MORPHOLOGY OF HIGHER PLANTS. 397 



Pollination and Fertilization. — Fertilization represents the 

 final stage in the work of the flower as a whole, and has already 

 been defined as the union of the egg-cell and a male nucleus. 

 Pollination may be considered to include the transferral of the 

 pollen grains from the anther to stigma and their subsequent 

 germination thereon, this latter process resulting in the produc- 

 tion of the male nuclei. Pollination thus represents but one series 

 of changes or processes which precede fertilization, for, while the 

 pollen grain is going through the various stages in development 

 which lead to the formation of the male nuclei, a series of com- 

 plex changes are going on in the embryo-sac leading to the develop- 

 ment of the egg-cell. 



Our special interest in pollination arises from the fact that the 

 pollen grains are not retained in the pollen sacs and are dependent 

 upon various agencies for transferral to the stigma. This is a 

 matter of great biological significance, for it is claimed that many 

 of the special characters of flowers have a direct relation to 

 pollination. 



The various ways in which the anthers open for the discharge 

 of the pollen when it is ripe have already been considered (Fig. 

 221), but it may be added that the manner in which this is done 

 usually appears to have a relation to the manner in which the 

 pollen is to be carried to the stigma. In order that pollination 

 may be effected, the stigma must be ripe or mature, when it is 

 said to be receptive. It then usually secretes a sticky, sugary 

 liquid which causes the pollen grains to adhere to the stigmatic 

 surface (Fig. 83), and which at the same time serves as a nutrient 

 to them. Usually the pollen grains begin to germinate in a short 

 time after reaching the stigma, which is made evident by the pro- 

 trusion of the pollen tubes. The stigma seems also to have the 

 power of selection, for in many cases the pollen does not germi- 

 nate as readily on the stigma of the same flower as on that of 

 another flower, provided it be of the same or a nearly related 

 species. 



When a flower possesses both stamens and pistils, — that is, is 

 bisexual or hermaphrodite,— and its pollen germinates upon its 

 own stigma, the process is known as close or self-pollination, 

 and if fertilization follows, this is known as self-fertilization. 



