About Green Leaves 193 



When a tree is stripped of its foliage, no new 

 wood is formed till new leaves bud forth and 

 unfold. 



With water and air to work on, and sunshine 

 as a helper, leaves make not only wood but cork, 

 the tender petals of flowers, the flesh of fruits, 

 the hard shells of nuts; and also the many vege- 

 table oils, gums, dyes, and essences which are used 

 by artists, doctors, manufacturers, and confection- 

 ers. Leaves are made to do this work well. 



Through every leaf there runs a network of 

 woody threads, which branch and branch again 

 till they are as fine as cobwebs. We call this net- 

 work the " skeleton," and it does serve one use 

 of the bones in an animal frame, for it supports 

 the leaf, giving it shape and strength. But these 

 woody threads have another use. They are path- 

 ways for sap, which is always creeping slowly 

 through them. All the leaf " veins " are joined, 

 through the leaf stalk, with long lines of vessels 

 and tubes, fibro-vascular bundles leading up from 

 the root and down again. 



With a magnifying glass we can see that the 

 green leaf tissue between the veins is made up of 

 cells, row after row. Those on the upper side of 

 the leaf are generally long and narrow, and they 

 stand upright, pressed together as closely as the 

 stakes in a palisade fence (Fig. 49). Hence this 

 is called " palisade tissue." But on the lower side 

 of the leaf the cells are of different shapes and 



