We have found another species which had not been before noticed in this country (M. Hedwigii, Lev.), on the leaves of the mealy guelder-rose in the vicinity of Darenth Wood, near Dartford, in Kent. The mealy character of the leaves of this plant, and the minute size of the conceptacles of the parasite, render it difficult to find; indeed, it could not be noticed unless it were sought for, as we sought it, lens in hand. It only occurs on the under surface of the leaves: the mycelium is very web-like and fugacious, the conceptacles minute, globose, and scattered (fig. 243). Four sporangia, each containing but four spores (fig. 244), are enclosed in each conceptacle, which is surrounded by a few appendages (about six) thrice dichotomous, and thickened at the tips of the ultimate branches, which are incurved (fig. 247).
The species of true Erysiphe are distinguished botanically from the foregoing by the floccose character of the appendages, in which feature they accord with the species found on the rose and the hop, but from which they differ in the conceptacles containing numerous sporangia instead of only one, as in those species.
One of the most common and conspicuous of these is found on the leaves and leaf-like stipules of the garden pea. Every leaf in a crop will sometimes suffer, and the gardener, to his great mortification, finds that the mildew is more prolific than his peas. The leaves become sickly and yellow as the mycelium of the fungus spreads over them, when they present a peculiar appearance, as if growing beside a chalky road in dry dusty weather, and had become covered with comminuted chalk. Soon the conceptacles appear, profusely scattered over the white threads, like grains of gunpowder (fig. 237), and after a brief struggle for existence the pea and its parasite die together. In this species (Erysiphe Martii, Lev.), the appendages are nearly transparent, short, and much interwoven with the mycelium (fig. 238), the globose sporangia containing from four to eight spores (fig. 239). It is not confined to peas, although that habitat has been here given for it, because it is so common upon them. Beans, melilot, St. John’s-wort, some umbelliferous plants, and the meadow-sweet, have all been found affected.
The species found on grasses, especially the cocksfoot, has been already alluded to. The conceptacles contain from twenty to twenty-four ovate sporangia, each enclosing eight spores. The appendages and mycelium are much interwoven.
Another of these “white mildews,” not only on account of its frequency of occurrence on certain plants, but also from the numerous species of phanerogamous plants on which it is found (fig. 240), may be truly designated “common” (Erysiphe communis, Lk.); many kinds of crow-foot, especially Ranunculus acris, are subject to its parasitism. It is found also on other plants of the same natural order, on the rest-harrow, trefoils, enchanter’s nightshade, bindweed, and knotgrass. There are from four to eight sporangia in each conceptacle, containing from four to eight spores (fig. 241). In this species, more especially, M. Tulasne found curious sucker-like processes developed on the threads of the mycelium (fig. 242): their office may probably be only that of attachment.
Of the other species found in Britain an enumeration will suffice, since they contain no feature of interest to the microscopist; and all the members of this section are far less beautiful than those in other genera (especially Microsphæria).
The leaves of the dogwood or cornel (figs. 245, 246) are the home of one species (E. tortilis, Lk.), and the burdock of another (E. Montagnei, Lev.). Both of these, in addition to the above, have sporangia which contain more than two spores. There are also two species in which only two spores are contained in each sporidium. One of these (E. Linkii, Lev.) is found on both surfaces of the leaves of the mugwort (figs. 248, 249); the other (E. lamprocarpa, Lev.) occurs on salsafy, scorzonera, weasel-snout, and plantain (figs. 250, 251).
These complete the Erysiphei; but there are allied species of too much interest not to be noticed in connection with them. Three very singular fungi are found on damp straw and paper; two on the former and one on the latter. Of the species occurring on straw, the most common one is figured, natural size, in our plate (fig. 257); but from this no idea can be formed of its structure, which in some points resembles an Erysiphe. The conceptacles are thin and brittle, and are clothed externally with long dark-coloured branched hairs (figs. 258, 259). The conceptacle contains long narrow sporangia, each enclosing dark, almost black, lemon-shaped sporidia. For low powers this is a very interesting object. The minute structure affords no feature of popular interest. This fungus (which bears the name of Chætomium elatum) is common on old straw, thatch, reeds, matting, &c., resembling small brown tufts of hair, visible to the naked eye.
Paper much exposed to damp will occasionally develop a similar “bristle-mould,” surrounded by a yellowish spot (Chætomium chartarum, Ehrb.); but it is not so common as the last. In habit and structure it is very similar (figs. 252, 253).
In 1838, the Rev. M. J. Berkeley announced the discovery by him of a singular production, for which he was unable to find a fitting location in any genus then established, and for which he accordingly characterized a new one, under the name of Ascotricha. This new species of paper mildew was found by him on some printed paper in a box. It somewhat resembles the other species above alluded to, at a casual glance; but more minute examination will reveal its differences. The author to whom we are indebted for this species thus describes its development. At first it appears as a minute branched mould interspersed with globose brownish conidia. As it advances in growth, globose black peridia become visible amongst the threads, clothed with and supported by alternately branched obscurely-jointed filaments, the branches of which generally form an acute angle with the stem (fig. 254). The ramification of these is very peculiar, the stem and main shaft of each sub-division being almost constantly shortened and surmounted by the branches given off near its apex; this, again, is often abbreviated and another branchlet given off, which again surpasses it; and occasionally the same circumstance takes place a third time. The apices are clavate and colourless; the rest of the filaments, when viewed by transmitted light, brown, even, and pellucid: a few globose conidia are usually attached to them (fig. 255). The conceptacle is thin, black to the naked eye, of an olive-brown under the microscope, filled with a mass of linear extremely transparent asci (fig. 256), each containing a single row of broadly elliptic chocolate sporidia. These have a paler border; sometimes the colour entirely vanishes, either from age or abortion, and there is only a minute globose nucleus or more probably a vesicle of air, in the centre; occasionally they become so transparent that the globular bodies alone are visible. After the conceptacles burst, several are frequently collected together into an irregular linear body, which consists principally of the conglomerated sporidia.