2. Ipomoea purpurea.
Thirty-one intercrossed plants raised from a cross between flowers on
the same plants were grown in ten pots in competition with the same
number of self-fertilised plants, and the former were to the latter in
height as 100 to 105. So that the self-fertilised plants were a little
taller than the intercrossed; and in eight out of the ten pots a
self-fertilised plant flowered before any one of the crossed plants in
the same pots. The plants which were not greatly crowded in nine of the
pots (and these offer the fairest standard of comparison) were cut down
and weighed; and the weight of the twenty-seven intercrossed plants was
to that of the twenty-seven self-fertilised as 100 to 124; so that by
this test the superiority of the self-fertilised was strongly marked. To
this subject of the superiority of the self-fertilised plants in certain
cases, I shall have to recur in a future chapter. If we now turn to the
offspring from a cross between distinct plants when put into competition
with self-fertilised plants, we find that the mean height of
seventy-three such crossed plants, in the course of ten generations, was
to that of the same number of self-fertilised plants as 100 to 77; and
in the case of the plants of the tenth generation in weight as 100 to
44. Thus the contrast between the effects of crossing flowers on the
same plant, and of crossing flowers on distinct plants, is wonderfully
great.
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