Many fungi, however, though they make their presence noticeable by conspicuous signs, cannot be said to do much damage to the individual plant attacked. The extraordinary malformations induced by parasites like Exoascus, which live in the ends of twigs of trees and stimulate the buds to put out dense tufts of shoots, again densely branched—Witches' brooms—are a case in point. Also the curious distortions of nettle stems swollen and curved by Æcidium, of maize stems and leaves attacked by Ustilago, and of the inflorescences of Capsella by Cystopus, etc., are not individually very destructive; it is the cumulative effects of numerous attacks, or of large epidemics, which tell in the end.
Some very curious effects are due to fungi such as Æcidium elatinum, which, living in the cortex of firs, stimulate buds to put out shoots with erect habit, and with leaves which are radially disposed, annually cast, and differently shaped from the normal—characters quite foreign to the species of fir in its natural condition.
Equally strange are the shoots of Euphorbia infested with the æcidia of Uromyces, those of bilberries affected with Calyptospora, etc. In all these cases we must assume a condition of toleration, so to speak, on the part of the host, which adapts itself to the altered circumstances by marked adaptations in its tissue developments, mode of growth and so forth.
This toleration is perhaps most marked in the case of those cereals which, though infected by the minute mycelium of Ustilago while still a seedling, nevertheless go on growing as apparently healthy green plants indistinguishable from the rest, although the fine hyphae of the parasite are in the tissues and keeping pace with the growth of the shoots just behind the growing points. As the grains of the cereal begin to form and swell, however, the hyphae suddenly assume the part of a dominant aggressor, consume the endosperm of the enlarging seed, and replace the contents of the grain with the well-known black spores known as Smut.
Notes to Chapter XII.
The reader will find a summary of such fungi as are here concerned in Massee, A Text-Book of Plant Diseases, 1899, or Prillieux, Maladies des Plantes Agricoles.
For further details the student should consult the works of Frank and Sorauer referred to in the [notes to Chapter IX.], and Tubeuf, The Diseases of Plants, Engl. ed. 1897, pp. 104-539.
For experiments on the effects of grass on orchard trees, see Report of the Woburn Experimental Fruit Farm, 1900, p. 160.
For the further study of weeds, the interesting bulletins of the Kansas State Agricultural College, 1895-1898, will show the reader what may be done in the matter of classifying them according to their biological peculiarities.
In regard to insects, the reader will find the following list embraces the subject: Somerville, Farm and Garden Insects, 1897; Theobald, Insect Life, 1896; Ormerod, Manual of Injurious Insects, 1890, and Handbook of Insects Injurious to Orchards, etc., 1898.
