MONOCOTYLEDONS AND DICOTYLEDONS 381 
251. Dicotyledons, Dicotyledons form the greatest group 
of plants in rank and in numbers, being the most highly 
organized, and containing about eighty thousand species. 
They represent the dominant and successful vegetation in 
all regions, and are especially in the preponderance in tem- 
perate regions. They are herbs, shrubs, and trees, of every 
variety of size and habit, and the rich display of leaf forms 
is notably conspicuous. 
Two great groups of Dicotyledons are recognized, the 
Arcliiclilamydem and the Sympetalce. In the former there 
is either no perianth or its parts are separate (polypeta- 
lous) ; in the latter the corolla is sympetalous. The Archi- 
chlamydeae are the simpler forms, beginning in as simple a 
fashion as do the Monocotyledons ; while the Sympetalae 
are evidently derived from them and become the most 
highly organized of all plants. The two groups each con- 
tain about forty thousand species, but the Archichlamydeae 
contain about one hundred and sixty families, and the 
Sympetalae about fifty. 
(1) ArcMcJilamydecB. In this great division of Dicoty- 
ledons are such groups as the great tree alliance which 
includes poplars, oaks, hickories, elms, willows, etc. ; the 
buttercup alliance, which includes buttercups, water-lilies, 
poppies, mustards, etc. ; the rose family, one of the best 
known and most useful groups of the temperate regions ; 
the pea family, by far the greatest family of the Archi- 
chlamydeae, containing about seven thousand species ; the 
parsley family, or umbellifers, containing numerous useful 
forms, and being the most highly organized family of the 
Archichlamydeae. 
(2) Sympetalce. These are the highest and the most 
recent Dicotyledons. While they contain numerous shrubs 
and trees in the tropics, they are by no means such a 
shrub and tree group in the temperate regions as are the 
Archichlamydeae. The flowers are constantly cyclic, the 
number five or four is established, and the corolla is 
