572 PART III. THE CLASSIFICATION OF PLANTS. 



leaves, and which frequently anastomose; a midrib is almost 

 always present, giving off lateral branches to right and left. 



The flowers, when they are lateral, are usually furnished with 

 two prophylla or bracteoles (see p. 505) : they differ very con- 

 siderably in their structure, and cannot be referred to any one 

 type. The following are the principle forms: 



1. In a considerable number the perianth, which is simple, and 

 the androecium are isomerous, consisting of four, five, or six 

 members; their arrangement is either spiral (), or whorled so 

 that the stamens are always superposed on the leaves of the 

 perianth ; the latter are all similar and are sepaloid. Formula 

 P5 | A5, or Pn + n, An + n, where n = 2 or 3. This structure 

 prevails in some of the Monochlamydeae (Urticales, Amentales). 



2. In a second group, all the parts of the flower are arranged 

 in a continuous spiral, the stamens, and sometimes the carpels, 

 being generally more numerous than the leaves of the perianth : 

 the perianth may consist only of a calyx, or a corolla may be 

 developed in place of the external stamens : when this is the case 

 the corolla alternates with the calyx, provided that it is isomerous 

 with it, as in most Banales. 



3. With these two types are connected by many intermediate 

 forms those flowers in which the biseriate perianth and the stamens 

 are in whorls: their formula is JCn, Cn, An + n, where n usually 

 = 5 or 4. This is the most common type of structure of the 

 flower; it occurs in most Polypetalae and Gamopetalae ; it may be 

 modified either by the suppression of one (usually the inner) 

 whorl of stamens, or by their multiplication, their branching, or 

 their cohesion, or by the suppression of the corolla. 



4. Finally, there remain certain flowers which cannot be directly 

 referred to any one of the above types, and they must therefore 

 be left unexplained for the present, and the relationships of their 

 families must remain uncertain. 



The sub-divisions in which the Dicotyledons are arranged in the 

 following classification are especially characterized by peculiarities 

 in the structure of the flower. It is impossible, however, to draw 

 sharp distinctions between the sub-classes, the cohorts, the orders, 

 and sometimes even between the families, for the position of a 

 plant in the system depends, not upon any one character, but upon 

 the aggregate of its characters. 



The principal orders of Dicotyledons may be arranged as 

 follows : 



