Fungi: Their Nature and Uses

by M. C. Cooke

Fig. 92.—Resting spore of Cystopus candidus with zoospores escaped.

The generation of the zoospores commences within from an hour and a half to three hours after the sowing of the conidia on water. From the oogonia, or resting spores, similar zoospores, but in greater number, are generated in the same manner, and their conduct after becoming free is identical. Their movements in the water usually last from two to three hours, then they abate, the cilia disappear, and the spore becomes immovable, takes a globose form, and covers itself with a membrane of cellulose. Afterwards the spore emits, from any point whatever of its surface, a thin, straight or flexuous tube, which attains a length of from two to ten times the diameter of the spore. The extremity becomes clavate or swollen, after the manner of a vesicle, which receives by degrees the whole of the protoplasm.

De Bary then proceeds to describe experiments which he had performed by watering growing plants with these zoospores, the result being that the germinating tubes did not penetrate the epidermis, but entered by the stomates, and there put forth an abundant mycelium which traversed the intercellular passages. Altogether the germination of these conidia or zoospores offers so many differences from the ordinary germination of the Uredines, and is so like that which prevails in Peronospora, in addition to the fact of both genera producing winter spores or oogonia, that we cannot feel surprised that the learned mycologist who made these observations should claim for Cystopus an affinity with Peronospora rather than with the plants so long associated with it amongst the Coniomycetes.

In passing from these to the Mucedines, therefore, we cannot do so more naturally than by means of that genus of white moulds to which we have just alluded. The erect branched threads bear at the tip of their branchlets spores, or conidia, which conduct themselves in a like manner to the organs so named in Cystopus, and oogonia or resting spores developed on the mycelium within the tissues of the foster plant also give origin to similar zoospores.

The conidia are borne upon erect, elongated filaments, originating from the creeping mycelium. These threads are hollow, and rarely septate; the upper portion divided into numerous branches, and these again are subdivided, the ultimate ramuli each terminated by a single conidium. This body when mature is oval or elliptical, filled with protoplasm, but there is a diversity in their mode of germination. In the greater part, of which P. effusa may be taken as an example, the conidia have the function of simple spores. Placed in favourable conditions, each of them puts forth a germ-tube, the formation of which does not differ in any essential point from what is known of the spores of the greater part of fungi.

The short oval conidia of P. gangliformis have little obtuse papillæ at their apex, and it is at this point that germination commences.

The conidia of P. densa are similar, but the germination is different. When placed in a drop of water, under favourable circumstances, the following changes may be observed in from four to six hours. The protoplasm, at first uniformly distributed in all the conidia, appears strewn with semi-lenticular, and nearly equidistant vacuoles, of which the plane face is immediately in contact with the periphery of the protoplasm. These vacuoles number from sixteen to eighteen in P. macrocarpa, but are less numerous in P. densa. A short time after the appearance of the vacuoles the entire conidium extends itself so that the papilla disappears. Suddenly it reappears, elongates itself, its attenuated membrane vanishes, and the protoplasm is expelled by the narrow opening that remains in place of the papilla. In normal cases the protoplasm remains united in a single mass that shows a clear but very delicate outline. When it has reached the front of the opening in the conidium, which is thus emptied, the mass remains immovable. In P. densa it is at first of a very irregular form, but assumes by degrees a regular globose shape. This is deprived of a distinct membrane, the vacuoles that disappeared in the expulsion again become visible, but soon disappear for a second time. The globule becomes surrounded with a membrane of cellulose, and soon puts out from the point opposite to the opening of the conidium a thick tube which grows in the same manner as the germ-tube of the conidia in other species. Sometimes the expulsion of the protoplasm is not completely accomplished; a portion of it remaining in the membrane of the conidium detaches itself from the expelled portion, and while this is undergoing changes takes the form of a vesicle, which is destroyed with the membrane. It is very rare that the protoplasm is not evacuated, and that the conidia give out terminal or lateral tubes in the manner that is normal to other species without papillæ. The germination just described does not take place unless the conidia are entirely surrounded by water; it is not sufficient that they repose upon its surface. Besides, there is another condition which, without being indispensable, has a sensible influence on the germination of P. macrocarpa, and that is the exclusion of light. To ascertain if the light or the darkness had any influence, two equal sowings were placed side by side, the one under a clear glass bell, the other under a blackened glass bell. Repeated many times, these experiments always gave the same result—germination in from four to six hours in the conidia under the blackened glass; no change in those under the clear glass up to the evening. In the morning germination was completed.

The conidia of P. umbelliferarum and P. infestans[L] show an analogous structure. These bodies, if their development be normal, become zoosporangia. When they are sown upon water, one sees at the end of some hours the protoplasm divided by very fine lines, and each of the parts furnished with a small central vacuole. Then the papilla of the conidium disappears. In its place appears a rounded opening, by which the parts of the protoplasm are expelled rapidly, one after the other. Each of these, when free, immediately takes the form of a perfect zoospore, and commences to agitate itself. In a few moments the sporangium is empty and the spores disappear from the field of the microscope.

The zoospores are oval or semi-oval, and in P. infestans the two cilia spring from the same point on the inferior border of the vacuole. Their number in a sporangium are from six to sixteen in P. infestans, and from six to fourteen in P. umbelliferarum. The movement of the zoospores ceases at the end of from fifteen to thirty minutes. They become motionless, cover themselves with a membrane of cellulose, and push out slender bent germ-tubes which are rarely branched. It is but seldom that two tubes proceed from the same spore. The same development of the zoospores in P. infestans is favoured by the exclusion of the light. Placed in a position moderately lighted or protected by a blackened bell, the conidia very readily produced zoospores.

A second form of germination of the conidia in P. infestans, when sown upon a humid body or on the surface of a drop of water, consists in the conidium emitting from its summit a simple tube, the extremity of which swells itself into the form of an oval vesicle, drawing to itself, little by little, all the protoplasm contained in the conidium. Then it isolates itself from the germ-tube by a septum, and takes all the essential characteristics of the parent conidium. This secondary conidium can sometimes engender a third cellule by a similar process. These secondary and tertiary productions have equally the character of sporangia. When they are plunged into water, the ordinary production of zoospores takes place.