92 



POLLINATION AND FERTILIZATION 



secretions and the exercise of these influences at the most opportune 

 times. 



Form. — The form of the flower is efficient when it resembles a form 

 attractive to an insect the visit of which is desirable, or when it is 

 one well calculated to display effectively the coloration; and it is not 

 impossible that certain forms, like certain colors, are attractive jmv se. 

 The forms of nectar-bearing flowers are, moreover, in most cases, such 

 as to facilitate the collection of the food by the visiting insect, or, 

 when otherwise, to eft'ect special objects to be considered farther on. 

 For example, it is usually a peripheral or central position of the nectaries 



which respectively determine the ex- 

 trorse or introrse dehiscence of the 

 anthers. 



Color. — Coloration also may be 

 attractive, through its simulation of 

 an insect or merely by its serving 

 to make known to the insect the 

 presence or position of the flower 

 concerned — as a white, light-colored, 

 or lustrous flower, in attracting in- 

 sects which fly only when there is 

 little light. 



Function of Neutral Floivers.— 

 Flowers are frequently modified in 

 size so as to effect these results, and 

 this modification is often secured at the expense of their own sexual 

 functions. Fig. 268 illustrates a cluster of Viburmnn flowers, the 

 marginal being large and light-colored and admirably adapted to 

 attract insects, but destitute of perfect reproductive parts. This 

 tendency to produce upon the same plant flowers of two kinds, the 

 one for display, the other for rei)roduction, is widely manifested. In 

 the Einphegus, the flowers produced respectively upon the lower and 

 upper portions of the stem exhibit this difference. In such heads of 

 flowers as the Daisy, the showy marginal flowers are very frequently 

 sterile, even though pistillate, and attract insects which then poIHnate 

 the inconspicuous central flowers. 



Odor. — The odors of flowers, while frequently offensive to the human 

 sense, are supposed to be attractive in most cases to the insects whose 

 visits favor their pollination. They result from the evaporation of 

 volatile oils. The glands by which these oils are excreted and in which 



Fig. 208. Inflorescence of Viburnum with 

 neutral marginal flowers. 



