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 

 ArchichlamydecB and the SympetalcB. 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) ArcMcUamydecB. — 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 limbellifers, containing numerous useful 

 forms, and being the most highly organized family of the 

 Archichlamydeae. 



(2) Sympet(d(B. — 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 



