other fruit-trees. There is not the least reason to believe that a branch
which has borne seed or fruit directly modified by foreign pollen is itself
affected, so as subsequently to produce modified buds: such an occurrence,
from the temporary connection of the flower with the stem, would be hardly
possible. Hence but very few, if any, of the cases of sudden modifications
in the fruit of trees, given in the early part of this chapter, can be
accounted for by the action of foreign pollen; for such modified fruits
have commonly been afterwards propagated by budding or grafting. It is also
obvious that changes of colour in the flower which necessarily supervene
long before it is ready for fertilisation, and changes in the shape or
colour of the leaves, can have no relation to the action of foreign pollen:
all such cases must be attributed to simple bud-variation.

The proofs of the action of foreign pollen on the mother-plant have been
given in considerable detail, because this action, as we shall see in a
future chapter, is of the highest theoretical importance, and because it is
in itself a remarkable and apparently anomalous circumstance. That it is
remarkable under a physiological point of view is clear, for the male
element not only affects, in accordance with its proper function, the germ,
but the surrounding tissues of the mother-plant. That the action is
anomalous in appearance is true, but hardly so in reality, for apparently
it plays the same part in the ordinary fertilisation of many flowers.
Gaertner has shown,[947] by gradually increasing the number of pollen-grains
until he succeeded in fertilising a Malva, that many grains are expended in
the development, or, as he expresses it, in the satiation, of the pistil
and ovarium. Again, when one plant is fertilised by a widely distinct
species, it often happens that the ovarium is fully and quickly developed
without any seeds being formed, or the coats of the seeds are developed
without an embryo being formed within. Dr. Hildebrand also has lately shown
in a valuable paper[948] that, with several Orchideae, the action of the
plant's own {403} pollen is necessary for the development of the ovarium,
and that this development takes place not only long before the pollen-tubes
have reached the ovules, but even before the placentae and ovules have been
formed; so that with these orchids the pollen apparently acts directly on
the ovarium. On the other hand, we must not over