I40 BOTANY AND PHARMACOGNOSY. 



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 i-elation to 

 pollination. 



The various ways in which the anthers open for the dis- 

 charge of the pollen when it is ripe have already been considered 

 (Fig. 8i), 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. 85), 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 germin- 

 ate 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. 

 While most hermaphrodite flowers are self-pollinated there are 

 some that are not, and this is brought about in several ways : 

 ( I ) As already pointed out the pollen may germinate better on 

 the stigma of another flower than on the stigma of the same 

 flower; (2) the anthers and pistils of the same flower may 

 mature at different times, and this is one of the commonest ways 

 of preventing self-pollination. Usually in such cases the stamens 

 mature first. The common plantain (Plantago) furnishes an 

 example of the maturing of the stigma before the anther. The 

 flowers of this plant are arranged in spikes (Fig. 87, illus. 3 

 and 4) which belong to the indefinite class, and hence the lower 

 flowers on the spike expand first. As stated, the pistil of each 



