DEVELOPMENT OF PLANTS 



411 



forming the conspicuous and characteristic features of the vege- 

 tation of the earth. 



The dicotyledons may be short-lived annual plants, or per- 

 ennials, and they include a great variety of climbers, epiphytes, 

 parasites, and saprophytes and comparatively few aquatics. The 

 most extreme forms of xerophytes are also found in this group. 

 The leaves are highly differentiated, usually consisting of a 



Fig. 294. Fig. 295. 



Fig. 294. Leaf of white birch, the blade, b, traversed by a network of 

 veins that end in free branches (the margin irregularly toothed or dentate) 

 and supported upon a petiole, p. 



Fig. 295. Diagram of a cross-section of a stem of black oak four years 

 old: p, pith; i, 2, 3, 4, annual rings of xylem; c, cambium cylinder; ph, 

 phloem; c, cortex; ck, cork; m, medullary rays. 



blade and petiole which is often associated with small leaf-like 

 organs, the stipules (Fig. 294). The blade varies in form but 

 is often characterized by teeth and various forms of lobing, owing 

 to the fact that the veins usually differ from those of the mono- 

 cotyledons in that they repeatedly branch, becoming smaller and 

 smaller and thus forming a network or reticulated venation with 

 free ends on the margins and other parts of the leaf. The stems 

 are markedly different from the previous class, owing to the 

 arrangement of the vascular bundles in a circle about the pith 



