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, scalariform 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 
