(_Continued from p. 27._)
The curious effect of insect attacks in stimulating the growth of
certain parts of the plant which they have damaged may be found
illustrated in hundreds of instances. We have just mentioned the
pine-apples on the spruce fir, but the currant gall on the male catkin
of the oak is yet more striking. In this case a structure which
is naturally very shortlived has its vigour enhanced and its life
prolonged by the presence of the parasite. In this instance the fly
attacks the pollen-bearing flowers and deposits its eggs. These flowers
would, in the ordinary course, wither and fall as soon as the pollen
has ripened and been blown away. Under the stimulating influence of
the larvæ, however, sap is attracted, their stems thicken and become
fleshy, and instead of withering, they produce what looks like a
handsome bunch of currants.
Another instructive instance of parasitism stimulating growth may be
observed at this season in any patch of the common field thistle. Some
of the plants are almost sure to be affected by a parasitic fungus. It
is present in the stole of the plant, and its influence will cause the
affected plants to put forth leaves earlier than the healthy ones. They
will also grow faster, and in the course of a month be twice the height
of their fellows. The fungus grows in the stem, and finally it will
flower out on the surface of the leaves. When this happens the plant
will die, but up to that period its growth has been notably vigorous.
The fungus is the _Puccinia suaveoleus_ (_see_ Plowright, p. 183.) In
some plants it is visible even in early spring, and if abundant will
dwarf the plant so affected instead of stimulating its growth.
* * * * *