2/6 OUTLINES OF PLANT LIFE. 



392. (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 fur- 

 ther 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 petals or sepals, 

 often curiously localized. Dusk- and night-blooming plants 

 often have heavy odors. 



393. (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 are 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 if 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. 



394. {d) Exclusion of unwelcome visitors. — In addition 

 to provision for welcome guests must be enumerated the 

 methods of excluding unwelcome guests, which on account of 

 their size and habits are unable to bring about the desired 

 transfer of the pollen, while at the same time they rob the 

 plant of nectar or pollen provided for more acceptable visitors, 

 (i) Various obstructions within the flower may render access to 

 the nectar impossible to the smaller and weaker insects, while 

 allowing others to reach it. Such obstructions are formed 



