my).
(Similarly crossing in the case of flowers of Pelargonium zonale, which
belong to plants raised from cuttings from the same parent, shows no
superiority over self-fertilisation.) Darwin, however, considered it
possible that this may sometimes be the case. "The sexual elements
in the flowers on the same plant can rarely have been differentiated,
though this is possible, as flower-buds are in one sense distinct
individuals, sometimes varying and differing from one another in
structure or constitution." ("Cross and Self fertilisation" (1st
edition), page 444.)

As regards the importance of this question from the point of view of
the significance of cross-fertilisation in general, it may be noted
that later observers have definitely discovered a difference between the
results of autogamy and geitonogamy. Gilley and Fruwirth found that in
Brassica Napus, the length and weight of the fruits as also the total
weight of the seeds in a single fruit were less in the case of autogamy
than in geitonogamy. With Sinapis alba a better crop of seeds was
obtained after geitonogamy, and in the Sugar Beet the average weight
of a fruit in the case of a self-fertilised plant was 0.009 gr., from
geitonogamy 0.012 gr., and on cross-fertilisation 0.013 gr.

On the whole, however, the results of geitonogamy show that the
favourable effects of cross-fertilisation do not depend simply on the
fact that the pollen of one flower is conveyed to the stigma of another.
But the plants which are crossed must in some way be different.
If plants of Ipomoea purpurea (and Mimulus luteus) which have been
self-fertilised for seven generations and grown under the same
conditions of cultivation are crossed together, the plants so crossed
would not be superior to the self-fertilised; on the other hand crossing
with a fresh stock at once proves very advantageous. The favourable
effect of crossing is only apparent, therefore, if the parent plants
are grown under different conditions or if they belong to different
varieties. "It is really wonderful what an effect pollen from a distinct
seedling plant, which has been exposed to different conditions of life,
has on the offspring in comparison with pollen from the same flower or
from a distinct individual, but which has been long subjected to the
same conditions. The subject bears on the very principle of life, which
seems almost to require changes in the conditions." ("More Letters",
Vol. II. page 406.)

The