CHAPTER XVIII 



THE FOLIAGE LEAF 



THE leaves and stems of the higher plants are 

 differentiated organs of the shoot. In most of the 

 lower chlorophyll-containing plants (Fucus, Pellia) the 

 shoot does not show this differentiation, but it is 

 established in all the vascular plants, except a few 

 peculiar forms which have lost it. The foliage leaf is 

 essentially the organ of photosynthesis, its principal 

 tissue, the mesophyll, containing the great majority of 

 the chloroplasts of the plant, though the parenchy- 

 matous tissue of the stem generally contain a certain 

 number. 



Essential Structure. The typical foliage leaf (Fig. 50) 

 is essentially a thin plate of mesophyll, thin enough to 

 allow light of sufficient intensity to reach all the cells, 

 with air spaces between the cells, communicating with 

 the outer air, and thus permitting free diffusion of gases 

 between the cells and the air. The mesophyll is inter- 

 penetrated by vascular bundles (veins) which carry water 

 and salts coming from the root (through the vessels and 

 tracheids of the xylem) to the mesophyll, and formed 

 organic substances away from it (through the sieve tubes 

 of the phloem) to other regions of the plant, principally 

 to growing regions and storage organs. The mesophyll 

 is protected by a layer of cells (the epidermis], typically 

 not containing chlorophyll and acting as a water tissue 

 (see p. 275), and the continuous outer layer of the 



