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flowers; others have only pistillate flowers. Hemp plants likewise usually 

 bear only staminate or only pistillate flowers. Such plants are said to be 

 dioecious (Greek "two households"), in contrast to monoecious (Greek 

 "one household") plants on which both staminate and pistillate flowers, 

 or perfect flowers, occur. Some plants are monoecious in certain environ- 

 ments and dioecious in others. 



Flower clusters. The flowers of some plants, such as those of tulip and 

 trillium, grow singly at the ends of solitary unbranched peduncles. But 



Fig. 158. Diagrams illustrating terms applied to flower clusters: A, spike; B, 

 spadix; C, catkin; D, raceme; E, panicle; F, head; G, head with disk flowers and 

 ray flowers; H, corymb; I, umbel; J, compound umbel. See also illustrations in 

 Chapter LI. 



in many species of plants the peduncle branches; and each of its ulti- 

 mate branches, the pedicels, terminates in a single flower. The flowers 

 are thus grouped in various types of clusters called inflorescences. (Fig. 

 158). Flowers mav also be clustered along the sides of the main floral 

 axis, or on numerous small lateral branches at the upper part of a plant. 

 The variety of flower clusters is so great that a special terminology is 

 necessary to describe them. The spike exemplified by the common plan- 



