me sterile; now it is
fertile[400] if kept in a dry stove during the winter. Other varieties
of pelargonium are sterile and others fertile without our being able to
assign any cause. Very slight changes in the position of a plant,
whether planted on a bank or at its base, sometimes make all the
difference in its producing seed. Temperature apparently has a much
more powerful influence on the fertility of plants than on that of
animals. Nevertheless it is wonderful what changes some few plants will
withstand with undiminished fertility: thus the _Zephyranthes candida_,
a native of the moderately warm banks of the Plata, sows itself in the
hot dry country near Lima, and in Yorkshire resists the severest
frosts, and I have seen seeds gathered from pods which had been covered
with snow during three weeks.[401] _Berberis Wallichii_, from the hot
Khasia range in India, is uninjured by our sharpest frosts, and ripens
its fruit under our cool summers. Nevertheless I presume we must
attribute to change of climate the sterility of many foreign plants;
thus the Persian and Chinese lilacs (_Syringa Persica_ and
_Chinensis_), though perfectly hardly, never here produce a seed; the
common lilac (_S. vulgaris_) seeds with us moderately well, but in
parts of Germany the capsules never contain seed.[402]

Some of the cases, given in the last chapter, of self-impotent plants,
which are fertile both on the male and female side when united with
distinct individuals or species, might have been here introduced; for
as this peculiar form of sterility generally occurs with exotic plants
or with endemic plants cultivated in pots, and as it disappeared in the
_Passiflora alata_ when grafted, we may conclude that in these cases it
is the result of the treatment to which the plants or their parents
have been exposed.

The liability of plants to be affected in their fertility by slightly
changed conditions is the more remarkable, as the pollen when once in
process of formation is not easily injured; a plant may be
transplanted, or a branch with flower-buds be cut off and placed in
water, and the pollen will be matured. Pollen, also, when once mature,
may be kept for weeks or even months.[403] The female organs are more
sensitive, for Gaertner[404] found that dicotyledonous plants, when
carefully removed so that