74 THE STRUCTURE OF ECONOMIC PLANTS 



stated that the facts "favor the contention that stipules are integral 

 portions of the base of the leaf, a view which is well expressed by 

 Eichler when he says that stipules arise without exception as a 

 product of the leaf-base of the primordial leaf. ' ' Recent work by 

 Cross (lo) on Morus, which attacks the problem from the stand- 

 point of foliar ontogeny, confirms this point of view. In some 

 instances, the stipules are very prominent as in Pisum, where 

 they are large and perform the same functions as the blade. Some 

 stipules are protective in relation to the axillary bud, others 

 differentiate as spines. 



The blade may be variously specialized; and differences in its 

 color, shape, marginal characteristics and venation result in a 

 great number of forms. Typically, the foliage leaf is green owing 

 to the presence of chlorophyll in the mesophyll; but this pigment 

 may be masked by the presence of accessory pigments, chiefly 

 anthocyanins; and, in variegated leaves, the chlorophyll may be 

 restricted to definite regions of the blade. 



The shape of leaves may range from the slender, needle-like 

 leaves of many conifers and the linear or lanceolate types found in 

 grasses, to the more expanded ovate, cordate, or orbicular leaves 

 which are commonly developed in dicotyledons. The degree of 

 marginal modification varies from those which are smooth and 

 entire through a progressive series of marginal indentations that 

 include the serrate, dentate, and crenate forms. Where the cutting 

 of the margin is more pronounced, the leaf is lobed; and this may 

 be palmate (Gossypium and Cucurbita) or pinnate (Raphanus). 

 If the blade is divided into segments or leaflets, the leaf is 

 compound; and the arrangement of the leaflets may be palmate 

 (Cannabis) or pinnate (Pisum and Apium). 



The venation of the blade may be parallel or netted. In the 

 former, the primary veins extend through the lamina parallel to one 

 another without conspicuous anastomoses; while in the netted sys- 

 tem the veins form a complicated reticulum. Studies of character- 

 istic systems of parallel venation (Zea and Allium) indicate that 

 there are many small cross-veins interconnecting the parallel mem- 

 bers of the system. In the net-veined types, the ultimate veinlets 

 commonly end blindly in a small area of chlorophyll parenchyma 

 which is termed a vein islet. 



The foliage leaf may persist throughout the life of the plant, 

 or through several seasons in perennial plants. Evergreen leaves 



