4 BRITISH FLOWERING PLANTS CHAr. 



sometimes tubular, sometimes composed of separate 

 leaves called petals; thirdly, of one or more stamens, 

 consisting of a stalk or filament, and a head or anther, 

 in which the pollen is produced ; and fourthly, a pistil, 

 which is situated in the centre of the flower, and consists 

 generally of three principal parts — the ovary comprising 

 one or more compartments at the base, each containing 

 one or more ovules ; the style ; and the stigma, which in 

 many familiar instances forms a small head at the top of 

 the style or of the ovary, and to which the pollen must 

 find its way in order to fertilise the ovule. But though 

 the pistil is thus surrounded by one or more rows of 

 stamens, and though most flowers are capable of fertilisa- 

 tion by their own pollen, still it is a great advantage to 

 a species that the flower should be fertihsed by pollen 

 from a different stock. How, then, is self-fertilisation 

 prevented ? There are three principal modes. Firstly, 

 in many species the stamens and pistil are in separate 

 flowers, sometimes borne on difl'erent plants. Secondly, 

 even when the stamens and pistil are in the same 

 flower, they are in many species not mature at the same 

 time. This was first observed by Sprengel in Eptlohium 

 angustifolium as long ago as 1790. In some cases the 

 stigma has matured before the anthers are ripe, while 

 in other and more numerous cases the anthers have 

 ripened and shed all their pollen before the stigma has 

 come to maturity. Thirdly, there are many species in 

 which, though the anthers and stigma are contained in 

 the same flower and are mature at the same time, they 

 are so situated that the pollen can hardly reach the 

 stigma of the same flower. 



Another circumstance which makes for cross-fertilisa- 

 tion is the prepotence of pollen from another plant of 

 the same species. If plants of several varieties are 

 grown together, the seeds cannot be reckoned on to 

 come true. Even after twenty -four hours Darwin 

 found that pollen from another plant exercised a pre- 

 dominant influence. He placed on several stigmas of a 

 long-styled Cowslip [Primula veris) plenty of pollen 



