412 THE DICOTYLEDONES 



and the formation of a cambium cylinder which brings about an 

 increase in the diameter of the stem (Fig. 295). This arrange- 

 ment gives the plant a great advantage, permitting the extensive 

 system of branching that characterizes the group and the con- 

 sequent increase in the display of foliage. 



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 

 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 

 the calyx and corolla become more or less tubular and the carpels 

 unite, forming a compound ovary. The cohesion of the organs 

 is often associated with other variations of the flower. Among 

 the more common of these changes is the adhesion of the stamens 

 to the corolla, which is usually associated with the basal growth 

 of the receptacle, forming the perigynous type of flower, and the 

 adhesion of the receptacle to the ovary so that the perianth and 

 stamens apparently arise from the top of the ovary, epigynous 

 type of flower (page 370). The irregularity that characterizes 

 the highest types of flowers in many of the orders is the most 

 helpful of all these variations, as is indicated by the abundance 

 and more common occurrence of such forms. 



The most characteristic feature of the dicotyledons is the 

 embryo which usually consists of a root, stem, and two laterally 

 attached cotyledons. The region of the stem above the cotyle- 

 dons is known as the epicotyl and frequently appears as a minute 



