186 THE BEAUTIES OF NATURE CHAP. 



forest trees. A slice across the stem of a 

 tree shows many different tissues with more or 

 less technical names, bark and cambium, med- 

 ullary rays, pith, and more or less specialised 

 tissue ; air-vessels, punctate vessels, woody 

 fibres, liber fibres, scalar if or m vessels, and 

 other more or less specialised tissues. 



Let us take a single leaf. The name is 

 synonymous with anything very thin, so that 

 we might well fancy that a leaf would consist 

 of only one or two layers of cells. Far from 

 it, the leaf is a highly complex structure. On 

 the upper surface are a certain number of 

 scattered hairs, while in the bud these are 

 often numerous, long, silky, and serve to 

 protect the young leaf, but the greater number 

 fall off soon after the leaf expands. The hairs 

 are seated on a layer of flattened cells the 

 skin or epidermis. Below this are one or 

 more layers of " palisade cells," the function 

 of which seems to be to regulate the quantity 

 of light entering the leaf. Under these again 

 is the " parenchyme," several layers of more or 

 less rounded cells, leaving air spaces and pas- 

 sages between them. From place to place in 



