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.
Intumescences are similar trichomatous outgrowths not associated with insects or fungi, and due to some disturbance of the balance between transpiratory and assimilatory functions of their leaves, as indicated by the less localised occurrence and by their non-appearance when the plant is under favourable cultural conditions. Structures not unlike these have been artificially induced by exposure to particular lights, and also by painting spots with dilute corrosive sublimate, indicating that poisons may impel the epidermis cells to grow out abnormally.
Corky warts.—Several forms of disease are known in which the pathological condition is expressed by the formation of cork in unwonted places and quantities. The Scab or Scurf of Potatoes is a case in point. The tissue of the lenticels absorbs water and the outermost cells are cut off by cork and die: the cells below them burst the dead bark-like masses thus formed, and again cork is formed and cuts off the outer masses, and the rough cork warts—Scab or Scurf—are the result.
The causes predisposing to scab have been variously assigned to dampness, want of lime, action of bacteria and fungi—e.g. Sorosporium, Oospora, Spongospora,—the latter making their way into the ruptured tissue of the lenticels and irritating the cells to further growth.
It seems probable that several different kinds of scab exist in Potatoes, as well as in roots—e.g. Beets, and the whole subject needs further investigation. The scab-like rough scaly bark of Pear trees in dry districts may also be mentioned here.
Cork-wings are well known on the young branches of Elms, Maples, etc., some varieties of which have received specific names on this account.
Corky excrescences on leaves occur occasionally in the Gooseberry, Holly and other plants, for which no cause has been discovered.
Lenticels are also formed on some leaf-galls, and are remarkable as being structures not normal on leaves.
Pustules.—This term may be employed generally for all slight upheavals of the surfaces of herbaceous organs, which subsequently burst and give egress to the spores, etc., of the organism causing them, or merely fray away at the top if no organism is discoverable. They are often due to fungi—e.g. Synchytrium, Protomyces, Cystopus, and Ustilagineae,—and we may extend the use of the general term also to those cases where the stroma of the fungus itself bursts through the cortex of older parts and forms the principal part of the pustule—e.g. Monilia, forming white or grey pustules on Apples, Roestelia and other Æcidia, forming yellow or orange pustules on leaves, etc.; Cucurbitaria and Nectria (red) breaking through the cortex of trees, and Phoma and numerous other Ascomycetes which form black cushions. Pustules on the leaves of Lysimachia, Ajuga, etc., are due to the parasitic Alga Phyllobium.