the chlorophyll-corpuscles which are the 

 machines in which the carbonaceous materials are manu- 

 factured may be secured. 



All these gas and vapour interchanges from cell to 

 intercellular space and from intercellular space to cell, 

 varying in rapidity and amount at different periods as 

 they do, are controlled in many ways ; and they must all 

 take place through the moist cell-wall and its living lining 

 of protoplasm. 



