FIELDBOOK OF ILLINOIS WILD FLOWERS 



Flowers pollinated by insects contrast sharply with those 

 that are pollinated by wind. A majority of them are perfect 

 flowers, and many have developed adaptations of form that 

 practically insure cross-fertilization. 



DIMORPHIC AND TRIMORPHIC FLOWERS 



Flowers of the same species which have in one form short 

 stamens and long pistils, and in another form long stamens 

 and short pistils, are said to be dimorphic^ meaning of two forms. 

 Introduction of an intermediate stage between these two forms, 

 making another combination of stamens and pistils possible, 

 produces trimorphic flowers, meaning of three forms. The 

 intermediate stage is the result of adding a second set of stamens 

 that alternate with and are never equal in length to the first 

 set. Thus in trimorphic flowers we have: form i — a short 

 pistil or pistils, one set of stamens of medium length and the 

 second set \ong\ form 2 — the pistil of medium length, one set of 

 stamens shorter and the other longer than the pistil; /or?7z j — 

 the pistil long and the stamens in two sets below it, one short 

 and the other medium. It is a general principle among flowers 

 that the lower the position of the anthers the smaller the pollen 

 grains; and, in trimorphic forms especially, the pollen will not 

 germinate well unless it is transferred to a stigma of the same 

 relative position as the anther from which the pollen came. 



There are other notable characteristics of insect-pollinated 

 flowers. Usually they are showy due to color, size, shape or 

 arrangement, and frequently they have an odor. They com- 

 monly produce nectar^ a sweetish secretion of glands at the base 

 of the ovary or corolla. The pollen grains, instead of being dry 

 and smooth, are often rough and sticky. Flowers that are wide 

 open and can be pollinated by nearly any kind of visiting insect 

 often have numerous stamens and abundant pollen, but those 

 which partly or wholly conceal the pollen in tubular or closed 

 corollas usually have few stamens and little pollen. 



INSECTS WHICH POLLINATE FLOWERS 



Honeybees and bumblebees are the most efficient among 

 pollinating insects. First, they have a twofold interest in the 

 flower, gathering pollen as well as nectar for food. Then they 

 are hardy and live longer than other insect visitors; also they 

 are stout fliers and visit a greater number of flowers over a 

 wider area than do, let us say, butterflies and carrion flies. 



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