426 DICOTYLEDONES 



dust particles. The fringed orchis (Blephariglottis) , Arethusa, 

 ladies' tresses (Gyrostachys) , . rattlesnake plantain (Peramium) , 

 grass pink {Limodorum) , rose pogonia, showy orchis (Galeorchis) , 

 moccasin flower (Cypripedium) are among the common and 

 more showy of our native orchids. 



Class B. Dicotyledones 



136. General Characters. — ^The structure of these plants is 

 more complex than that of the monocotyledons and their varia- 

 tions have been more extensive and successful, over 100,000 

 species being known. For this reason they are adapted to a 

 greater range of conditions and have become the dominant plants, 

 forming the conspicuous and characteristic features of the vege- 

 tation of the earth. 



The dicotyledons may be short-lived annual plants, or per- 

 ennials, and they include a great variety of climbers, epiphytes, 

 parasites, and saprophytes and comparatively few aquatics. The 

 most extreme forms of xerophytes are also found in this group. 

 The leaves are highly differentiated, usually consisting of a 

 blade and petiole which is often associated with small leaf -like 

 organs, the stipules (Fig. 294). The blade varies in form and 

 is often characterized by teeth and various forms of lobing- 

 This is due to the fact that the veins usually differ from those of 

 the monocotyledons in that they repeatedly branch, becoming 

 smaller and smaller and thus forming a network or reticulated 

 venation with free ends on the margins and other parts of the leaf. 

 The stems are markedly different from the previous class, owing 

 to the arrangement of the vascular tissues in a circle about the 

 pith (p. 76) and the formation of a cambium cylinder which 

 brings about an increase in the diameter of the stem (Fig. 295). 

 This arrangement gives the plant a great advantage, permitting 

 the extensive system of branching that characterizes the group 

 and the consequent increase in the display of foliage. 



The flower is subject to the same modifications as noted in the 

 monocotyledons. Among the lower orders, the flowers are quite 

 as simple as those of the primitive monocotyledons and the de- 

 velopment of imperfect and wind pollinated flowers is of common 



