390 THE STRUCTURE OF ECONOMIC PLANTS 



dons before wall thickenings can be observed in the primary 

 xylem. 



After emergence above the ground, the cotyledons enlarge and 

 thicken for about two weeks, persisting for a longer period than 

 do many seedlings; but, in about thirty days, they become yellow 

 and die. The mesophyll of the mature cotyledon resembles that of 

 a foliage leaf, except that it is thicker and the intercellular spaces 

 are smaller. Stomata occur in considerable numbers in both the 

 upper and the lower epidermis and are subtended by large air spaces. 

 The venation is closed, resembling that of the leaf; and, except at 



Fig. 199. A-C, diagrams showing venation: A, double leaf; B, cotyledon; C, typical 



foliage leaf. (After Crooks.) 



the base of the median vein, all of the bundles are collateral. (Fig. 

 199, B.) 



Ontogeny of the Leaf. — The epicotyledonary axis develops 

 very slowly for the first eight or ten days and is surrounded by 

 the bases of the cotyledons. The leaf primordium involves at 

 least three of the outermost cell layers at its point of origin, arising 

 from the growing point as a conical mass of cells which elongates 

 and broadens, due to general meristematic activity, and becomes 

 a somewhat flattened subconical projection. (Fig. x6.) Con- 

 tinued general cell division increases the length of the flattened 

 primordium, until it is about one-fifth the length of the mature 

 leaf; and then a localization of meristematic activity to the lateral 



