OAK-Leaf Galls. These are of several kinds. The remarkable species called the mushroom gall is never found on any other vegetable substance than these leaves; and, besides this, there is a great number of other kinds.
The double gall of these leaves is very singular, because the generality of productions of this kind affect only one side of a leaf or branch, and grow all one way; whereas this kind of gall extends itself both ways, and is seen upon each side of the leaf, in the form of two protuberances, the one opposite to the other. These are of differently irregular shapes; but their natural figure seems that of two cones, with broad bases and very obtuse points, though they are sometimes round, or very nearly so.
These make their first appearance on the leaf in April, and remain on it till June, or even longer. They are at first green, but afterwards yellowish, and are softer to the touch than many other of the productions of this kind. They are usually about the size of a large pea, but sometimes they grow to the size of a nut. When opened, they are found to be of that kind which is inhabited each by one insect only, and contains but one cavity. The cavity in this, however, is larger than in any other gall of the size, or even in many others of three times its size; the sides of it being very little thicker than the substance of the leaf.