A GREEN LEAF 



59 



upon the activity of a certain green-coloured sub- 

 stance which fills one layer of tiny cells in the 

 leaves of plants, the temptation to exclaim " Mar- 

 vellous ! " is great. 



It is not enough, of course, merely to gaze upon 

 the magnified structure; we must at the same time 

 endeavour mentally to analyse a leaf and learn of 

 what chemical elements it is chiefly composed. 

 Having thus gained a knowledge of its structure, 

 and of the matter of which it is principally built 

 up, we are in a position to trace the connection of 

 the two and the consequences of that connection. 



In Fig. 39 is shown part of a magnified section 

 of the blade of a laurel leaf, made to exhibit its 

 internal structure. As the laurel is an evergreen, 

 the upper surface of its leaf has a protective layer 

 of a varnish-like substance, probably to protect 

 the leaves from injury by the frost of winter. 

 Immediately below this varnish layer is situated 

 another layer of large cells, which botanists call 

 the " epidermal tissue." These cells usually are 

 transparent and colourless, and full of water, and 

 serve to protect the internal tissues of the leaf 

 from excessive evaporation and external injuries. 

 Boys are fond of tearing a laurel leaf crookedly so 

 that this layer of colourless cells extends like a 

 fringe beyond the green part ;i then, placing the 

 torn fragment between their two thumbs and 



