104 PLANTS AND MAN 



species (it is difficult to get exact estimates in any of the groups 

 larger than the family), among which are found all our broad- 

 leaved and deciduous trees. On the basis of flower structure the 

 Dicot families fall within three well defined groups; (7) those in 

 which the flowers lack petals and sepals or possess undiflerenti- 

 ated ones when present, and thus are chiefly wind-pollinated ; (2) 

 those in which the petals and sepals are separate from each other, 

 forming a corolla and calyx of clearly defined segments; {3) 

 those in which the petals are fused to form a tubular corolla, and 

 the sepals are similarly fused to form a funnel-shaped calyx. 



In the first group* there are about 20 common families and 

 some 20,000 species. Among them are the catkin plants, found in 

 the Willow, Walnut, Birch and Beech Families; in which are 

 found many of our important hardwood trees. Also in this group 

 are some herbaceous families of general interest, such as the 

 Mistletoe," Pepper, Buckwheat and Pink Families. Few are of 

 ornamental value since the flowers are inconspicuous or small in 

 the majority of species. 



In the second group of the Dicots we find the greatest number 

 of families (about 100) and species (some 50,000). Many native 

 and ornamental flowers are found in the Buttercup, Water Lily, 

 Poppy, Violet, Saxifrage, Rose and Mallow Families. A few 

 families include native trees, such as the Magnolia, Rose, Maple, 

 Horse Chestnut and Holly Families. Food plants are numerous 

 in this group, headed by the Pea and Rose Families (see Chapters 

 8, 9 and 10), and including the Mustard, Rue and Grape Fami- 

 lies. All of the citrus fruits are members of the Rue Family. In 

 this group is also the Cactus Family, one of the most interesting of 

 the succulent families. 



The third group is made up of those species with the most 

 highly evolved flowers found among the entire Dicot sub-class; 

 here some 35,000 species are found distributed through about 50 

 families. Trees and food plants are relatively fewer than in the 

 other two groups; but because of the complex flower, more species 

 are cultivated as ornamental garden plants. Ornamental shrubs 

 such as the rhododendrons and azaleas are found in the Heath 



* For additional illustrations of Dicot trees see Chapter 17; jor herbaceous Dicots, see 

 Chapters 26 and 27. 



