THE LEAF 71 



the means whereby the water taken up by the roots is 

 brought to the leaf, and the substances manufactured in 

 the leaf are carried away to the other parts of the plant. 



The veins of leaves are arranged in two main ways ; 

 netted, as in the examples above ; parallel, as in the lily, 

 wheat, barley, and all grasses where the veins run side 

 by side and do not form an interlacing network. These" 

 two types of vein arrangement netted and parallel 

 are, on the whole, characteristic of the leaves of dicoty- 

 ledons and monocotyledons respectively, and with certain 

 exceptions; for instarice, the black bryonyr^-may be 

 taken as indicating to which of these two groups a plant 

 belongs. 



It is impossible, without' the use of a microscope, to 

 obtain very much information concerning the internal 

 structure of leaves. If, however, we select some thicks 

 leaved plant, such as the iris, we find that both upper and 

 lower surfaces of a leaf are covered with a colourless skin, 

 which, with a little care, can be stripped off*. The main 

 mass of the leaf is seen to be made up of comparatively 

 soft tissue, through which harder, fibrous strands (the 

 veins) run. The thin skin makes a kind of waterproof 

 coating to the leaves, but has an enormous number of 

 minute openings, called stomata (too small to be seen 

 without a magnifying glass), through which the gases of 

 the atmosphere can pass in and out, and so reach the 

 spongy tissue of the inside of the leaf. This is most 

 important, for it is in this inner part that the real work 

 of the leaf, the breathing and building up of new matter, 

 goes on, and for these processes a free interchange of 

 gases with the outside air is absolutely necessary. 



