CHAPTER XII. POLLINATION AND FERTILIZATION. 89 



is the general rule. Most flowers are so constructed that exter- 

 nal agencies of various kinds are utilized for this purpose, and 

 the appliances by means of which the result is secured and close- 

 fertilization prevented, are sometimes very elaborate and won- 

 derful. Even where the anther and the stigma are in the closest 

 juxtaposition in the same flower, the pollen, in many cases, is 

 effectually prevented from reaching the stigma, while the arrange- 

 ments at the same time are such as to insure its being brought 

 to it from another flower. It is evident from the pains Nature 

 has taken to secure the result, that some great advantage must 

 accrue to the plant or its offspring from allogamy, and this has 

 also been proved to be the fact by careful and extended experi- 

 ments. It is proved that there are some plants that utterly refuse 

 to set seed when supplied with only their own pollen, that there 

 are others which greatly prefer pollen from another plant, and 

 refuse to utilize their own when that from another of the same 

 species is placed upon the stigma, and it is proved that in the 

 great majority even of those plants which are capable of close- 

 fertilization, stronger, hardier and more numerous offspring result 

 from cross-fertilization. 



The external agencies utilized by the plant to bring about 

 cross-fertilization are chiefly the wind and insects. Humming- 

 birds, and some other species of birds that habitually visit 

 flowers, are occasionally of service, and in the case of some 

 aquatic plants currents are made use of; but these agencies are 

 comparatively unimportant. 



Flowers whose pollination is effected by means of the wind 

 are called anemophilous. Such flowers differ markedly in appear- 

 ance and structure from those in which insects are the agents. 

 They are usually provided with stigmas that expose a good deal 

 of surface to the wind ; they produce great abundance of dry, 

 powdery pollen; they are without showy floral envelopes; they are 

 without nectar, and they are destitute of perfume. Frequently, 

 also, but not always, the pistils and stamens are in separate 

 flowers, thus making self-fertilization impossible. 



The difference between the extent of surface exposed by the 

 stigmas of anemophilous flowers and those pollinated by insect 

 agency will be seen by reference to Figs. 274 to 279, inclusive. 

 The three figures at the left represent, respectively, the pistils of 



