A GILBERT WHITE PAGE.
(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.