ODORS 189 



Wind-pollinated flowers secure pollination by growing in 

 dense masses like wheat, rye and the grasses, by producing 

 enormous amounts of buoyant pollen, by forming more 

 staminate than pistillate flowers and by producing branching 

 stigmas. In entomophilous flowers cross-pollination is accom- 

 plished by birds and insects. 



How Plants Attract Birds and Inisects. Plants use various 

 means in order to bring about cross-pollination. Some 

 attract by odor, by nectar, by color, by form, by the time 

 of flowering, by modification of the floral structure. 



Odors. Odor is an aid to pollination since it serves to 

 bring insects to the flower. The odor of a flower is not always 

 pleasant; it is frequently quite disagreeable, as in wake- 

 robin and skunk cabbage. In contrast to these disagreeable 

 odors there is the wonderful fragrance of roses, violets, 

 carnations and hundreds of sweet-scented wild flowers. 

 Many of the odorous flowers are very inconspicuous. 



The petals of most flowers secrete a nectar or sweet sub- 

 stance which provides a pleasing food for the visiting insects. 

 The color of the flower serves to attract insects and there 

 are innumerable shades and tints of practically every color. 

 There is also considerable variation in the color of a flower 

 at different stages of its growth between the bud, mature 

 and after-fertilization stage of the flower. Color is unques- 

 tionably a very great aid to the plant in attracting insects. 

 Experiments have proved that certain insects w T ill visit only 

 flowers of certain colors, and in most instances the insects 

 visiting such flowers are perfectly adapted to bring about 

 cross-pollination. In thorn apple the flowers are white or 

 bluish-white and are conspicuous at night, a fact of great 

 service to the plant because the insect that brings about its 

 cross-pollination flies only at night. In certain flowers the 

 form will attract certain insects only and those attracted are 

 the only ones that can bring about cross-pollination. 



