542. THE STRUCTURE OF ECONOMIC PLANTS 



In the mature blade, there is one layer of palisade cells and three 

 to five rows of spongy tissue. (Fig. x88, A.^ 



Differentiation of the vascular system is not completed until 

 the blade is fully developed. It is similar in general details to 

 that of the stem except for the position, number, and size of the 

 vascular elements. In the leaf primordium small crescentic groups 



^. ep 



Fig. i88. A, transection of margin of leaflet showing mesophyll and small veins ; B, transec- 

 tion through midvein of leaflet: ^^ p/i, abaxial phloem ; ^^/7/j, adaxial phloem ; w, cambium; 

 co/, collenchyma ; ?/», epidermis ; ^;-, hair; /«f, midvein; pa/, pnlisa.de; jpo, spongy tissue; 

 sto, stoma ; v, lateral vein ; xy, xylem. 



of procambial cells occur, surrounded by the larger cells of the 

 fundamental parenchyma; and, at about the center of each group, 

 a few protoxylem elements are differentiated. This is followed 

 by the appearance of phloem initials which form a band of smaller 

 cells at the periphery of the procambial strand. The adaxial 

 (inner) phloem is the first to be differentiated, and these groups, 

 though few in number, make up the larger portion of the phloem 



