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. The admirable series of publications of the U.S. Department of Agriculture under the editorship of Riley and Howard, and entitled _Insect Life_, 1888-1895, also abounds in information. Further, Taschenberg's _Praktische Insektenkunde_, 1879-1880, and Judeich and Nietsche, _Lehrbuch der Mitteleurop. Forst. Insektenkunde_, 1889. For an elementary introduction to the study of fungus diseases, see Marshall Ward, _Diseases of Plants_, Soc. for Promoting Christian Knowledge, London. CHAPTER XIII. NATURE OF DISEASE.