Plate X.
W. West imp.
The summer spores, or pulverulent spores of the first generation, which are analogous to the Uredospores of Aregma, are also capable of germination, for, if placed in favourable circumstances, they will develop very long filiform processes, which either remain simple or become more or less branched, but always nearly uniform in their diameter. M. Tulasne states that he has observed this germination many times, though we have been less fortunate.
Of the six species of this genus known to occur in Great Britain, the majority may be commonly met with. That very widely-diffused plant, the wood cow-wheat (Melampyrum pratense), known well to all amateur botanists for its pertinacity in drying black, and presenting anything but an inviting appearance to claim for it a place in the herbarium—also becomes the matrix for the development of a member of this genus; i.e., the cow-wheat rust (Coleosporium Rhinanthacearum, Lev.), and which is found on other allied plants, as the little eyebright (Euphrasia officinalis), &c. ([Plate VIII.] fig. 176). In colour and habit it resembles the last-named species, and its free, echinulate spores ([Plate VIII.] fig. 177) form a pretty object for the microscope.
Another equally common species is found inhabiting the leaves of the sow-thistles (Sonchus arvensis and S. oleraceus), and in the autumn may generally be found on either of those plants, presenting the appearance delineated in our plate ([Plate VIII.] fig. 178). The permanent spores resemble in many points those of the first species, as will be seen from the figure from De Bary’s treatise on this subject ([Plate VIII.] fig. 179). This is certainly one of the most showy of uredinous fungi, and could not be well overlooked.
The butter-bur rust (Coleosporium petasites, Lev.) and the Campanula rust (Coleosporium Campanulæ, Lev.) are found, the former on the leaves of the butter-bur, and the latter on those of the harebell and other Campanulæ, less frequently. We have, however, indicated sufficient, since their great similarity in unprofessional eyes will furnish, in one or two species, all that is desirable for the microscopist.
Unless some similar plan to the following be adopted for examining the species of this genus, it may result in disappointment; for the slight attachment of the joints to each other will otherwise present only a mass of simple echinulate cellules, if a portion be only removed from the leaf on the point of a lancet. This method consists in making a thin vertical section of a pustule in which the spores are contained; by this means the arrangement of the fruit and the mucedinous threads from whence they proceed may be observed. Any person possessed of the cardinal virtues of microscopy—patience and perseverance—will be rewarded in this instance; whilst those who are deficient will lose an object worthy of the virtues they dare not boast. But few instances have occurred in this and the preceding chapters in which the exercise of any great ingenuity or application has been called for; the most juvenile or truest tyro at the microscope may see for himself much of what has been indicated, whilst a few opportunities have occurred for more practised manipulists to prove that they are neither juveniles nor tyros.
CHAPTER X.
WHITE RUSTS.
ALLUSION has already been made to the important memoir recently published by Dr. de Bary. “White rusts” occupy a conspicuous position in that memoir, and the experiments therein detailed, with the conclusions arrived at, will be largely drawn upon in furnishing the present chapter. Whilst believing that we have fairly represented the views, and faithfully narrated the story of research, if not literally, but denuded of some technicality, yet in such manner as to convey the sense of our author, we claim no originality or merit save for the garb in which it appears, without addition, stricture, or confirmation of our own.
What is the external appearance presented by the “white rust” of cabbages, and allied cruciferous plants, is soon told. During summer and autumn it occupies the surface of the leaves and stems of the shepherd’s-purse (Capsella bursa-pastoris), with elongated narrow white spots like streaks of whitewash ([Plate X.] fig. 198), and later in the season the leaves of cauliflowers and cabbages become ornamented with similar patches, arranged in a circular manner ([Plate X.] fig. 199), forming spots as large as a sixpence. Wherever these spots appear, the plant is more or less deformed, swollen, or blistered, even before the parasite makes its appearance at the surface. These white pustules have a vegetative system of ramifying threads which traverse the internal portion of the plants on which they are found: these threads constitute what is termed the mycelium. Not only when the plant is deformed and swollen with its undeveloped parasite do we meet with the threads of mycelium in its internal structure, but also in apparently healthy portions of the plant, far removed from the evidently infected spots. These threads are unequal in thickness, much branched, and often with thick gelatinous walls filled with a colourless fluid. They creep insidiously along the intercellular passages, and are provided with certain appendages in the form of straight thread-like tubes, swollen at their tips into globular vesicles ([Plate X.] fig. 204). These threads do not exceed in length the diameter of the mycelium which bears them. The appendages communicate in their interior with the mycelium, and contain within them the same fluid, which at length becomes more watery, and the terminal vesicles have their walls thickened, so as to resemble, on a casual observation, granules of starch. Dr. de Bary conceives that these appendages serve a similar purpose to the tendrils or suckers of climbing phanerogamic plants; i.e., to fix the mycelium to the cells which are to supply the parasite with nourishment. As these appendages are always present, it is easy to discover the mycelium wherever it exists amongst the tissues of an affected plant.