e northern hemisphere, when contrasted with
those procured from the southern, as alluded to by Dr. Seemann, seems
also to point to the effect of cultivation in producing these flowers.
Now, although this is, to a large extent, due to the selection that has
been for so long a period practised by gardeners, still that process
will not account for the appearance of double flowers where no such
selection has been exercised; as in the case of wild plants. Some double
peas, observed by Mr. Laxton, appeared suddenly; they had not been
selected or sought for, but they were produced, as it would appear, as a
result of high cultivation, and during the period when the plant was in
greatest vigour; and as the energies of the plant failed, so the
tendency to produce double flowers ceased. Indeed, in reference to this
subject, it is always important to bear in mind the time at which double
flowers are produced; thus, an annual plant subjected to cultivation,
will, it may be, produce single flowers for the firet year or two, then
a few partially double flowers are formed, and from these, by careful
selection and breeding, a double-flowered race may be secured.
Sometimes, as in the peas before alluded to, in the same season the
earlier blossoms are single, while later in the year double blossoms are
produced. This happens, not only in annuals, but also in perennials, and
is not infrequent in the apple; an illustration of this occurrence in
this tree is given in the 'Gardeners' Chronicle' for 1865, p. 554.[570]
Sometimes the flowers on a particular branch are double, while those on
the rest of the plant are single.[571] On these points, the evidence
furnished by a double white hawthorn in the Royal Botanic Gardens at
Edinburgh is important. Professor Balfour kindly wrote as follows in
reply to an inquiry respecting this plant:--"A double white hawthorn in
the Royal Botanic Gardens produced double flowers in spring. It retained
its leaves during autumn and winter, until the following spring. It then
flowered in the second spring, but produced weak single flowers only,
and has continued to do so ever since. The flowering has been always
weak, since this change of flowers from double to single. Mr. M'Nab
attributes the change in the duration of the leaves to the filling up of
the ground round the tree, to the height of a foot and a half on the
stem. He is now trying the effect of extra manure in giving extra vigour
to the plant." Here, at le