PLANTS, FUNGI AND INSECTS 7 



the leaf- stalk from the main veins, then ramify between the 

 palisade and spongy mesophyll, thus providing a continuous 

 connection by way of the stem and branches of the tree 

 between the finest roots and every part of the leaves. The leaf- 

 cells, except those of the epidermis, are abundantly supplied 

 with chlorophyll, which plays a most important part in nutrition. 

 The epidermal cells of young leaves have thin walls, which are 

 permeable to gases and water vapour, but, as the leaf gets older, 

 the outer walls of the cells become cuticularised and impermeable. 



Transverse Section of Blade. 



e, upper epidermis ; s, stomata ; int, intercellular spaces ; pp, 

 palisade parenchyma with chlorophyll ; sp.p, spongy parenchyma ; 

 e' ', lower epidermis; g.c., guard cells. 



FIG. 3. THE LEAF. 1 



At this stage communication with the outer air is provided for 

 by means of stomata, which are so very numerous that a single 

 leaf may possess millions. As a general rule the majority of the 

 stomata, sometimes all of them, are on the underside of the leaf, 

 but the distribution of them between the two surfaces varies in 

 different plants. 



During the winter most trees enter upon a resting period, and, 

 to reduce the physiological functions as far as possible, generally 

 shed their leaves. Preparatory to this, all useful substances 

 are passed back from the leaves to the branches, and frequently 

 waste products, which are not required by the plant, are left in 



1 From Laurie's Flowering Plants. 



