Rust, Smut, Mildew, & Mould: An Introduction to the Study of Microscopic Fungi

by M. C. Cooke

Let not the reader imagine, from what we have just stated, that it is our intention to burden him with a dry series of botanical descriptions; as much of this as we deem essential to render the book available to the botanical student, we have preferred to add in the form of an Appendix. Useful as these may be to some, we hope to be enabled to furnish for others something more; and although we at once disclaim any intention of including all the microscopic, or even the epiphytal fungi, in our observations, yet we trust, by a selection of common and typical species for illustration, and by an adherence to certain well-defined groups and sections, to demonstrate that the microscopist will find an eligible field for his observations in this direction, and the botanical student may gain some knowledge of their generic and specific distinctions.

It is exceedingly difficult to give a logical definition of what constitutes a fungus. It is no less difficult to furnish a popular description which shall include all and nothing more. If, for example, we particularize the spots and markings on the leaves and stems of herbaceous plants, so commonly met with from early spring till the fall of the last leaf, and even amongst the dead and decaying remains of the vegetation of the year, we may include also such spots and marks as result from insect depredations or diseased tissue. It is not always easy, with a cursory observation under the microscope, to determine whether some appearances are produced by fungi, insects, or organic disease: experience is the safest guide, and until we acquire that we shall occasionally fail.

If we take a stroll away from the busy haunts of men, though only for a short distance,—say, for example (if from London), down to New Cross,—and along the slopes of the railway cutting, we shall be sure to find the plant called the goatsbeard (Tragopogon pratensis) in profusion. In May or June the leaves and unopened involucres of this plant will present a singular appearance, as if sprinkled with gold-dust, or rather, being deficient in lustre, seeming as though some fairy folk had scattered over them a shower of orange-coloured chrome or turmeric powder. Examine this singular phenomenon more closely, and the poetry about the pixies all vanishes; for the orange powder will be seen to have issued from the plant itself. A pocket lens, or a Coddington, reveals the secret of the mysterious dust. Hundreds of small orifices like little yellow cups, with a fringe of white teeth around their margins, will be seen thickly scattered over the under surface of the leaves. These cups (called peridia) will appear to have burst through the epidermis of the leaf and elevated themselves above its surface, with the lower portion attached to the substratum beneath. In the interior of these cup-like excrescences, or peridia, a quantity of the orange-coloured spherical dust remains, whilst much of it has been shed and dispersed over the unoccupied portions of the leaves, the stems, and probably on the leaves of the grass or other plants growing in its immediate vicinity. These little cups are fungi, the yellow dust the spores,[^[1]] or ultimate representatives of seed, and the epiphytal plants we have here found we will accept as the type of the group or order to which we wish to direct attention ([Plate I.] figs. 1-3).

Plate I.
W. West imp.

[1]. Protospores they should be called, because, in fact, they germinate, and on the threads thus produced the true spores, or fruit, are borne.

Amongst the six families into which fungi are divided, is one in which the spores are the principal feature, as is the aurantiaceous dust in the parasite of the goatsbeard. This family is named Coniomycetes, from two Greek words, meaning “dust-fungi.” This group or family includes several smaller groups, termed orders, which are analogous to the natural orders of flowering plants. Without staying to enumerate the characteristics of these orders, we select one in which the spores are enclosed in a distinct peridium, as in our typical plant they are contained within the cups. This order is the Æcidiacei, so called after Æcidium, the largest and most important of the genera included within this order.

The Æcidiacei are always developed on living plants, sometimes on the flowers, fruit, petioles, or stems, but most commonly on the leaves: occasionally on the upper surface, but generally on the inferior. The different species are distributed over a wide area; many are found in Europe and North America, some occur in Asia, Africa, and Australia. When the cryptogamic plants of the world shall have been as widely examined and as well understood as the phanerogamic plants have been, we shall be in a better position to determine the geographical distribution of the different orders of fungi. In the present incomplete state of our knowledge, all such efforts will be unsatisfactory.

But to return to the goatsbeard, and its cluster-cups. The little fungus is called Æcidium tragopogonis, the first being the name of the genus, and the last that of the species. Let us warn the young student against falling into the error of supposing because in this, and many other instances, the specific name of the fungus is derived from the plant, or one of the plants, upon which it is found, that therefore the species differs with that of the plant, and that, as a rule, he may anticipate meeting with a distinct species of fungus on every distinct species of plant, or that the parasite which he encounters on the living leaves of any one plant is necessarily specifically distinct from those found on all other plants. One species of Æcidium, for instance, may hitherto have been found only on one species of plant, whereas another Æcidium may have been found on five or six different species of plants. The mycologist will look to the specific differences in the parasite without regard to the identity or distinctness of the plant upon which it is parasitic.

Before the Æcidium breaks through the epidermis, the under surface of the leaves of the goatsbeard will appear to be covered with little elevations or pustules, paler at the apex; these soon become ruptured, and the fungus pushes its head through the opening, at the same time bursting by radiating fissures. The teeth thus formed resemble those of the peristome of some mosses. All around the orifice of the peridium the teeth become recurved, and the orange spores are exposed, crowded together within. At first, and while contained within the peridium, these spores are concatenate or chained together, but when dispersed they are scattered singly about the orifice, often mixed with the colourless cells arising from the partial breaking up of the teeth of the peridium.