Habitat, Specialised Forms of Parasites.
Habitat.—The habitat of vegetable parasitic fungi is extremely variable. Fungi are found everywhere, living and flourishing on all the families of the vegetable and animal kingdoms. They attack our houses, foods, clothes, utensils of every kind, wall papers and books, the paste of which, to my astonishment, affords a sufficient supply of nourishment. Members of the parasitic tribe of bacteria, by a combined effort of countless myriads, have given rise to a sense of supernatural agency. Bacillus prodigiosus, described also as Palmella mirifica and Zoogalactina imetropia, from its attacking milk and other alimentary substances, the spores of which are often of a deep red colour, have been found to cover whole tracts of country in a single night with what is called a “gory dew,” changing in daylight to a deep green colour. This was at one time regarded with superstitious awe as a miracle, as it has been known to attack bread and even the sacred wafer, and which in mediæval ages was described as the “bleeding-host.” This parasitic plant belongs to anærobic bacteria, and is only developed in the dark. The nitrogen required for nutrition must be derived from the air. An algal form gives rise to the red scum seen in ponds and reservoirs in the autumn. The discharge from wounds is coloured blue by Bacterium pyocyanine. There are many other forms, some of which have an orange colour, and the genus is recognised as “chromogenic microbes.”
Fig. 274.—Fungi and Moulds.
Description of Figures.—d. Puccinia graminis on wheat; c. Polycystis spore of rye-smut; f. Alder fungus spores, Microspheria penicellula; g. Dactylium roseum, rose-coloured mould; h. Verticillium distans, whorled mould found on herbaceous plants; i. Botrytis, vine and lily fungus; j, j′. Peronospora infestans, potato fungus; k. P. gangliformis, mould of herbaceous plants; l. Various Penicillium and other spores taken in a bean-field.
A cryptogam belonging to anærobic bacteria, described as Protococcus invalis, on being set aside in a bottle, and a little rain water added, was seen to set up spontaneous fermentation, and in a very short time exhibited remarkable activity. The colour of the infusion changed, it assumed a delicate pink hue in direct light, which deepened to a red in reflected light. The fluid contents were now observed to be dichoric, and the spectroscopic appearance subsequently presented was one of much interest. The spectrum was a well-marked one, and might be taken to determine the presence of a nitrogenous element or of glucose.
Among all the various plants known to suffer from the attacks of parasites, the vine has been the greatest sufferer. The oïdium, or Erysiphe Tuckeri, so called from the name of the discoverer by whom it was first described, has been longest known to the vine grower. This really belongs to the group Ascomycetes, and appears to have been brought from America in 1845, whence it was passed on to France, where it soon threatened to entirely destroy the vineyards. This was followed by another parasite, belonging in this instance to the animal kingdom, Phylloxera vastatrix. This oïdium appears on the grape in the form of greyish filaments, terminating in an enlarged head, which contains an agglomeration of spores, not free or in a chaplet, as in Aspergillus ([Fig. 273]). These spores when ripe burst from the capsule as fine dust, and are diffused by the air in all directions, thus spreading the disease far and away. Another of the parasitic moulds, Peronospora viticola, is a kind of mildew, differing from oïdium. The hyphæ penetrate more deeply than that of oïdium. On the upper surface of the leaf brown patches appear; these branch out and ramify as seen in the potato-fungus, P. infestans ([Fig. 274]). The parasite destroys the tissue of the leaf, and it withers and dies. There are other well-known parasites, the black-rot, Phomauvicola, belonging to the Ascomycetes. This appears in early shoots in the form of round black spots, and gradually spreads over leaves and young fruit. This same rot, one year, devastated the American vineyards.
Fig. 275.—Fungi, Moulds.
a. Clustered Spores, Gonatobotrys simplex; b. Spore of Puccinia coronata, the mildew of grapes; c. Barley smut; d. Puccinia althæa; e. Penicillium glaucum; m. Ixodes farinæ, found in damaged flour together with smut.