ADJUSTMENT TO TEMPERATURE 127 



important, and are very many times more numerous than all 

 other zoophilous species. Indeed, more species are pollinated by 

 insects than by all other agents combined. It is highly probable 

 that all flowers with corollas owe the development of the latter 

 to insect-pollination, and this is true also of many species which 

 possess sepals alone. The form, color, and fra2;rance of petaloid 

 flowers in particular seem to be due almost wholly to insects. 

 They have modified their form to afford landing-places for welcome 

 visitors, to enable the latter to find their way quickly to the nectar 

 and pollen, and to confuse or repel unwelcome visitors. As would 

 be expected, those flowers which have been most strikingly modified, 

 such as the mints, figworts, orchids, and many composites, are 

 the ones which are dependent for pollination upon insects of a 

 certain type. 



Insects are attracted by color or fragrance, and in many flowers 

 both methods of attraction occur. Inconspicuous flowers which 

 are scentless are nevertheless often visited by insects for the sake 

 of the pollen they afford, and the pollen is the real attraction in 

 brilliant scentless flowers. These are termed pollen flowers; in 

 them a part of the pollen is sacrificed to insure the transfer of the 

 remainder. In nectar flowers, nectar constitutes the attraction, 

 and the removal of the pollen is incidental. Flowers have pro- 

 duced a great many devices to effect the sprinkling or loading of 

 insects with pollen, and to insure the deposition of the pollen in 

 the proper manner. Furthermore, the opening of the flower at a 

 certain time of the day or season is an adaptation to the habits 

 of the insects upon which it depends for pollination. 



149. Self-pollination. Autogamy exists as the alternative 

 method of pollination in the majority of plants that are regularly 

 cross-pollinated. It is the sole method in cleistogamous flowers 

 and in those whose size, structure, or position makes them little 

 adapted to cros.s-pollination, or whose habitats present conditions 

 unfavorable to the latter, as in the case of many arctic and alpine 

 plants. Autogamy is direct in most cleistogamous flowers, and 

 in those where contiguity of stamens and pistils, or the position 

 of the stamen above the pistil, permits the pollen to fall directly 

 upon the stigma. It is indirect when the transfer of pollen is the 

 result of movement or growth, as in the majority of autogamous 

 plants. Indirect autogamy is brought about by various methods, 

 of which the movements of stamens or style, their elongation or 



