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 - eg. Monilia, forming white or grey pustules on Apples, Roestelia and other Aecidia, 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.
Cylindrical stem swellings are caused by Calyptospora: they are due to the hypertrophy of the cortex of Bilberry stems permeated by the hyphae. Epichloe, 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.