ANGIOSPERM^E. 421 



531. Pollination. As the pollen-grains are entirely want- 

 ing in means of locomotion, they are dependent for trans- 

 portation to the stigma, upon (1) the wind (anemophilous 

 flowers) ; (2) certain contrivances, by means of which insects 

 (or rarely birds) are made to carry the pollen from anther to 

 stigma (entomophilous flowers) ; (3) the favorable position of 

 the anthers and stigmas, bringing the pollen in the open- 

 ing anther into contact with the stigmatic surface (auto- 

 gamous flowers). The grasses and sedges, and the oaks, 

 beeches, chestnuts, walnuts, birches, and their allies, and a 

 few others, have anemophilous flowers. In these the pollen 

 is produced in great abundance, and the flowers are small, 

 uncolored, and destitute of nectar (honey). An immense 

 number of plants have entomophilous flowers ; these are, as 

 a rule, large, colored, and provided with nectar-secreting 

 glands; the nectar acts as a bait, and the showiness as a 

 guide to honey-loving insects, which, by various structural 

 contrivances in the flowers, are made to come successively 

 in contact with the anthers of one flower and the stigmas of 

 the next, in the first dusting their bodies with pollen, which 

 in the second adheres to the stigmas. Autogamous flowers 

 are much less numerous than either of the foregoing, and it is 

 doubtful Avhether there are any species of plants all of whose 

 flowers exhibit constant autogamy. There are a good many 

 plants, however, which have two forms of flowers, viz., large, 

 showy, nectar-bearing, entomophilous ones, and small, in- 

 conspicuous autogamous ones, generally with a rudimentary 

 perianth. Flowers exhibiting this form of autogamy are 

 said to be cleistogamous. Examples are to be met with in 

 Viola, Lithospermum, Impatiens, etc. ; early in the season 

 these have large flowers, which are entomophilous, but later 

 only small cleistogamous ones appear, and in some species of 

 Viola these are subterranean. Without doubt it frequently 

 happens that the pollen of anemophilous and entomophilous 

 flowers- falls upon their stigmas, resulting in accidental auto- 

 gamy, but too frequent a recurrence of this is guarded against 

 by various structural devices.* 



* Upon tliis interesting subject the student is referred to Mr. Dar- 

 win's works, " The Various Contrivances by which Orchids are Fertil- 



