DISTRIBUTION OF SPORES AND SEEDS. 359 



cross-pollination must be considered the far more desirable 

 process. Flowers are adapted to insect visitors in the follow- 

 ing ways : 



485. (</) Food. — They provide for their visitors edible 

 substances, such as nectar and pollen,* material for nest 

 building, shelters, or breeding places. 



486. (b) Advertisements. — They advertise the presence 

 of such attractions in two ways, which are sometimes com- 

 bined, and insects accustomed to visit flowers quickly learn 

 to know what the advertisements mean. (i) By color. 

 Flowers are so colored as to attract notice ; and this is 

 further secured by the large size of individual flowers or by 

 massing many small flowers into close clusters. (ii) By odor. 

 Odors are due to volatile oils, usually in the epidermis of the 

 petals or sepals, often curiously localized. Dusk- and night- 

 blooming plants often have heavy odors. 



487. (c) Form and position of parts. — Many plants by 

 the form of their flower leaves provide landing places for 

 welcome visitors. Guides to the location of the nectar, in 

 the form of grooves, folds, hairs, lines of color, etc., are 

 often present. The form and position of the stamens and 

 pistils is often such as to insure the desired transfer of pollen. 

 These positions may be permanent or they may be secured 

 by movements at opportune times. Among the movements 

 are those due to turgor and those due to the presence of 

 motor organs. In a very large number of cases, by the form 

 of the flower-leaves and the essential organs the plant is 

 adapted to visitation by particular insects, and if these are 

 not present, or it their access is denied, constant failure to set 

 seeds is the result. Thus one may distinguish plants adapted 

 to bees, moths, butterflies, flies, birds, or even snails. 



488. (d) Exclusion of unwelcome visitors. — In addition to 



* The microspores are often produced in great excess of the plant's own 



needs. 



