THE PLANT BODY 



37 



The root cap is an important morphological feature of the root. It is 

 absent in only a few monocotyledons, where it seems to have been 

 lost as a part of adaptation to an aquatic habitat. 



Ontogeny of the Leaf. The development of the leaf follows patterns 

 related as a whole to the type of leaf that is formed. Variety in details 

 of meristem activity and tissue specialization is great, and only general 

 features of form and anatomy are discussed here. 



In the dicotyledons, the leaf primordium is initiated close to the 

 shoot apex in die tunica or in an area involving cells of both tunica 



m 



n 



nr 



Fig. 15. Diagrams showing types of marginal growth in the leaf blade. E, epidermal 

 initial; S, subepidermal initial; P, procambium. {After Hara.) 



and outer corpus. A lateral shoulder, or "buttress," increases in size by 

 apical growth and by lateral expansion around the apical meristem. The 

 lateral extent of the more or less crescent-shaped mound so formed 

 varies with form of leaf and with stipule form and position. Median 

 growth builds up a somewhat flattened, fingerlike projection, on which 

 marginal meristems soon appear. Apical growth continues, but increase 

 in size is soon largely a result of the activity of the marginal meristems 

 (Figs. 15 and 16), which early lay down the foundation of the blade, 

 often outlining its general form very early. Below the marginal meristems 

 a basal region, which shows less activity, later becomes an "intercalary" 

 or "basal" meristem, which builds the petiole. If the leaf is stipulate, 

 stipule meristems arise below this meristem on the shoulders of the 

 buttress, or, where adnate to the petiole, on the petiole meristem when 



