190 GROWTH AND WORK OF PLANTS 



into one part of the flower and out at the other in such a way as 

 to bring about cross-pollination. 



305. Imperfect flowers. In those flowers where the stamens 

 and pistils are in different flowers, often in different flower clusters 

 or on different plants, cross-pollination is necessitated. Many 

 of these are wind-pollinated as stated above in many of the trees 

 with catkins. Others of this group are insect-pollinated, as in 

 the willow and chestnut. Here the stamens and pistils are the 

 showy parts, and the nectar also attracts the insects. 



306. Flowers with stamens and pistils of unequal length. 

 In some flowers the stamens and pistils are of unequal length. 

 When the insect visits such a flower one part of its body comes 



Fig. 142. 

 Dichogamous flowers of Primula. 



in contact with the pistil and another part comes in contact with 

 the anthers. The part which comes in contact with the anthers 

 brushes off and carries away some of the pollen. In the next 

 flower, or one of the succeeding ones which the insect visits, the 

 length and position of the stamens and pistils will be reversed, 

 so that the pollen from one of the previously visited flowers 

 will be brought in contact with the stigma and its viscid surface 

 will pull off some of the pollen. The common bluet (Houstonia) 

 and the bellflower (Campanula) are examples, as well as the 

 primrose so commonly grown in greenhouses. This plant can 

 be used for study and demonstration. Not only do the length 

 and position of the stamens and pistils in such flowers favor 



