for. The vegetative mycelium of most of the Uredineæ is very similar, although the spores are very diverse. The mycelium, being present in the appropriate host-plant, may give origin to several different kinds of spores, according to the nature of the Uredine under examination, each kind of which was regarded by the older botanists as being a distinct genus (Æcidium, Uredo). While it is true that the relationship between the various spore-forms was suspected as early as the beginning of the present century, yet its actual demonstration has been accomplished only within the last thirty years. From the investigations of Tulasne and De Bary, we now know that each spore-form has a life-history of its own, that they are often products of the same mycelium grown among different environments, and that they all arise from some antecedent spore-form. The ultimate condition in which all the Uredineæ are encountered is the teleutospore, which, after a longer or shorter period of quiescence, manifests its vitality by germinating and producing a body-the promycelial spore. The promycelial spore may then be regarded as the beginning of the series of spore-forms of which the teleutospore is the end. The actual number of spore-forms intervening between the promycelial spore and the teleutospore is subject to considerable variation in different species. 1. The promycelial spore, after emitting a germ-tube, which enters the tissues of its appropriate host-plant, may give rise to a mycelium, which produces teleutospores exactly similar to the teleutospore from which it originated (Leptopuccinia, Micropuccinia). 2. The promycelial spore in a second group of the Uredineæ, in like manner, gives rise to a mycelium which produces uredospores, and subsequently teleutospores (Hemipuccinia). 3. The promycelial spore may give rise to a mycelium which produces, first, spermogonia, then æcidiospores, then uredospores, and finally teleutospores (Auteupuccinia). Certain minor variations in the life-cycle occur; for example, the uredospores may be suppressed, so that we have only æcidiospores and teleutospores (Pucciniopsis), or the spermogonia and æcidiospores may occur on one plant, and the uredospores and teleutospores upon another of a totally distinct kind (Heteropuccinia). Each of the above spore-forms has its own peculiarity of structure, as well as origin, so that it will be better, for clearness, to describe them separately. The spore relations may be thus tabulated : PROMYCELIAL SPORE. Teleutospore Uredospore Spermogonium Teleutospore Uredospore Teleutospore Spermogonium Teleutospore CHAPTER II. MYCELIUM OF THE UREDINEA. THIS is, of course, common to all the different sporcforms, inasmuch as it is that part of the fungus which develops them. It consists of a number of hyaline tubes that extend themselves principally between the cells of the host-plant. In some instances these tubes send short branches into the cells (haustoria), but the haustoria are not so common nor so well developed as in some other parasitic fungi (Peronosporeæ, Ustilagineæ). In the tropical genus Hemileia the haustoria are unbranched, thin-stemmed vesicles, very like those of Cystopus. Bagnist has figured the haustoria of Puccinia malvacearum, but his figure is of a doubtful character; and Barclay‡ has depicted an arborescent haustorium on the mycelium of Æcidium urticæ, var. himalayense. 录 The mycelial tubes (Plate I. Figs. 1 and 2) consist of hyaline, membranous walls, containing usually a colourless watery fluid. They are rendered more distinct by the * Marshall Ward, "On Hemileia vastatrix," Linn. Soc. Jour. Bot., vol. xix., and Quart. Jour. Micr. Science, new series, vol. xxi. + Bagnis, "Obs. Vita et Morphol. Funghi Uredinei," t. i. fig. 11. Barclay, "Scientific Memoirs by Medical Officers of the Indian Army" (1887), t. iv. fig. 4. action of caustic potash (KHO). The tubes themselves are rather irregular in their outline, and branch at frequent intervals. These branches unite with other mycelial tubes, so as to form an anastomosing irregular network, which pervades more or less widely the tissues of the affected plant. At rare intervals transverse septa are seen. function of this mycelial network is to utilize the elaborated material which the host has prepared for its own use, and to turn it into a suitable pabulum for the sustenance of the fungus. The The extent to which the mycelium permeates the tissues of the host-plant varies; in the majority of cases it is localized and confined to a limited area. The germtube from a single spore having once entered the tissues of a leaf, the tendency of the mycelium thereby produced is to spread equally in all directions in a centrifugal manner. Many causes, however, come into operation which tend to prevent the ultimate spore products being equidistant from the centre. This may be due, in part, to the mycelial hyphæ growing more luxuriantly in one direction than in another; but to a great extent it arises from a want of uniformity in the tissues of the host-plant. Still, however, we are often able to observe that the fructification of those Uredines which have a limited. mycelial growth, is arranged either in a circle or in more or less circular manner (Puccinia lychnidearum, Uromyces scillarum, Caoma orchidis, Ecidium zonale). In those cases in which the mycelium is developed in a leaf with strongly marked venation, this tends to exert a directive influence upon its extension; for example, the primary uredospores of Triphragmium ulmarie and Uromyces alchemillæ. The same directive influence of the tissues of the host-plant is seen in the linear arrangement of the sori of Puccinia magnusiana, graminis, rubigo-vera, etc., especially when they occur upon the stems or cauline sheaths of their graminaceous host-plants. The presence of the mycelial hyphæ generally acts as a local stimulant to the tissues in which they are present, as is evinced by the increased thickness which is often associated with a concave arching or vaulting of the affected places (Acidium grossularia, berberidis, Ræstelia cancellata). When the stems are affected, this is usually shown very markedly by the development of swellings and distortions; as a rule these are more or less fusiform, and often induce considerable bending of the attacked stem (P. difformis, Kze.; Ur. trifolii). Even upon these cauline tumefactions may often be traced the concentric arrangement of the sori. The stimulation of the affected part may be carried to such an extent as to kill the invaded tissues; thus we often find the older leaves of Malva sylvestris and Althea rosea with numerous circular holes, punched as it were out of them. Each of these has been the seat of the very localized mycelium of Puccinia malvacearum, which has by its presence killed a circumscribed portion of the entire thickness of the leaf tissue, so that, as the leaf itself expands, the dead area above described becomes separated round its circumference and falls out, leaving a circular hole. The above-described perforated foliage is most observable after a period of drought; in rainy weather the reproduction of the parasite is so rapid that the entire leaf tissue is quickly invaded by the fungus and totally destroyed. The same dropping out of mycelial areas occurs upon the stem; here, however, the hyphæ only penetrate the external parts, so that when the affected spots drop out an elongated or fusiform wound is left, at the bottom of which the central woody part of the stem is exposed. Schröter* has pointed out that a somewhat * Schröter, "Cohn's Beitrage," vol. ii. p. 88. |