PINE GALLS 209 



on the twigs — sometimes called oak balls instead of oak 

 galls — are common. These spherical enlargements or 

 outgrowths form around grubs which hatch out from eggs 

 deposited in the young tender bark of growing twigs in 

 the spring. Galls, in addition to consisting of a large 

 amount of extremely light spongy tissue, contain also a 

 larger proportion of tannin than other parts of the oak, 

 and for this reason mature galls have been used for cen- 

 turies as a source of ink. 



Other species of insects cause the formation of 

 galls on leaves of oak (Fig. 53) and other plants. The 

 same oak-leaf may form two or more different kinds 

 of gall. It is obvious from this fact that the diseased 

 structures called galls take on a form which is due not 

 only to the host but to the species of insect depositing the 

 egg which is to hatch into the parasitic grub. We have 

 here, then, a very interesting demonstration of the specific 

 action of the different agents of disease acting on the 

 same organism, the host reacting in characteristic though 

 different fashion to the attacks of the different individual 

 organisms. One may often see as many as half a dozen 

 different kinds of gall on the same white oak leaf, due to 

 the invasion of this leaf by as many different species of 

 gall-flies. 



The formation of galls with their characteristically 

 diverse shapes, the tissues of various sorts composing 

 these galls, the substances which act as stimuli resulting 

 in these overgrowths, the nature of the association, all 

 these are subjects of extreme interest. 



Pine Galls. — In this group may be mentioned an- 

 other peculiar gall, formed without mechanical injury, but 

 due to the deposit of its eggs by a fly at the base of and 

 between the needles of the Monterey pine as these come 

 out in the spring. The grubs hatch as the very young 

 leaves begin to grow out into the light from the sheath 

 which encloses them. There is no wound, there is mere 

 contact between grub and tender leaf-base. The leaves 



