200 APPLIED BIOLOGY 



self-pollination (pollen on stigma of its own flower). Also 

 in some cases flowers are not fertile with their own pollen, 

 or at least not so fertile as when cross-pollinated. Why 

 these things are so is unexplained by biology; but it is a 

 well-established fact that in all groups of animals and plants 

 there are special arrangements for producing new individuals 

 from egg-cells fertilized by unrelated sperm-cells or pollen- 

 grains. The remarkable contrivances, described in the 

 following pages, some to aid cross-pollination and some to 

 prevent self-pollination, can only be interpreted as meaning 

 that for some unknown reason it is important that plant 

 ovules should be fertilized by pollen-grains from other flowers 

 and very commonly from flowers on other plants. 



However, it must not be understood that self-pollination is 

 always guarded against, for there are some flowers which 

 do not open and which must be self-pollinated, and yet they 

 produce good seeds. There are some cases of flowers which 

 do not open, but which may possibly be pollinated by insects 

 which bite holes in searching for food. 



193. Adaptations against Self-pollination. Most famil- 

 iar of these is the fact that in many species certain flowers 

 have only stamens and others have only pistils. In the case 

 of cultivated strawberry plants, some varieties produce 

 only pistils in their flowers and are called pistillate flowers. 

 Other varieties have both stamens and pistils in each flower 

 and are said to be perfect or bi-sexual flowers. There are no 

 varieties of strawberries which have only stamens (staminate 

 flowers) ; but some other kinds of plants have such flowers. 

 Strawberry plants which are known to produce only pistillate 

 flowers must be planted near plants which have perfect 

 flowers. In buying plants from dealers one must learn the 

 name of the variety and then find out from catalogues or 

 garden books whether the variety is perfect or pistillate. 

 A very instructive experiment consists in planting some 

 strawberry plants of a pistillate variety in several pots or 



