VEGETATIVE AND SEXUAL REPRODUCTION 167 



Kinds of pollination. The term self-pollination will be used 

 to indicate the transfer of pollen from the anthers of a given 

 flower to the stigma of the same flower. Close-pollination will 

 be interpreted as the transfer of pollen from the anthers of 

 one flower to the stigma of another flower or flowers on the 

 same plant. Close-pollination thus defined is often designated 

 as cross-pollination ; but since the practical effects of close- 

 pollination in plant breeding are usually different from those of 

 cross-pollination as defined below, it is thought best to retain 

 the above definition of close-pollination. Cross-pollination will 

 be used to designate all cases in which the pollen from flowers 

 on one plant is transformed to the stigma or stigmas of flowers 

 on another plant. 



Since pollination is essential in the higher plants before fer- 

 tilization can take place, it is necessary for the perpetuation 

 of any given race or species of plants which is not adapted to 

 maintaining itself by vegetative reproduction. The researches 

 of Darwin also established the fact that cross-pollination is of 

 distinct advantage to many species in producing stronger and 

 better offspring. It is not surprising, therefore, that nature has 

 evolved a great variety of novel and interesting devices for insur- 

 ing both self-pollination and cross-pollination in flowers. In the 

 following section the papilionaceous flowers of the pea family 

 have been selected to illustrate some devices for insuring self- 

 pollination and cross-pollination. 



Inflorescence and pollination. In some species the flowers are 

 borne singly from the axils of ordinary leaves, but in a large 

 number of plant species the flowers are clustered, and such flower 

 clusters are termed inflorescences (Fig. 90, c?). This flower clus- 

 ter is evidently a modified branch system, in which the central 

 axis, termed the axis of inflorescence, corresponds to the central 

 stem of a shoot. The leaves have been reduced to small bracts, 

 and the flowers replace branches which ordinarily spring from 

 the axils of the leaves. This agrees with the statement made 

 above that flowers are really modified branch buds. The dis- 

 tinct advantage of such an inflorescence as that of the locust 

 (Fig. 90, d) in securing pollination is easily understood if one 



