DEVELOPMENT OF PLANTS 



427 



occurrence. The spiral arrangement of the numerous organs 

 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. 



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. 



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 mass growth so that in the higher types the calyx 

 and corolla become more or less tubular and the carpels form a 

 compound ovary. The basal growtli of tlie receptacle also causes 

 the frequent occurrence of the perig}nous and epigynous t>pes of 

 flowers. Irregularity of the flower is more common than in the 



