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.
Plate XII.
W. West imp.
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.
CHAPTER XI.
MOULDS.
TWENTY years since, and some of these little pests were altogether unknown, whilst others were only recognized and partly understood by a few scientific men. During the period to which we have alluded more than half the present species contained in the genus Peronospora had never been observed, and amongst these the most devastating of its tribe, the associate and undoubted cause of the potato disease.
Parasitic fungi are far more numerous, both in individuals and species, than most persons are aware, and cultivated plants of all kinds are more or less subject to their ravages. Some are more susceptible than others, of which the corn and grass tribe, or Graminaceæ, as they are termed by botanists, is an example. Not less than thirty species have been recorded upon plants of this natural order, and of these nearly one-half are found upon the living plants. Upon the potato plant, again, no less than ten different kinds of fungi have been described; whilst upon other and more fortunate plants only one or two parasites of this nature establish themselves.
Plate XIII.