Cylindrical stem swellings are caused by Calyptospora: they are due to the hypertrophy of the cortex of Bilberry stems permeated by the hyphae. Epichloë, which clothes the sheaths and halms of grasses with its stroma, at first snowy white and later ochre-yellow as the perithecia form, is another example.
The cylindrical layer of eggs of a moth such as Bombyx on a twig must not be confounded with these cases.
Frost-blisters are pustule-like uprisings of the cortex, where the living tissues below have formed a callus-like cushion into the cavity beneath the dead outer parts of the cortex which were killed by the frost; they occur on the stems of young Apples, Pears, etc.
Galls in the narrower sense are tissue outgrowths usually involving deeper cell-layers. They are so varied and numerous that classification is difficult. For symptomatic purposes we may divide them as follows:
Leaf-galls.—A well-marked type is that of the pocket-galls or bladders in which the whole thickness of the leaf is as it were pushed up like a glove-finger at one spot, so that if the upper surface of the leaf forms the outside of the gall the lower surface is its lining. Such galls are common on Limes (Phytoptus), Glechoma (Cecidomyia), Elms (Tetraneura), etc. Similar localised extension of the leaf surface, compelling it to rise up like a pocket, are caused by fungi—e.g. Taphrina on Poplars, Exoascus on Birches, etc., Exobasidium on Bilberries, Rhododendrons, etc.
Another type is that of the Gall-apple, so well known on Oaks, where the spherical swelling is solid—except for the inner cavity containing the eggs—Neurotus, Cynips, Hormomyia, etc. These are comparable in general characters to the nodules on roots.
Fungus galls with similar external features when young are found on Maize (Ustilago Maydis), and betray their nature by the black powdery spores as they mature.
Bud galls on Willows are due to Cecidomyia, which causes several internodes to swell out into a greenish barrel-shaped mass, from which leaves may spring.
Small irregular excrescences on Willow stems are referred to Phytoptus, and another species of the same insect induces similar swellings on Pines which are not surcharged with resin.
American Blight, or Woolly Aphis, on Apples especially, causes the tumour-like swellings covered with sticky white fluff, which is a waxy excretion of the insect. Galls on Pilea, in Java, are due to an Alga—Phytophysa.