Arrangements for Securing Union of Sexes 315 



In the first place what is it which prevents close pollination in 

 flowers where both sexes are present? Against this obvious 

 difficulty, however, floral evolution has made ample provision. 

 The simplest method is a physiological one, viz., a flower is 

 sterile to its own pollen, that is, 

 a given stigma will not permit the 

 growth of its own pollen thereon, 

 doubtless for some chemical 

 reason; while another phase of 

 the same thing is the fact true of 

 some plants, that if close and cross 

 pollen happen to fall simultane- 

 ously on a stigma, the cross pollen 

 is the one that grows fastest and 

 produces the fertilization. But 

 far commoner is the sunple and 

 perfectly effective device of mak- 

 ing the stamens and pistils of 

 each flower ripen at different times, an arrangement called 

 dicJiogainy (figure 115), and found in a good many common 

 plants. Again, close pollination is prevented by mechanical 

 arrangements, usually the interposition between anther and 

 stigma of some specialized outgrowth, as shows very well, for 

 example, in the common Blue Flag, or Iris (figure 116). Still 

 another arrangement is displayed by the Primroses, Bluets, and 

 Ma^^flowers, which possess two kinds of flowers bearing stamens 

 and pistils in different positions, with corresponding differences in 



Fig. 115. — Dichogamous flower of Clero- 

 dendron, on two successive days, show- 

 ing the different time of ripening of 

 stamens and pistils. (Reduced from 

 Gray's Structural Botany.) 



German. The best general account of the subject, admirably written and beauti- 

 fully illustrated, is contained in Kerner's Natural History of Plants (another transla- 

 tion from the German), while his smaller volume, Flowers and their Unbidden Guests, 

 is a charming presentation of that subject. Brief and popular summaries have been 

 given by various writers, notably by Asa Gray in his all too brief How Plants Behave, 

 by Lubbock, in his Flowers, Fruits, and Leaves, and by W. H. Gibson in his Blossom 

 Hosts and Insect Guests, which is the most readable of all the works on the subject. 

 All of these books should be found in the public libraries. 



