But these were only
occasional admixtures, since after artificial fertilization of the
typical specimens the strain at once became absolutely pure, and
remained so for a series of generations, as long as the experiment was
continued. Seeds of trees often contain large quantities of impurities,
and the laciniated varieties of birch, elder and walnut have often been
observed to come true only in a small number of seedlings.
In the case of new or young varieties, seed merchants often warn their
customers as to the probable degree of purity of the seeds offered, in
order to avoid complaints. For example the snow-white variety of the
double daisy, _Bellis perennis plena_, was offered at the start as
containing [196] as much as 20% of red-flowered specimens.
Many fine varieties are recorded to come true from seed, as in the case
of the holly with yellow fruits, tested by Darwin. Others have been
found untrue to a relatively high degree, as is notorious in the case of
the purple beech. Seeds of the laciniated beech gave only 10% of
laciniated plants in experiments made by Strasburger; seeds of the
monophyllous acacia, _Robinia Pseud-Acacia monophylla_, were found to be
true in only 30% of the seedlings. Weeping ashes often revert to the
upright type, red May-thorns (_Crataegus_) sometimes revert nearly
entirely to the white species and the yellow cornel berry is recorded to
have reverted in the same way to the red berries of the _Cornus Mas_.
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