The various genera of these endophytes owe their distinctions to the form, or mode of development of their true spores. In one instance these spores are united in pairs, or divided by a septum, so that they are two-celled: these are named Puccinia. In another instance the spores are one-celled, and at first borne upon a stalk or peduncle, from which they are detached in ripening: such are called Trichobasis. It is unnecessary here to indicate all the variations to illustrate the fact that the generic distinctions are based upon the characters of the true spores. How unsatisfactory such a mode will appear, when subjected to experience day by day, a botanist would suspect. In the same pustule, resting upon the same cushion of mycelium, the spores of an Aregma will be found with those of a Lecythea, and those of a Puccinia with Trichobasis. More than this has even been affirmed. The alternation of generations, known to students in the animal world, is here repeated in the vegetable. Dr. de Bary declares that certain data appear to indicate that Æcidium constitutes not a genus by itself, but are organs in the development of some other germs and species, possessing its spermogonia, its Æcidium; its Uredo, and its spores, properly speaking; whilst in others the Uredo-form the Puccinia-form, and the Æcidium-form may alternate. It is not our intention to enter deeply upon the discussion of this subject, of so little interest to the beginner, and so out of place in an introduction to the study. That forms and conditions are multifarious, and that an entire revision of the classification is inevitable, are facts which do not require many words to establish. Already it is to be feared that in this brief chapter we have said too much, and must recommend its perusal again, when the names and characters of the genera alluded to have been rendered more familiar.
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.
[3]. 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.
The stylospores of the cluster-cups possess the irregular, globular form and structure of their congeners. They are filled with orange granular matter, and provided with a colourless, finely-punctated epispore. When these stylospores are sown on the moistened epidermis of a favourable plant, the germ-tube at first creeps along the surface, but as soon as its extremities find a stomate, it enters it and elongates itself in the air-cavity below the orifice, receives the contents of the original stylospore and exposed portion of its tube, then separates itself from those parts, which become dispersed. The active part increases and ramifies, and produces a mycelium which spreads through the intercellular passages of the parenchyma. At the end of from six to eight days, the whitish spots appear on the surface of the fostering plant, and indicate that the fructification of the parasite is about to commence. The epidermis is elevated and broken, and little brown pustules appear through the openings. These are the stylospores of Uredo, which are produced in immense quantities, and soon cover the pustules with a deep brown dust. Later, the formation of the stylospores is arrested, and the true germinating spores appear in the same pustules.
The stylospores of Uredo are borne singly at the top of short filaments. On arriving at maturity they detach themselves. They are of a globular form, with a reddish-brown epispore, provided with little pointed prominences, and three pores at equal distances. After maturity they germinate in precisely the same manner as the stylospores of the cluster-cups. They enter only through the stomata of the epidermis. The pulvinules are identical with those which the stylospores of Æcidium originate, and they also produce true spores at the end of their vegetation. No other fruit arises from them. These organs, therefore, always reproduce the same form to which they owe their origin. The result of these investigations shows that the bean rust (Uromyces appendiculatus), besides spermogones, possesses four sorts of reproductive organs, which all serve to propagate the species, but that one alone of them produces it in a form always identical, whilst the others present well-marked alternations of generation. Hence it is concluded that there are,
I. Spores which produce in germinating the promycelium, and
II. Sporidia.—These give place to a mycelium, which bears afterwards—