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

by M. C. Cooke

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.