Galls, in the broad sense, are not always preceded by a wound, however. Insects on the outside of young tissues may cause such irritations that the parts in contact with the animal are arrested in their growth, while those further away grow more rapidly--_e.g._ where Mites, etc., cause puckers and leaf-rolling. In true galls the hypertrophy may consist merely in the enlargement of cells already present, and no new cell-divisions and, still less, changes in the nature of the tissues result--_e.g._ some pocket galls on _Viburnum_, _Pyrus_, etc., and the hairy outgrowths of the epidermis known as _Erineum_. In other cases there is not only hypertrophy of existing cells, but new cell-divisions are instituted: these cell-divisions may be confined to the direction perpendicular to the epidermis, and the tissues grow only in the direction of the surface, producing puckerings--_e.g._ the Aphis galls on _Ribes_, Phytoptus galls of _Salvia_, leaf galls on _Tilia_, _Acer_, _Alnus_, etc., and the curious galls on Plums due to _Cecidomyia Pruni_, and which must not be confounded with the "pocket plums" and similar galls due to Exoasci. In a third series of cases, cell-divisions occur parallel to the surface of the leaf, and galls are formed which grow in thickness, and develop the most extraordinary and complicated new tissues--proteid-cells surrounding the egg or larva deposited inside, followed by a protective layer of sclerenchyma encasing this food layer, and around this again softer tissues which may assume the structures and functions of respiratory tissues, water-storing tissues, starch reservoirs, assimilatory, or protective tissues of various kinds, and over all may be a well-marked epidermis, with stomata, or cork with lenticels. The chief seat of these hypertrophies and--what is more remarkable--development of new tissue elements not found elsewhere in the leaves, or even in the species, is the mesophyll, and various speculations and hypothesis have been founded on these curious phenomena. _Erineum._--The simplest excrescences on plants are certain hair-like developments of epidermal cells due to the irritation of species of _Phytoptus_, and similar insects which rise in clusters on the surfaces of leaves and by their colours, consistence, arrangement in patches, spots, etc., so simulate fungi that Persoon was deceived by them and gave them the genus name _Erineum_. They occur on most of our trees, _e.g._ Poplar, Lime, Oak, and are very common in the Tropics. Usually pale or even white at first, they turn brown as the hair-like outgrowths die and lose their sap, but since the latter may be bright coloured--yellow, red, purple,--the patches are sometimes very conspicuous objects on smooth leaves. In many cases these hairs exactly resemble in shape and other characters the abnormal root-hairs found on roots exposed to the effects of poisonous reagents, or of unsuitable food-materials, or the rhizoids developed from wounded Algae, etc.