78 CROSS-POLLINATION 



Consequently, flowers which possess stamens, but have no 

 carpels, do not bear seeds. Whereas flowers devoid of stamens, 

 but endowed with carpels, may produce seed. Experiment has 

 shown that the ovules do not change into seeds unless pollen 

 grains have previously been conveyed on to the stigma (or, in 

 Gymnosperms, into the micropyle of the ovule). The pollen 

 in some way exerts a fertilising influence on the ovule. 

 The transference of pollen from an anther to the receptive 

 part of the carpel of a flower is referred to under the term 

 pollination. 



Cross - Pollination and Self- Pollination. — The simplest 

 method of pollination is the transference of the pollen from 

 the anther to the stigma of the same flower ; this is described 

 as self-pollination, and the flower is said to be self-pollinated. 

 When, on the other hand, the stigma of a flower receives pollen 

 from the flower of another individual-plant of the same kmd, it 

 is, said to be cross-pollinated. Lastly, pollen may be transferred 

 from one flower on to the stigma of another flower of the same 

 individual-plant; this is obviously a stage between cross-pollina- 

 tion and self-pollination, but there is no simple word or term 

 in use by which to distinguish it. 



CROSS-POLLINATION. 



It has been proved that in many species of plants cross- 

 pollination leads either to larger crops of seeds, or to the pro- 

 duction of seeds which are better in quality than is the case 

 when the same plants are self-pollinated. Accordingly, many 

 arrangements exist in flowers which are calculated to hinder 

 self-pollination and facilitate the more invigorating cross- 

 pollination. 



Arrangements for hindering Self-pollination: — 



(i.) Sometimes the stamens and carpels do not occur in the 

 same flowers, consequently the pollen must be conveyed from 

 one flower to another. The stamen-bearing (staminate) and 

 carpel-bearing (carpellary) flowers may occur on the same 

 individual-plant, as in the Hazel, Oak, and Pine. Or the two 

 kinds of flowers may be on different individual-plants, as in the 

 Willows, in which case the stamens are borne on one tree and 

 the carpels on another tree. 



(ii.) Sometimes the stamens and carpels in one flower 

 ripen at different times; the consequence is that, though the 



