3IO FLOWERS 



nearly all conspicuous or odorous flowers, though " corre- 

 sponding structures " between flowers and insects are the 

 exception rather than the rule. Most insect-polKnated 

 flowers may be pollinated by any one of a considerable 

 number of kinds of insects. In the case of dandelion, for 

 example, over one hundred different kinds of insects have 

 been observed visiting its flowers. Quite commonly one 

 kind of insect appears to be the favorite visitor of one kind 

 of flower, even though pollination by other insects is possi- 

 ble. This is the case with red clover, whose favorite visitor 

 is the bumblebee. One condition necessary for a good crop 

 of clover seed is an abundance of bumblebees in the clover 

 field. Evidently bumblebees are of commercial value to 

 the farmer who grows clover for seed. 



As you have noted, nearly all insect-polUnated flowers 

 are monoclinous. Willows (see page 264) and some of 

 the Compositm are exceptions to this. Evidently mon- 

 ocliny is an advantage in insect-pollination ; for one reason, 

 to effect an equal amount of pollination, only half as many 

 visits are necessary to monoclinous flowers as to diclinous 

 flowers. 



a. Why do Insects visit Flowers ? — Pollen and nectar 

 are the two things for which insects visit flowers. Both 

 are used for food, though many insects use one and not 

 the other. Nectar is a sweet liquid which is secreted by a 

 part of the flower called the nectary. It is usually so placed 

 that in order to reach it the insect must brush the stamens 

 or stigma or both. In some cases the nectar drips from 

 the nectary and collects in a protected extension of the 

 corolla called the spur. (See Figure iji.) Insects also 

 visit flowers for shelter, and, in some cases, to lay their 

 eggs. 



