132 



PRACTICAL BOTANY 



Usually, as in some mallows (Fig. 121) and in Olerodendron 

 (Fig. 122), the stamens mature first. An insect visitor to a 

 flower in the staminate condition becomes somewhat covered 

 with pollen. Then flying to a flower in the pistillate condition, 



he is sure to leave pollen 

 on the stigmas and thus in- 

 sure cross-pollination. 



It is common to find the 

 stamens of a flower matur- 

 ing a few at a time, as in 

 " nasturtium," buckwheat 

 (Fig. 114), and many other 

 flowers. This gives more 

 opportunities for insects to 

 carry away the pollen than 

 would be possible if it all 

 matured at once. 



124. Prevention of self- 

 pollination: dimorphism. A 

 means of preventing self- 

 pollination, even more ef- 

 fective than is dichogamy, 

 is found in the structure 

 of flowers m which some 

 have a long pistil and short stamens, others a short pistil 

 and long stamens. This condition occurs in the flo\\'ers of 

 bluets (Fig. 123), the partridge berry, the primrose, and some 

 other common flowers. It is easy to see that the head of an 

 insect smeared with pollen by contact with the anthers of 

 Fig. 128, A, would just come into contact with the stigma 

 of B, and that the insect's abdomen covered \vith pollen in 

 B would just touch the stigma of .1. All the flowers on an 

 individual plant are of one kind (either long-styled or short- 

 styled), and the pollen is of two sorts, — each kind sterile 

 on the stigma of any flo\ver of similar form to that from 

 which it came. 



Fig. 122. Dichogamous flower of Olero- 

 dendron in two stages 



In A (the earlier stage) tlie stamens are 

 mature, wliile the pistil is still undevel- 

 oped and bent to one side ; in B (the later 

 stage) the ■ stamens have withered and 

 the stigmas have separated, ready for the 

 reception of pollen 



