CHAPTER XVII 
DICOTYLEDONS: ARCHICHLAMYDEA 
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 Archichlamydea, whose sepals and petals are either want- 
ing or entirely separate; and the Sympetala, 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 Archichlamydezx 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 
Archichlamydee 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 moneecious or 
yet) 
