CHAPTER X.

_WHITE RUSTS._

ALLUSION has already been made to the important memoir recently published by Dr. de Bary. “White rusts” occupy a conspicuous position in that memoir, and the experiments therein detailed, with the conclusions arrived at, will be largely drawn upon in furnishing the present chapter. Whilst believing that we have fairly represented the views, and faithfully narrated the story of research, if not literally, but denuded of some technicality, yet in such manner as to convey the sense of our author, we claim no originality or merit save for the garb in which it appears, without addition, stricture, or confirmation of our own.

What is the external appearance presented by the “white rust” of cabbages, and allied cruciferous plants, is soon told. During summer and autumn it occupies the surface of the leaves and stems of the shepherd’s-purse (_Capsella bursa-pastoris_), with elongated narrow white spots like streaks of whitewash (Plate X. fig. 198), and later in the season the leaves of cauliflowers and cabbages become ornamented with similar patches, arranged in a circular manner (Plate X. fig. 199), forming spots as large as a sixpence. Wherever these spots appear, the plant is more or less deformed, swollen, or blistered, even before the parasite makes its appearance at the surface. These white pustules have a vegetative system of ramifying threads which traverse the internal portion of the plants on which they are found: these threads constitute what is termed the _mycelium_. Not only when the plant is deformed and swollen with its undeveloped parasite do we meet with the threads of mycelium in its internal structure, but also in apparently healthy portions of the plant, far removed from the evidently infected spots. These threads are unequal in thickness, much branched, and often with thick gelatinous walls filled with a colourless fluid. They creep insidiously along the intercellular passages, and are provided with certain appendages in the form of straight thread-like tubes, swollen at their tips into globular vesicles (Plate X. fig. 204). These threads do not exceed in length the diameter of the mycelium which bears them. The appendages communicate in their interior with the mycelium, and contain within them the same fluid, which at length becomes more watery, and the terminal vesicles have their walls thickened, so as to resemble, on a casual observation, granules of starch. Dr. de Bary conceives that these appendages serve a similar purpose to the tendrils or suckers of climbing phanerogamic plants; _i.e._, to fix the mycelium to the cells which are to supply the parasite with nourishment. As these appendages are always present, it is easy to discover the mycelium wherever it exists amongst the tissues of an affected plant.

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.

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.

The antheridia are somewhat blunt-shaped or obovate cellules, considerably smaller than the oogonia, with slightly thickened walls, and containing a finely granular protoplasm. These are produced upon branches of the mycelium which do not bear oogonia. The obtuse extremities of these branches, which are to be developed as antheridia, are applied to the surface of the growing oogonia, to which they adhere, become distended, assume their obovate form, and by the formation of a septum at their base, their contents are isolated from those of the threads of the mycelium, and thus the antheridia are perfected.

When these bodies have attained their full dimensions, the large granules which are contained in the oogonium accumulate at its centre, and form an irregular, somewhat spherical mass, which is called by De Bary a _gonosphere_. This gonosphere having been formed, a straight tube shoots out from the antheridium which perforates the wall of the oogonium, passes through the fluid which surrounds the gonosphere, elongating itself until it touches that body. From this period a membrane begins to be formed about the gonosphere, which thenceforth maintains a regular spheroidal form. It may be observed that the extremity of the tube which proceeds from the antheridium does not open, and the fecundation, if such it be, is produced solely by contact. After this contact of the two bodies, the gonosphere acquires a new name, and is called an “oospore.” The membrane which at first invests this organ is very thin, but by deposits from the surrounding fluid it attains to a greater thickness, and is at length of a yellowish-brown colour, having its surface studded with large obtuse warts (Plate X. fig. 206). One of these warts, larger than the rest, forms a kind of thick sheath around the fecundating tube.

The oospores do not give evidence of any appreciable change for some months. For instance, those collected by De Bary in June did not attain their ulterior development until the commencement of December. The method adopted was as follows:—Parts of the plants containing ripe oospores were preserved in the dried state. When examination was considered desirable, the portion to be employed was immersed in water for a day or two; it was then placed on a humid soil, or mould covered with blotting-paper. The tissues enclosing the oospores were decomposed, and at the end of from four to eight days their germination might be observed when placed in a drop of water. This method again corresponds with the ordinary processes by which the plant naturally decays on exposure to the influences of the atmosphere, and the oospores germinate under the favour of a shower of rain.

If the oospore, after the decay of the tissues, is isolated and placed in a drop of water, the brown investing membrane will be seen to rupture irregularly, and its contents (enclosed in a transparent inner membrane) issuing from the orifice. As in the case of the conidia, this body at first contains vacuoles, and is afterwards divided into polyhedric portions; these pass into zoospores, which congregate at the centre into a globular mass (Plate X. fig. 207). They afterwards separate, and for some minutes float about in the vesicle in which they were generated. Ultimately the membrane ruptures, and the zoospores swim about in water just as those produced from the conidia had done. The number contained in each oospore is considerable, and may be estimated at not less than one hundred.

The zoospores, whether produced from conidia or from oospores, appear to be the same. The movements of both in the water last from two to three hours; then they cease, the ciliæ disappear, and the zoospores remain at rest, taking meanwhile a globular form. Afterwards these spores (for having ceased all motion they are no longer zoospores) emit a thin tube from some portion of their surface, such tube attaining a length of from two to ten times that of the spore whence it proceeds. The extremity of these tubes swells and forms a kind of cell, into which the contents of the spore pass through the medium of the tube (Plate X. fig. 209).

Thus far, and thus far only, has Dr. de Bary been enabled to trace the development of the zoospores in a drop of water. Another series of experiments was instituted by this mycologist having especial reference to the parasitism of the “white rust.” He made numerous observations to ascertain whether the spores, or the germinating tubes, entered by the roots of growing plants, and satisfied himself that they did not. Plants of garden-cress, mustard, and shepherd’s-purse had their roots immersed in water impregnated with zoospores. After one or two days, though the surfaces of the roots were covered with zoospores that had emitted their germinating tubes in all directions, none had penetrated or showed the least tendency to penetrate the epidermis. Other plants were planted in flowerpots and watered at the roots with water charged with zoospores, and for two days the pots were left standing in the water similarly charged, then the plants were removed, cultivated in the ordinary manner, grew up healthy, and gave no signs of the white rust. Care had been taken that neither stems nor leaves should come in contact with water containing zoospores.

If a drop of water thus charged is placed on the surface of a living leaf of the shepherd’s-purse, for instance, and left at rest for a few hours and examined minutely at the end of that period, they will be found to have germinated. Let the epidermis be removed carefully and placed on a glass slide and submitted to the microscope. Many zoospores will be found to have produced from that point of their surface which is nearest to one of the stomata, or pores of the leaf, its slender tube, and to have thrust it through those openings, with the swollen extremity resting in the air-cavity situated beneath the pore. If many days, or even weeks, are allowed to pass, and the leaf is examined again, or another leaf similarly treated, and kept in a living and vigorous condition by remaining attached to the parent plant, still no further change or advance will be observed, the germs will appear fresh, and still in the same condition. Hence it is concluded that plants are not infected through the medium of their leaves.

If the cotyledons, or seed-leaves, are watered with similar impregnated water, a different result has been observed to take place. The germination of the tubes till their entrance at the stomata is the same; but, having entered, the swollen extremity elongates, becomes branched, and takes all the appearance of mycelium such as we at first described. If the infected plant endures through the winter, the mycelium endures with it, to recommence vegetating in the spring.

The experiments which Dr. de Bary performed were all upon plants of the common garden-cress. It will be unnecessary to repeat all the details of these, as given in the memoir recently published on the subject, but it will suffice to give a summary of results. In two series of plants cultivated at different periods from good seeds, one hundred and five plants which, had not received the water impregnated with zoospores upon their cotyledons vegetated without any indications of the parasite. Amongst the eighteen plants which were inoculated by watering the cotyledons, four only were not attacked by the parasite, fourteen bore the “white rust.” In six of these it did not extend beyond the cotyledons; in the others it also appeared on the stems and leaves.

From these experiments it may be deduced that plants are not infected by spores of the parasite entering at the roots, or by their leaves, but that inoculation takes place through the medium of the cotyledons, or seed-leaves; that the agents in this inoculation are the zoospores produced either from the conidia or the oospores; that they do not enter the stomata or pores themselves, but thrust out a germinating tube, into the extremity of which the contents of the zoospores pass; that when these tubes have entered the stomata of the cotyledons they branch and ramify, becoming a true mycelium, from which fruitful parasites are developed; that if a plant so infested lives through the winter, the parasite lives with it, to vegetate again in the spring.

The immense number of zoospores capable of being produced from a single infested plant is almost beyond calculation. It is easy for a million of conidia to be developed from such a plant, each producing from five to eight zoospores, besides a large number of oospores, each containing a hundred zoospores. It can scarcely be considered marvellous that the white rust should be so common on plants favourable to its development, the marvel being rather that any plant should escape.

Until recently it was doubtful whether more than one or two species of _Cystopus_ (white rust) were known. It is now certain that we have three in Great Britain, and three or four others are found elsewhere. Of the British species one is found on many cruciferous plants, as the shepherd’s-purse, garden-cress, mustard, radish, and plants of the cabbage kind. This is the _Cystopus candidus_. Another occurs on the goat’s-beard, salsify, and scorzonera, which is called _Cystopus cubicus_. Both have great external resemblances, but both possess specific internal differences. In the Goat’s-beard rust (Plate X. fig. 201) the terminal conidia in the bunches or fascicles of conidia which are produced within the pustules are spheroidal, large, and of a yellow-brown tint, whilst the residue are cylindrical, smaller (Plate X. fig. 202), and more or less compressed. In the crucifer rust the conidia are all equal in the pustules and globose. The oospores in the former of these are subglobose and the warts on their surface are solid; whilst in the latter the oospores are truly globose, and the warts on the surface are hollow (Plate X. fig. 210). The third species is the Sandspurry white rust (_Cystopus Lepigoni_), which was found on the common sandspurry (_Spergularia rubra_) by Mr. R. G. Keeley, in Swanscombe Marshes (September, 1864). Of the other species it is not improbable that one or two may yet be found in this country. Without attempting to indicate their microscopic differences, it may be serviceable to name the species of phanerogamic plants on which they are likely to be found. The Purslane white rust (_Cystopus Portulacæ_, D. C.) should be sought on the purslane, which, though of limited cultivation, is exceedingly liable to attack from this parasite, and the Thistle white rust (_Cystopus spinulosus_, D. By.) may probably be met with on the leaves of the common thistle (_Cnicus arvensis_) or some of its allies.

Considerable interest is now attached to these parasites, which, as far as we at present know, differ materially in their reproduction from the other dust-like or uredinous fungi with which they have long been associated. Dr. de Bary proposes the union of these with the mould-like fungi of the genus _Peronospora_, to which the mould infesting the potato belongs, so as to constitute by themselves a group apart from the genera with which both have heretofore been associated. Whether his views will be accepted by mycologists time will speedily prove. Under any circumstances, microscopical and botanical science will reap considerable benefit from his researches.