38 
which can be seen as a thick structure below the petals, 
and others producing only pollen. In such cases, self- 
pollination is impossible and cross pollination must be 
effected either by insects or by the gardener. It is curious 
that in spite of the many adaptations to cross fertilisation 
there are many plants in which self-fertdzsation 1s 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 in flowers, apparently specially adapted to the visits 
of insects such as the Sweet Pea, which possesses both 
scent and colour, the immature pod is already 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. While 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-fertile 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\in 
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 in 
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 more favourable will 
enable it to reproduce its kind. : 
The flowers of many varieties 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 
