"blind" strawberries has been accounted
for. Mr. R. Thompson, writing on this subject, speaks of a plantation of
Hautbois strawberries which in one season were wholly sterile, and
accounts for the circumstance as follows: the plants were taken from the
bearing beds the year previous, and were planted in a rich well-manured
border, in which they started rapidly into too great luxuriance, the
growth being to leaves rather than to fruit. The following season these
same plants bore a most abundant crop, hence these plants were
accidentally prevented from perfecting their female organs.[195]

Mr. Darwin[196] cites from various sources the following details
relating to strawberries which it may be useful to insert in this place,
as throwing some light upon the production of unisexual flowers.
"Several English varieties, which in this country are free from any such
tendency, when cultivated in rich soils under the climate of North
America commonly produce plants with separate sexes. Thus, a whole acre
of Keen's seedlings in the United States has been observed to be almost
sterile in the absence of male flowers; but the more general rule is,
that the male plants over-run the females.... The most successful
cultivators in Ohio plant, for every seven rows of pistillate flowers,
one row of hermaphrodites, which afford pollen for both kinds; but the
hermaphrodites, owing to their expenditure in the production of pollen,
bear less fruit than the female plants."

_Stratiotes aloides_ has been said to produce its carpels with greater
abundance towards the northern limits of its geographical distribution,
and its stamens, on the other hand, are stated to be more frequently
developed in more southern districts.

_Honckenya peploides_ affords another illustration of the sexual
arrangements in the flower being altered as it would seem by climatal
conditions. Thus, in the United States, according to Professor Asa Gray,
the flowers are frequently hermaphrodite, while in this country they are
usually sub-dioecious.[197]

Treviranus[198] says that the flowers of _Hippuris_ and _Callitriche_
are apt to be hermaphrodite in summer, but female only at a later
period.

For further remarks on this subject, see sections relating to
suppression of stamens and pistils.

=Change from unisexuality to hermaphroditism.=--This occurrence depends
on one of two causes, either organs are developed (stamens or pistils as
the case may be), which are habi