INTRODUCTION 



21 



between the outside air and the intercellular spaces in 

 the leaf-tissue, which are specially developed on the 

 lower face of the leaf (see Fig. 11 below). Through 

 the stomata also escapes the surplus water in which the 

 nitrogenous and other mineral food has been carried up 

 from the roots to the leaves. Some of the water is 

 required for nutrition and as cell-sap to maintain the 

 turgescence of the tissues, but a large proportion serves 

 only as a carrier, and is ultimately given off, or trans- 

 pired, through the stomata. They are very numerous. 



Fig. 6. — Loiigitadiual section through the growing point of a winter bud of Abies 

 peotinata. x aliout 200. .s; apex of growing point ; b, b, youngest leaves ; 

 r, cortex ; m, pith. 



It is estimated, for instance, that on such a leaf as 

 that of the Oak there are not less than 2,000,000. 

 The carbon is assimilated and the oxygen released, at 

 least in part. It is remarkable that plants do not take 

 up carbonic acid from the soil. It might have been 

 expected that the roots, ramifying as they do in earth 

 more or less saturated with water containing carbonic 

 acid in solution, would absorb what is so important 

 an element in their food. This function is, however, 

 mainly performed by the leaves, and especially under 

 the influence of daylight. It is carried on by 

 protoplasm containing " chlorophyll granules." These 

 are roundish green corpuscles, which give their peculiar 



