WITCHES' BROOMS, the name, in botany, by which peculiar broom-like outgrowths found on the branches of a num ber of trees are known. They consist of a closely set mass of short branching twigs formed at one place on a branch as a result of the irritating action of an insect or a fungus. They are very common on the birch, being conspicuous when the tree is in a leafless condition ; the "brooms" have then the appearance of birds' nests. They may be caused by the attack of a mite
(Eriophyes rudis) ; in other cases they are due to the species of fungus, Exoascus turgidus, attacking the birch. An allied fungus (E. cerasi) causes brooms of the same kind on cherry.