THE ROLE OF CxREEN PLANTS 



251 



of the leaf, they branch into bundles of ever smaller and smaller 

 diameter to form the veins of the leaf. In the monocotyledonous 

 leaves, these veins run in a more or less parallel direction as seen in 

 grass blades or palm leaves. In the case of the dicotyledonous plants 

 characteristic irregular and netted veins 

 are found, reminding one of the branch- 

 ing of the capillaries in the human body. 

 These veins are made up structurally 

 of tracheids and tracheal vessels, ser\'ing 

 as water-conducting tissues ; sieve tubes, 

 which carry out food materials from the 

 leaf; and supporting tissue, which 

 makes up the mechanical framework of 

 the veins. Thus the veins act as a sup- 

 porting skeleton for the leaf as well as 

 conduits. 



The Structure of the Leaf 



The outer covering of the leaf (epi- 

 dermis) is composed of a layer of 

 irregularly shaped cells, usually rather 

 flattened. In some plants, like the 

 mullein, these cells are prolonged into 

 hairs, or again the layer, as a whole, is 

 frequently covered with a waxy cuticle 

 which is impermeable to gases and 

 water. The under surface of the leaf, 

 as seen through the compound micro- 

 scope, shows many tiny oval openings, 

 which are called stomata. The position 

 of the stomata varies in different leaves. 

 Some plants, as, for example, water 

 lilies, whose leaves float on the surface 

 of the water, have them in the upper 

 epidermis. Others have them on the 

 under .side, and .still others have them 

 on both surfaces. The estimated num- 

 ber of these openings varies. Mac- 

 Dougal estimates that as many as two 

 million are on the under surface of an 



Stomata from the loaf of an 

 Easter lily (Lilium lonyiflorum) : 

 Above, a stoma, as seen in sur- 

 face view, showing the two 

 kidney-shaped guard cells {g), 

 which enclose the stomatal aper- 

 ture (s), the more deeply shaded 

 portion representing the central 

 slit ; note the chloroplasts in the 

 guard cells; (b) subsidiary cells. 

 Below, a stoma, as seen in cross 

 section ; note the guard cell (g) 

 next to the subsidiary cell (6) ; the 

 outer slit (o) is enclosed between 

 the cutinized outer guard-cell 

 ridges (r), the enlarged area just 

 below being the outer vestibule 

 (o') ; below the central slit (s) is 

 the inner vestibule (('), which 

 here opens directly into the 

 cavity (c) underneath the stoma. 



