The extraordinary malformations known as Witches Brooms, caused by the repeated branching and tufting of twigs in which the mycelium of Exoascus (on birch) or Aecidium (on silver fir) are living, may be borne in considerable ntimbers for years without any very extensive apparent injury to the tree.
Irritation and hypertrophy of cells are common signs of the presence of parasites, as ovinced by the numerous malformations, galls, witches-brooms, &c., on diseased plants.
Under the head of malformations we place cases of atrophy of parts or general dwarfing, due to starvation, the attacks of Fungi or minute insects, the presence of unsuitable food-materials and so on, as well as cases of transformation of stamens into petals, carpels into leaves, and so forth.
When we come to phenomena such as proliferations, vivipary, the development of Lammas shoots, adventitious buds, epicormic branches, and to those malformations of flowers known as peloria, phyllody, virescence, &c. while assured that definite, and in many cases recognizable, physiological disturbances are,at work, we find ourselves on the borderland between patho~gical and physiological variation, where each case must be examined with dtie regard to all the circumstances, and no generalization seems possible beyond what has been sketched.
Malformations of the pelvis, accidental injuries and the diseases and displacements to which the uterus is liable, on the one hand; and, on the other, various morbid conditions of the ovum or placenta leading to the death of the foetus, are among the direct local causes.
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