Sugar-peas and sugar-corn
afford further instances. Strawberries without runners have come true
from seed ever since their first appearance, over a hundred years ago.
Many garden-varieties, the stability of which under ordinary
circumstances is doubtful, because of their being sown too close to
other varieties of the same species, have been tested in [159] respect
to their stability by different writers and at different times. In doing
this it is plain that it is very essential to be sure of the purity of
the seed. Specimens must be grown in positions isolated from their
allies, and if possible be pollinated artificially with the exclusion of
the visits of insects. This may be done in different ways. If it is a
rare species, not cultivated in the neighborhood, it is often sufficient
to make sure of this fact. Pollen may be conveyed by bees from distances
of some ten or twenty meters, or in rare cases from some hundred meters
and more, but a greater distance is ordinarily sufficient for isolation.
If the flowers fertilize themselves, as is more often the case than is
generally supposed, or if it is easy to pollinate them artificially,
with their own pollen or in small groups of similar individuals, the
best way is to isolate them by means of close coverings. When flowering,
the plants are as a rule too large to be put under bell-glasses, and
moreover such coverings would keep the air moist, and cause the
flower-buds to be thrown off.
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