It could scarcely have been permitted that the student should go far without being cautioned that there is such a thing as di-morphism in microscopic fungi; and the explanation of such a phenomenon must presuppose a certain amount of knowledge which, thus far, the reader could not have acquired. Hence an anomaly, to escape from which an ultimate return to the subject will be necessary.
In a recent account of Dr. de Bary’s experiments,[3] an interesting history is given of the development of a rust-like fungus, which is common on many plants of the pea and bean tribe. As it may serve to illustrate some of the preceding, as well as subsequent, remarks on development, an abstract shall close this chapter.
De Bary—“Annales des Sciences Naturelles,” ser. 4, vol. xx.
The spores of this species (_Uromyces appendiculatus_) are oboval cells, terminated by a rounded point, provided with a deep brown, smooth, _epispore_, or outer coating, and a distinct, but colourless _endospore_, or inner coating. These enclose a granular matter, which surrounds what has been termed the nucleus, but which appears to be a vacuole. At the top of the epispore is a pore which is characteristic of the genus. The spores are supported on a colourless, or slightly-tinted pedicel of considerable length (Plate VII. fig. 150). By means of this pedicel, the spores are attached to the fostering plant, on which they form pustules or sori of a blackish colour, and variable extent. These spores are ripened towards the end of summer or beginning of autumn. During winter they remain in a state of repose, but in the following spring the faculty of germination developes itself. At this period, when moistened or placed on a humid soil, they germinate at the end of a few days. The spore then emits a curved and obtuse tube, which soon ceasing to elongate itself, gives origin to three or four sporidia, of a reniform or kidney shape. When cultivated on moistened glass, these sporidia also emit a short, thin, slender tube, which produce in turn secondary sporidia. Here vegetation ends in the artificial culture above indicated.
When the sporidia are sown upon the epidermis of a favourable plant, the germ-tube being emitted, penetrates the wall of any approximate cellule, swells and increases into a cylindrical tube equal in thickness to the original sporidia, and therefore four or five times the diameter of the germ-tube before it entered the cellule. The contents of the sporidia and external portion of its germ-tube pass into the portion within the cellule, and then these external portions perish, and all evidence of the entry is obliterated, except a very minute point at which the tube remains attached to the inner surface of the wall of the cellule. The enclosed tube soon elongates, divides, and becomes branched. These branches perforate the inner walls of the epidermis, and pass into the intercellular spaces of the parenchyma to become mycelium. This takes place within 24 hours. A few days afterwards the mycelium is spread through the parenchyma. At length the surface of the same spots which had been sown in the first instance with the sporidia, become of a whitish tint, rapidly increasing and intensifying. Three days after, little protuberances appear on the surface of the white spots. These are of an orange colour, and many of them are surmounted by a little drop of mucilaginous fluid. These are _spermogones_. Their number daily increases, and a little time after appear numerous large globular protuberances intermingled with them. These soon rupture the epidermis, and take the orange colour and cylindrical form of cluster-cups (_Æcidium_). At length the summit of the peridia opens to allow the escape of the _stylospores_. It is easy to assure oneself that the spermogones and cluster-cups proceed from the mycelium of the sporidia which had been sown. During several days the length and number of the peridia of the _Æcidium_ continue to increase. One month after sowing, brownish or blackish points make their appearance upon the whitish spots, around, or intermingled with the cluster-cups. These increase rapidly in number and magnitude. Examined by the microscope, they present the ordinary fructification of _Uromyces_, mingled with stylospores. Thus the mycelium of the cluster-cups engenders at the end of its vegetation fruits equal in all points to those from whence they are in the first instance derived.