38 



which can be seen as a thick structure below the petals, 

 and others producnig- only pollen. In such cases, self- 

 pollination IS hnpossible and cross pollination must be 

 effected either by insects or by the gardener. It is curious 

 that m spite of the many adaptations to cross fertilisation 

 there are many plants m which self -fertilisation is the 

 normal process of seed production. This is the case with 

 most of our cultivated grasses. In the wheat and other 

 cereals, though the flowers open and the light powdery 

 pollen IS carried from plant to plant, the stigma of the 

 seed vessel is usually pollinated before the flowers open, 

 and as there is only one seed in each seed vessel it is 

 usually self-fertilised before the foreign pollen arrives. 

 Even m fiowers, apparently specially adapted to the visits 

 of insects such as the Sweet Pea, which possesses both 

 scent and colour, the immature pod is alreadv pollinated 

 in the bud stage of the flower, and if we wish to effect 

 any crossings between different varieties of this plant, we 

 must cut away the stamens before the flowers open and 

 introduce pollen from another plant. Wliile such normal 

 self-pollination has for horticulturists the advantage that 

 it preserves the purity of the varieties we cultivate, it is 

 probable that it may gradually cause a weakening of the 

 race, and in nature even occasional crossing probably re- 

 invigorates the strain. 



Quite a number of members of the Pea Family are 

 self -fertile, including the commonly cultivated forms of 

 peas and beans, while many other leguminous plants, such 

 as the clovers are self-jertile and cannot be fertilised with 

 their own pollen even when it is placed on the stigma of 

 the flower. An interesting analysis has been made of the 

 behaviour in this respect of the various members of this 

 Family, and it has been found that all the annuals m 

 this group of plants are self-fertile, while the perennial 

 forms are self-sterile. It would seem as if it were of more 

 importance to ensure regular and uninterrupted produc- 

 tion of seed in the former, while in perennials, even if m 

 one season owing to the absence of insect visitors, seeds 

 are not produced, the persistence of the plant to the next 

 season when conditions may be m.ore favourable will 

 enable it to reproduce its kind. 



The flowers of many ^'arieties of Apple and Pear are 

 self-sterile, and disappointment has often been caused by 

 the failure of a fruit tree to bear when grown singly with 



