The benefits which so generally follow from a cross between two plants
apparently depend on the two differing somewhat in constitution or
character. This is shown by the seedlings from the intercrossed plants
of the ninth generation, when crossed with pollen from a fresh stock,
being as superior in height and almost as superior in fertility to the
again intercrossed plants, as these latter were to seedlings from
self-fertilised plants of the corresponding generation. We thus learn
the important fact that the mere act of crossing two distinct plants,
which are in some degree inter-related and which have been long
subjected to nearly the same conditions, does little good as compared
with that from a cross between plants belonging to different stocks or
families, and which have been subjected to somewhat different
conditions. We may attribute the good derived from the crossing of the
intercrossed plants during the ten successive generations to their still
differing somewhat in constitution or character, as was indeed proved by
their flowers still differing somewhat in colour. But the several
conclusions which may be deduced from the experiments on Ipomoea will be
more fully considered in the final chapters, after all my other
observations have been given.
CHAPTER III.
SCROPHULARIACEAE, GESNERIACEAE, LABIATAE, ETC.
Mimulus luteus; height, vigour, and fertility of the crossed and
self-fertilised plants of the first four generations.
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