76 



THE STRUCTURE OF ECONOMIC PLANTS 



(many of the Gramineae), the stomatal frequency may be approxi- 

 mately the same for both surfaces. Stomatal counts are often at 

 variance, owing to the fact that the number of stomata per unit 

 area for a given species or even for a single plant may vary depend- 

 ing upon the age of the leaf and the environmental conditions under 

 which it has developed. Expressed in approximate ratios, Pisum 

 has about twice as many stomata on the abaxial surface as the 

 adaxial one; in Triticum, the ratio of adaxial to abaxial stomata is 

 about ID to 7; and in Zea, 3 to 5. In Olea, olive, and Ficus, rubber 

 plant, no stomata occur on the adaxial surface. 



spo 



Fig. 2.8. Transection through a portion of the blade of rhe tobacco leaf : g/^, epidermis; i>r, 

 epidermal hair; pal, palisade cells; ph, phloem; spo, spongy parenchyma; sto, stoma; xy, 

 xylem. 



The mesophyll is usually differentiated into palisade and spongy 

 cells; but in some cases it consists of parenchymatous cells that are 

 more or less uniform in size, shape, and arrangement (Zea, 

 Lactuca). Where differentiation exists, the palisade zone lies 

 immediately below the adaxial epidermis, consisting of one, some- 

 times two, or infrequently more, layers of cells which are compactly 

 arranged with their long axes at right angles to the surface of the 

 blade. The palisade cells are more compactly arranged than are 

 the spongy cells, but there are more intercellular spaces in this 

 region than are ordinarily shown in transections of a leaf. The 

 cells of the spongy parenchyma are approximately isodiametric and 



