174 Chapters in Modern Botany CHAP. 



despaired of. The varying pull of the epidermic cells upon 

 the stomata has also to be considered, and so on. 



We regard the stomata then as automatic ventilators, 

 which have especially to do with regulating the transpira- 

 tion of water-vapour from the leaf. They lead into spaces 

 between the loosely packed cells which form the lower half 

 of the leaf. 



But where within the leaf is the important work of 

 building up starch and proteids carried on? The answer 

 is not difficult, for starch is a very readily recognisable 

 substance, occurring in characteristic granules, and we 

 know that it occurs most abundantly in the layer or layers 

 of green cells lying immediately below the protective upper 

 epidermis, which are so closely crowded together towards 

 the light as to give a section the aspect of a palisade. This 

 "palisade parenchyma" is the chief laboratory of the plant. 

 Here it is that the radiant energy of the sun streaming 

 through green colouring matter is used by the living matter 

 of the plant in the splitting up of the absorbed carbonic 

 acid, and in building up complex materials whose chemical 

 energy is well known to us when we use them as food or as 

 fuel. 



When we think of the leaves manufacturing starch all 

 the day long and all the summer through, the question 

 must arise, Why do not the leaves become too full ; how 

 is it that they are not overloaded with starch? It is plain 

 that the starch must in some way pass away from the 

 leaves, and it is not difficult to prove that leaves which 

 were richly filled with starch when the sun set may have 

 very little next morning when the sun rises. When the 

 leaves wither and die in the autumn there is again a dis- 

 appearance of almost all of their starch. How does the 

 starch disappear? in what form? by what path? 



At this stage in our studies we can answer these questions 



