194 BOTANY: PRINCIPLES AND PROBLEMS 



transferred thereto directly, and this may sometimes happen 

 even before the flower opens. In the great majority of eases, 

 however, this transfer is brought about by some external agency, 

 and pollen from the flowers of one plant is thus frequently carried 

 to the flowers of another. 



The two most important agencies in effecting pollination are 

 the wind and insects. Wind-pollinated or anemophilous flowers 

 (Figs. 107 and 237) are prominently exposed on the plant but are 

 generally small, inconspicuous and unisexual, possessing a poorly- 

 developed perianth, abundant dry and light pollen, and feathery 

 stigmas. Insect-pollinated or entomo'philous flowers (Figs. 104, 

 109, 240, etc.), on the other hand, are conspicuous or possess 

 marked odor, and are characterized by a well-developed corolla, 

 pollen grains which tend to adhere in masses, stigmas which are 

 sticky, and in many cases by the presence of nectaries secreting a 

 sugary liquid. The insect is guided to the flower by its color or 

 odor and visits it either to secure nectar, the source of honey, or 

 pollen, the source of "bee bread". Pollen readily adheres to 

 the hairy bodies of these insects, and as it is thus carried about 

 from flower to flower it often comes in contact with a stigma, to 

 the sticky surface of which it is transferred. Insects belonging 

 to the order Hymenoptera (the bees and their allies) are more 

 important than any others in effecting pollination. 



In many cases we have evidence that offspring which arise 

 from a cross, or union of sexual cells contributed by two different 

 parents, are superior in vigor to those in which both gametes 

 came from the same plant. Perhaps in response to this fact, 

 there are an enormous number of devices among flowering plants 

 which tend to insure cross-pollination, or the transfer of pollen from 

 one flower or plant to another, and to prevent self -pollination, or 

 the transfer of pollen from anther to stigma of the same flower. 

 Anthers and stigmas, for example, may ripen at different times, 

 with the result that the anthers liberate their pollen either before 

 the stigma of the same flower is ripe for pollination or after it 

 has become no longer receptive. In many cases, also, pollen 

 from another plant is better able to effect fertilization than the 

 plant's own pollen; and in extreme instances the plant may be 

 actually self-sterile, its own pollen having no effect whatever 

 upon its stigma. More striking than these methods, however, 

 are the multitude of structural devices in entomophilous flowers 

 whereby self-pollination through insect agency is rendered diflS- 



