CHAPTER XVII 



DICOTYLEDONS: ARCHICHLAMYDB-SJ 



162. The two great divisions of Dicotyledons. The 



Dicotyledons are a much larger group than the Mono- 

 cotyledons, containing more than 200 families and about 

 100,000 species. Most of them are easily recognized by the 

 floral number five or four, the net-veined leaves, and the 

 arrangement of the vascular bundles of the stem in a hollow 

 cylinder. There are two great divisions of Dicotyledons: 

 the Archichlamydece, whose sepals and petals are either want- 

 ing or entirely separate; and the Sympetalce, whose corollas 

 are sympetalous ( 133). This is by no means the only dif- 

 ference, but it is the one used to form the names. 



The Archichlamydese comprise about three-fourths of the 

 families and three-fifths of the species of Dicotyledons, and 

 the group is so extensive and intricate that only a slight 

 acquaintance with it is possible at first. Five conspicuous 

 families or groups are selected on account of their repre- 

 sentative character and common occurrence. 



163. The tree group. In the lower stretches of the 

 Archichlamydese there are a number of small families that 

 include our most common hardwood or deciduous trees, 

 and this assemblage of conspicuous forms may be considered 

 together, without selecting any special family. They in- 

 clude elms (Fig. 44), sycamore, walnuts, hickories, oaks 

 (Fig. 43), chestnuts, willows, poplars (cottonwoods), birches, 

 beech, etc. These trees are all characterized by their simple 

 and inconspicuous flowers, which are usually monoecious or 



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