Green Leaves at Work 89 



shaken themselves after their long winter sleep, 

 they set to work, for, fair though they are, beauty 

 is not their sole excuse for being, and there is 

 plenty for them all to do in Nature's great gar- 

 den. 



Through every leaf there runs a network of 

 delicate woody threads, curving, branching, and in- 

 terlacing. 



Its ramifications continue beyond the limits of 

 unaided vision. We call it the " skeleton," and 

 it does fulfil an office similar to that of the bones 

 in the human frame, for it supports the leaf and 

 gives it shape and strength. 



But it also serves the leaf as veins and arteries 

 serve the body, for the life-giving sap creeps 

 through these woody threads in slow but continu- 

 ous circulation. 



With the aid of a powerful microscope we can 

 see that the green pulp of the leaf looks some- 

 what like a honeycomb, as it consists of number- 

 less cells laid row above row. Those on the 

 upper side of the leaf are generally long and 

 narrow, and stand upright, pressing together al- 

 most as closely as the bricks in the side of a 

 house (Fig. 16). 



But the lower leaf cells differ greatly one from 



