The present cases must
not be confounded with those to be given in the next chapter relatively
to flowers which are sterile when insects are excluded; for such
sterility depends not merely on the flowers being incapable of
fertilisation with their own pollen, but on mechanical causes, by which
their pollen is prevented from reaching the stigma, or on the pollen and
stigma of the same flower being matured at different periods.
In the seventeenth chapter of my 'Variation of Animals and Plants under
Domestication' I had occasion to enter fully on the present subject; and
I will therefore here give only a brief abstract of the cases there
described, but others must be added, as they have an important bearing
on the present work. Kolreuter long ago described plants of Verbascum
phoeniceum which during two years were sterile with their own pollen,
but were easily fertilised by that of four other species; these plants
however afterwards became more or less self-fertile in a strangely
fluctuating manner. Mr. Scott also found that this species, as well as
two of its varieties, were self-sterile, as did Gartner in the case of
Verbascum nigrum. So it was, according to this latter author, with two
plants of Lobelia fulgens, though the pollen and ovules of both were in
an efficient state in relation to other species. Five species of
Passiflora and certain individuals of a sixth species have been found
sterile with their own pollen; but slight changes in their conditions,
such as being grafted on another stock or a change of temperature,
rendered them self-fertile.
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