PISUM SATIVUM 



359 



' — spo 



/.— ff c 



the principal veins, does not exceed more than six or eight cells in 

 thickness. (Fig. i8i, A.') Both of the somewhat papillate epi- 

 dermal surfaces are thinly cutinized, and the cells are sinuous in 

 outline, except for those lying above the veins, which are elongated 

 parallel to the long axis of the leaf. 



Stomata are approximately twice as numerous on the lower sur- 

 face as the upper, Holman and Robbins (i6) reporting counts of 

 ■L16 and loi per sq. mm. 

 respectively. They are 

 formed by the interposition 

 of a wall which cuts off 

 one of the lobed portions 

 of an epidermal cell to form 

 the stomatal mother cell. 

 Subsequent division of this 

 mother cell produces the 

 two guard cells, which are 

 somewhat depressed below 

 the surface of the adjacent 

 epidermal cells. 



The palisade layer con- 

 sists of a single row of 

 slender cells and comprises 

 about one-third of the 

 thickness of the mesophyll. 

 The cells are loosely organ- 

 ized so that numerous in- 

 tercellular spaces occur be- 

 tween adjacent groups and 

 this is also true of the 

 spongy tissue, which con- 

 sists of from four to six 

 layers of parenchymatous cells. (Fig. 181, A.^ The principal 

 veins are collateral, and the abaxial surface of the leaflet is ridged 

 along the line of the midvein because of the development of a 

 strand of mechanical tissue which parallels it. Supporting tissue 

 is also developed on the adaxial face of the large bundles and 

 there may be a few phloem fibers at the abaxial margin of the 

 phloem region of the bundle. The larger veins have a cambium, 

 and a limited amount of secondary vascular tissue is formed, the 



Fig. 181. A, transection of marginal portion of 

 blade of leaflet ; B, transection through midvein : 

 cu, cuticle ; ep, epidermis ; g c, guard cells ; mech, 

 mechanical tissue; pal, palisade; ph, phloem; 

 spo, sponge cells; sto, stoma; xy, xylem. 



