394 



THE DICOTYLEDONES 



The flower is subject to the same modifications as noted in the 

 monocotyledons. Among the lower orders, the flowers are quite 

 as simple as those of the primitive monocotyledons and the de- 

 velopment of imperfect and anemophilous flowers is of common 

 occurrence. The spiral arrangement of the numerous organs 



Fie. 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 zone; ph, 

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



of the flower will also be noted. The majority of the orders of 

 the dicotyledons, however, are characterized by perfect, cyclic 

 flowers and the various sets of organs usually consist of four or 

 five members each. The perianth, when present, is generally- 

 differentiated into a green calyx and a variously colored corolla. 

 The stamens are more frequently arranged in one or two whorls, 

 equalling or twice the number of the sepals, and the pistils usually 

 form a single whorl, equalling or less than the number of sepals. 

 The crowding of the various organs, and the lateral growth of 

 the receptacle results in the reduction in the number of organs 

 and in their cohesion with one another so that in the higher types 



