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.