DEVELOPMENT OF PLANTS 393 



out endosperm and surrounded by a delicate bladder-like integu- 

 ment. They are exceedingly small and produced in enormous 

 numbers; in fact, the seeds of the rattlesnake orchid, Pcraniiuiii, 

 weigh but two millionths of a grain each and float in the air like 

 dust particles. The fringed orchis (BIcphariglottis), Arcthusa, 

 ladies' tresses (GyrostacJiys), rattlesnake plantain (Peramium), 

 grass pink (Limodorum) , rose pogonia, showy orchis (Gale- 

 orchis), 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 form 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 but 

 is often characterized by teeth and various forms of lobing, owing 

 to the fact that the veins usually differ from those of the mono- 

 cotyledons in that the}' 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 bundles in a circle about the pith 

 and the formation of a cambium zone which brings about an 

 increase in the diameter of the stem (Fig. 295). This arrange- 

 ment gives the plant a great advantage, permitting the extensive 

 system of branching that characterizes the group and the con- 

 sequent increase in the display of foliage. 



