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

by M. C. Cooke

The white pustules already alluded to contain the fruit of the parasite. Bundles of clavate or club-shaped tubes are produced upon the mycelium beneath the epidermis of the infested plant, forming a little tuft or cushion, with each tube producing at its apex reproductive cells, designated “conidia.” These conidia appear to be produced in the following manner:—The tips of the clavate tubes generate them in succession. At first a septum, or partition, divides from the lower portion of the tube a conidium cell; this becomes constricted at the septum and assumes a spherical shape, at length only attached by a short narrow neck. Beneath this again the same process is repeated to form another and another conidium in succession, until a bead-like string of conidia surmount each of the tubes from which they are produced ([Plate X.] fig. 200). At length the distended epidermis above is no longer able to bear the pressure of the mass of engendered conidia within, and is ruptured irregularly, so that the conidia, easily separating from each other at the narrow neck, make their escape.

As long since as 1807, M. Prevost described the zoospores, or moving spores, of these conidia, and his observations were confirmed by Dr. de Bary three years since, and are now adverted to by him again in further confirmation. If the conidia (white spherical bodies ejected from the pustules of the “white rust”) are sown in a drop of water on a glass slide, being careful to immerse them entirely, they will rapidly absorb the water and swell; soon afterwards a large and obtuse papilla, resembling the neck of a bottle, is produced at one of the extremities. At first vacuoles are formed in the contents of each conidium; as these disappear, the whole protoplasm (granular substance filling the conidium) becomes separated by very fine lines of demarcation, into from five to eight polyhedric portions, each with a faintly coloured vacuole in the centre. These portions are so many zoospores. Some minutes after the internal division, the papilla swells and makes itself an opening, through which the zoospores are expelled one by one, without giving any signs of movement of their own. They take a flat disk-like or lenticular form, and group themselves about the opening, whence they have been expelled, in a globular mass. Soon, however, they begin to move, vibratile ciliæ show themselves, and by means of these appendages the entire globule oscillates, the zoospores disengage themselves from each other, the mass is broken up, and each zoospore swims off on its own account ([Plate X.] fig. 208).

The free zoospores are of the form of a planoconvex lens, obtuse at the edge. Beneath the plane face, out of the centre, and towards that point of the margin which during the movement of the zoospore is foremost, is a disk-shaped vacuole, with two ciliæ of unequal length attached to its margin; the shorter cilia is directed forwards, and the longer in the opposite direction, during the evolutions of the zoospores.

The zoospores are produced within from an hour and a half to three hours after the sowing of the conidia in water. They are never absent if the conidia are fresh, or even a month old, but beyond this period their artificial generation is very uncertain. This little experiment is a very simple and interesting one, and may be performed by any one who will take the trouble to follow out these instructions.

From this simple experiment, let us turn for a moment to the plant in its natural condition when affected by the white rust. If, after rain or dew, when the little drops of moisture hang like pearls about the sickly pallid leaves of the shepherd’s-purse, bespattered with the white pustules of the rust, we collect and examine a drop of water from the immediate neighbourhood of one of the pustules, we shall commonly find empty conidia and zoospores in different stages of development.

Plate XI.
W. West imp.

Water alone seems to be essential to them, and for this the conidia may remain unchanged for a month, and literally burst into activity at the first gentle shower, till the whole surface of the plant is swarming with zoospores. We may no longer doubt that a true vegetable produces from itself bodies endowed with active motion, resembling low forms of animal life, and yet in themselves not animalcules, as some would suggest, but essentially vegetable, as we shall hereafter demonstrate. To scientific men this is not new, except as regards fungi, for in algæ such bodies have long been recognized.

A second kind of reproductive organs are described by Dr. de Bary; and if future examinations confirm his observations, as they doubtless will, this feature is an important one. It is true that M. Caspary long since detected similar bodies in moulds (allied to that which produces the potato disease), but he only knew them in a limited sense compared with what De Bary has revealed. These fruits are hidden amid the tissues of the plant on which the “white rust” is parasitic, and only betray their presence by the coloration of those tissues. To these bodies it is proposed to give the name of “oogonia” and “antheridia,” on account of their presumed sexuality, the “oogonia” representing the female, and the “antheridia” the male organs.

The oogonia are large spherical or ovoid cells, with a thickish membrane containing a granular protoplasm, or formative fluid. They are produced either terminally or laterally upon the threads of the mycelium, from which they are separated by septa or partitions.