LEAVES 



17 



tribution it is evident that stomata are definitely related to 

 air; and that where there is difference of illumination on the 

 two surfaces they occur chiefly on the less illuminated sur- 

 face. Stomata are not peculiar to the epidermis of leaves; 

 for they are found in the epidermis of any green part, as 

 young stems, fruit, etc., and even on the colored parts of 

 flowers. 



(2) Mesophyll. A cross-section of a leaf such as that of 

 the lily shows the single layer of epidermal cells bounding 

 the section above 

 and below, pierced 

 here and there by 

 stomata, recognized 

 by their guard-cells 

 (Fig. 12). An en- 

 larged view of a sec- 

 tion of a single sto- 

 ma may be seen in 

 Fig. 20. Between 

 these two epidermal 

 layers is the mass 

 of green tissue mak- 

 ing up the body of 

 the leaf, and known 

 as mesophylL This 

 comprises cells con- 

 taining the numer- 

 ous small green 

 bodies (chloroplasts) 

 that give color to the whole leaf. Usually the mesophyll 

 cells are arranged differently in the upper and lower regions 

 of the horizontal leaf. In the upper region the cells just 

 beneath the epidermis are elongated at right angles to the 

 surface of the leaf, and stand in close contact, forming the 

 palisade tissue. In the lower region of the leaf the cells 



FIG. 12. Cross-section of a lily leaf, showing epi- 

 dermal layers <>> with stomata () ; mesophyll 

 made up of palisade tissue (p) and spongy tissue 

 (p) with air-spaces (a), and containing chloro- 

 plasts ; and sections of veinlets (r) 



