278 TEXTBOOK OF BOTANY 



no seeds are formed. If, as in the cucumber and the Indian 

 corn, staminate and pistillate flowers are separate but 

 borne on the same plant, only cross-pollination can occur, 

 but it may be between flowers on the same plant. However, 

 under such conditions some pollen is quite certain to be 

 carried from one plant to another. 



Cross-pollination between flowers on different plants is 

 sometimes made more likely by the fact that the flowers of 

 the two sorts on a particular plant ripen at different times ; 

 either the staminate flowers ripen and shed their pollen be- 

 fore the pistillate flowers are ready for pollination ; or else, 

 as in the oaks and the walnuts, the pistillate flowers ripen 

 and are pollinated before the staminate flowers are ripe. 

 Among plants with perfect flowers (flowers containing both 

 stamens and pistils) there are also many conditions that favor 

 cross-pollination. One of the commonest, something like 

 what we have just seen in plants with separate staminate 

 and pistillate flowers, is the habit of ripening the stamens 

 and the pistil of the same flower at different times. Some- 

 times, as in the onion, the gladiolus, and the members of the 

 composite family, the stamens ripen first ; in other cases, 

 as in the horse-chestnut and in some varieties of tobacco, the 

 pistil ripens before the stamens. 



In those flowers whose stamens and pistil ripen at nearly 

 the same time, the parts of the flower are often so arranged 

 that pollen falling from the anthers is not likely to land 

 upon the stigma. An arrangement of this sort has been 

 described in the bean, and the same result is brought about 

 in different flowers in a great variety of ways. 



A peculiar arrangement that favors cross-pollination be- 

 tween flowers on different plants is seen in many species of 

 primroses (Fig. 164). These have perfect flowers, but 

 they are of two different kinds, which are borne on different 

 plants. The flowers of some plants have high stamens (that 

 is, stamens set high up on the petals) and low (short) pistils: 



