SECT. II 



PHANEROGAMIA 



491 



The stem of most Dicotyledons is more or less profusely branched. 

 (For an account of the arrangement (Fig. 437) of the vascular bundles 

 and of their structure, ef. pp. 117 and 102. The secondary thickening 

 is described on p. 120.) 



The leaves are alternate or whorled, in the former case assuming 

 various arrangements. They often have stipules, but rarely leaf- 

 sheaths. The lamina is simple or compound, entire or more or less 

 irregular in outline. 



In the majority of cases the structure of the flowers may be 

 referred to the PENTACYCLIO, pentamerous type, although flowers with 

 whorls, consisting of two to six or 

 more members, also occur. When 

 more than five members are present 

 in a whorl the modification of the 

 normal structure is usually due to 

 splitting ; when less than five, to 

 suppression. There are also some 

 flowers which normally have less 

 than five members in the floral 

 whorls. The median sepal, with few 

 exceptions (Papilionaceae, Lobeliaceae), 

 occupies a posterior position. In 

 the most simply constructed Dicoty- 

 ledonous flowers (Amentacme) the 

 number of members composing the 

 whorls is subject to variation. Such 

 a condition does not result from a 

 modification of flowers of the pen- 

 tamerous type occurring in the 

 course of phylogenetic development ; 

 but it must be assumed rather that 

 the numerical relations existing be- 

 tween the parts are not yet thoroughly 

 fixed. 



In the oldest forms a corolla is typically absent; in the more 

 highly developed the perianth is usually differentiated into a calyx and 

 corolla. More rarely, by the suppression of one whorl, the perianth is 

 simple or developed as a double calycoid or corollaceous perigone. 



The Dicotyledons are divided into the two groups, Choripetalae and 

 Sympetalae. 



Fig. 438. — Leaf with reticulate venation 

 (| nat. size). 



A. Choripetalae 



Perianth single or double, and then usually polyphyllous. 

 The group contains the following orders : Amentaceae, Urticinae, 

 Polygoninae, Centrospermae, Polycarpicae, Blweadinae, Cistiflorae, Passi- 



