268 



THE CONTINUITY OF THE RACE 



The structure popularly regarded as a flower in this group is really a very 

 dense mass of flowers, called a head. In such heads, the small individual 

 flowers are often of two sorts — those in the center (the disk flowers) per- 

 fect, with inconspicuous corollas; 

 and those around the margin (the 

 ray flowers) imperfect and sterile, 

 the tubular corolla of each forming 

 a conspicuous strap-shaped projec- 

 tion or ray, which resembles a 

 single petal of an ordinary flower. 

 Monoecious and dioecious con- 

 ditions. All plants that possess 

 perfect flowers are hermaphrodi- 

 tic, since male and female gametes 

 or sex cells are both produced by 

 the same plant. They are said to 

 be monoecious, just as in the in- 

 stance of hermaphroditic animals. 

 In plants that have imperfect 

 flowers, the staminate (pollen-pro- 

 ducing or male) flowers and the 

 pistillate (seed-producing or female) 

 flowers may occur on different 

 parts of the same plant, as in 

 maize, oak, and maple. Here the 

 plant as a whole is still monoecious, 

 although the male and female 

 gametes are produced in different 

 flowers. In some species, however, 

 each individual plant produces 

 only staminate or pistillate flowers ; 

 this is true, for example, in the 

 date palm, papaya, holly, and 

 willow. Such species are said to 

 be dioecious, since the male and 

 separate individuals, as in most 



Fig. 17.14. The imperfect and incomplete 

 flowers of hazelnut (Corylus americana, 

 birch family, Betulaceae). The small pistil- 

 late flower appears near the tip of the twig. 

 The large staminate flower (catkin) occu- 

 pies the center of the picture. The male and 

 female gametophytes are borne in separate 

 flowers, but the plant as a whole is monoe- 

 cious since it produces both types of flowers. 

 {Photo by Prof. E. B. Mains.) 



female gametes 

 animals. 



are produced in 



The Alternation of Generations in the Flowering Plants 



The whole body of a flowering plant, including all of the flower itself 

 except certain minute structures that develop in the stamens and pistils, 

 is composed of sporophyte tissues with the diploid number of chromo- 

 somes. The gametophyte generation is reduced to the minute pollen 



