648 THE STRUCTURE OF ECONOMIC PLANTS 



by Eames and MacDaniels (9), the guard cells belong to the type 

 in which the wall is unevenly thickened and the exposed margins 

 of the two cells project as thickened ridges. (Fig. 340, B.) 

 Epidermal hairs occur at the margin of the leaf between the teeth, 

 and also along the veins. These are numerous in young leaves 

 but break up and wither as they mature. The hairs are multi- 

 cellular and usually capitate, the column consisting of either a 

 single or double row of cells arising from one or two short basal 

 epidermal cells. 



Fig. 340. A, transections of segments of young foliage leaf ii days old showing character 

 of mesophyll, development of hairs and lactiferous ducts, Ix d; B, side section of stoma and 

 guard cells, variety New York Regular. 



The mesophyll is uniform; and, in the young leaf, the paren- 

 chymatous cells which comprise it are approximately isodiametric 

 and rather compactly arranged. (Fig. 340, A.') As the leaf 

 enlarges, the mesophyll becomes more porous and large inter- 

 cellular spaces are formed. Even at maturity, there is little or 

 no differentiation of the mesophyll into a palisade and spongy 

 region, the chief difference between the adaxial and abaxial cells 

 being that the former are somewhat more compact, while the latter 

 tend to elongate in a plane parallel to the flat surface of the blade. 

 The distribution of chlorophyll is variable, depending upon the 

 variety as well as upon the position of the leaf. In the heading 

 types, many of the inner leaves are practically devoid of pigment, 

 while in leafy types and in the leaves arising on the floral axes, 

 chlorophyll is present in the guard cells and in all of the paren- 

 chyma of the mesophyll with the exception of the cells adjacent 

 to the veins. These usually have no chlorophyll although those 



