208 BOTANY FOR BEGINNERS CHAP. 



to reach the honey. In fact any part of the flower may be 

 modified for the secretion and reception of honey. 



Insects cannot live on honey alone for it contains no nitrogen, 

 and nitrogen is just as necessary for the life of animals as for 

 plants. Many coloured flowers do not produce honey, but 

 plenty of pollen, the pollen being collected by insects for their 

 food. The pollen, since there is a fair amount of protoplasm in 

 it, contains nitrogen. Bees, for instance, possess small brushes, 

 on the end of their appendages or limbs, which are used to 

 brush the pollen from the surface of the body. The pollen is 

 then moistened and rolled up into little balls, which are stored 

 in a little sac in one of the limbs until the hive is reached. The 

 Broom is a plant which does not produce honey, but which is 

 visited by crowds of bees for the sake of its pollen, which it 

 produces in large quantities. Insects are of service to plants 

 because they distribute the pollen from flower to flower, and the 

 plants in return supply them with honey and pollen for food. 



Contrivances to Prevent S elf-Pollination. The im- 

 portance of cross-pollination to many plants has produced many 

 contrivances to prevent self-pollination, and to enable the far 

 more successful cross-pollination to take place. The principal 

 arrangements by which flowers facilitate cross-pollination must 

 now be described. 



(1) The stamens and carpels may be produced in different 

 flowers. In such a case it is necessary for the pollen to be 

 carried from flower to flower. Such flowers are said to be 

 diclinous. The stamen-bearing flowers and the carpel-bearing 

 flowers may be produced on the same plant, as in the Birch, 

 Hazel, and Pine, when it is said to be monoecious. The stamen- 

 bearing flowers may be produced on one individual, and the 

 carpel-bearing flowers on another individual plant, as in the 

 Dog's Mercury and Willow, when the plant is said to be 

 dioecious. 



(2) Both stamens and carpels are present in most of the 

 common flowers, and the flower is said to be monoclinic. In 

 such a flower self-pollination may be prevented by the stamens 

 and carpels ripening at different times. If the stamens ripen 

 and distribute their pollen before the carpels of the flower 

 bearing them are ready for pollination, the flower is said to be 

 protandrous, as in the Dog Daisy, Stitchwort, and Harebell 



