GROUP V. ANGIOSPERMLE ; DICOTYLEDOXES. 511 



The leaves exhibit infinite variety both in their relative position 

 and in their form. The foliage-leaves almost always consist of 

 petiole and blade ; amplexicaul leaf-bases are comparativelv rare, 

 but stipules, on the contrary, are very common. Branching or 

 segmentation of the leaves is common, and is frequently indicated 

 by the incision of the margin. The usually reticulate venation 

 of the leaves is characterized by the presence of a large number 

 of veins which project on the under surface, except in thick, 

 fleshy 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. 443) : they differ very consider- 

 ably in their structure, and cannot be referred to any one type. 

 The following are the principal 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 (f ), 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 | J5, or Pn + n, An + n, where n = 2 or 3. This structure pre- 

 vails 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 Ranales. 



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 Kn, Cn, An + n, where n usually 

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

 flower ; it occurs in most Polypetalse 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. 



