Gradually the forms of both flowers and insects were modified. 

 The need of protecting the nectar from rain led to the development 

 of tubular nectaries, and this in turn to the lengthening of the 

 tongues of many insects. The bees, of which there are some 4,000 

 species in North America, are dependent on pollen for brood-rearing 

 and are thus compelled to constantly visit a great number of flowers. 

 It was a momentous epoch in the history of the plant world when 

 the ancestors of the bees became flower-visitors. More than any 

 other group of insects they have played an important part in the 

 evolution of conspicuous flowers. As a result of their numbers, fre- 

 quent visits and mental acuteness, many flowers are dependent on 

 them for pollination, and are known as bee-flowers. 



Bee-flowers occur in a great number of plant families. The pulse 

 family, for instance, to which belong the clovers, alfalfas, peas, 

 beans, vetches, locusts and lupines, contain several thousand. They 

 are also common in the mint and figwort families. Among bees 

 the bumblebees have the longest tongues and many flowers are 

 adapted to them alone, as the larkspurs, columbines, monkshoods, 

 snapdragons, red clover and turtlehead. Bee-flowers are usually 

 red or blue. In the German and Swiss flora there are 482 bee- 

 flowers, of which 330 are red and blue and 152 white and yellow. 



A small number of flowers are pollinated by butterflies and are 

 called butterfly-flowers, as the pinks, some primroses, lilies, heaths, 

 orchids and species of phlox. The floral tube of butterfly-flowers 

 is usually so long that the nectar cannot be reached by bees, and the 

 corolla is commonly red-colored. Blue butterflies, however, seem 

 to favor the blue flowers of Phyteuna. 



Another group of flowers open only in the evening and are pol- 

 linated by hawk-moths, as the thorn apples, various cacti, the evening 

 primrose, the night-flowering catchfly, bouncing bet, the evening 

 lychnis, and various species of tobacco, gentians and lilies. Hawk- 

 moth-flowers are almost invariably white or yellow, and have very 

 long nectaries or floral tubes. 



Fly-flowers are widely different from any of the groups described. 

 They comprise pit-fall flowers, prison flowers, pinch-trap flowers 

 and flowers with deceptive colors, odors and nectaries. Familiar 

 fly-flowers are the carrion flower, the arums, Jack-in-the-pulpit, 

 Dutchman's Pipe and the milkweeds. The insects are held pris- 

 oners for a time, and then are permitted to fly away, carrying more 

 or less pollen on their bodies, to another flower. 



Only a few bird-flowers are found in North America, but they 

 are very common in tropical South America. In the two' Americas 

 they are pollinated by humming-birds. In eastern North America 

 only one species occurs, the ruby-throated humming-bird, but in 

 South America more than 400 kinds have been described, and they 



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