_Excrescences_, or out-growths of more or less abnormal character from the general surface of diseased organs, are very common symptoms, and widely recognised. They are due to hypertrophy of the tissues while the cells are young and capable of growth, and may be induced by a variety of causes, among which the stimulus of insect-punctures and of the presence of insect eggs are best known; but that of fungi, though less widely recognised, plays an equally important part, and, as we shall see, galls and other excrescences may be due to widely different agents. _Galls_ or _Cecidia_ are protuberances of the most varied shapes, colours, and sizes found on herbaceous parts attacked by insects, fungi, etc. In the simplest cases the insects only pierce and suck the young cellular tissue--_e.g._ _Phytoptus_, Aphides, etc.--but in others the stimulus to hypertrophy starts by the puncture of the embryonic tissue of a leaf, root, etc., by the ovipositor of the female insect, which then lays an egg--_e.g._ _Cynips_, _Cecidomyia_, etc.--the presence of which appears to intensify the irritating action, or such only occurs when the young larva escapes. Our knowledge of the primary cause of gall-formation amounts to very little. Generally speaking, only embryonic or very young cellular tissue reacts, and galls on adult leaves and branches have usually been initiated long before. The same gall-insect may induce totally different galls on different plants, or even on different parts of the same plant, and different insects call forth different galls on any one plant. These facts point clearly to the co-operation of both plant and insect in the gall-formation, and the best hypothesis yet to hand is to the effect that a gall is a hypertrophy of cells, the normal nutrition, growth, and division of which have been disturbed owing to the action of some poison or other irritant derived from the insect, or fungus, or other organism. Attempts have been made to reproduce galls by injecting the juices of similar galls into the tissue, but as yet without success, and this may point to the co-operation of mechanical irritation during the hypertrophy in normal gall-formation.