There are a few practical conclusions which may be drawn from these discoveries. In the first place, it is clearly shown by the production of the spots that the fungus is capable of _causing_ the disease, a fact which has been disputed, but now placed beyond doubt. The inference is, that not only is it capable of producing, but is really the cause of the potato murrain. With bodies so minute and active as the zoospores, there can no longer be difficulty in accounting for their penetrating the tissues of the plant. They are most active and productive in wet weather, especially when it is also warm. Moisture appears to be essential, and a dry season the greatest enemy to the spread of the disease. That bodies so minute and subtle should have baffled all efforts to destroy or eradicate, is not now surprising. Whether any method will be found to contend successfully with it, is now more doubtful than ever. A careful reperusal of the old facts by the aid of this new light will tend to the elucidation of much of the mystery in which the subject has been involved. All who have hitherto been sceptical of the mycological source of one of the greatest pests of modern times should study M. de Bary’s pamphlet. Plate XV. [Illustration: 266.—PEA MOULD. _Peronospora Viciæ._ ] [Illustration: 267.—ANEMONE MOULD. _Peronospora pygmæa._ ] The potato mould has been judiciously named _Peronospora infestans_, or, as it was at first called, _Botrytis infestans_; but on a revision of the genera _Botrytis_ and _Peronospora_, it was transferred to the latter genus, in which it remains. Three names were given to it, within a short period of each other, by different mycologists, in ignorance of its having already received a name. The one we have adopted appears to have the priority, at least of publication, and was given by Dr. Montagne. That of _Botrytis devastatrix_ was given by Madame Libert, and _Botrytis fallax_ by M. Desmazières. The principal feature in this species seems to consist in the branches becoming alternately thickened and constricted, so as to resemble a moniliform string or necklace of little bladders or vesicles. The branches are also more erect than in the allied species, and the spores are solitary on the tips or from the sides of the branches, and not in pairs or clusters, and the tips are simple, and not bifid or trifid, as in most of its allies. It need scarcely be remarked, that a high power of the microscope is necessary to make out the distinctive features of the different members of this genus, and that to the naked eye they only appear as a minute whitish mould. As already stated, this little fungus makes its first appearance on the under surface of the leaves, especially the lower ones, of the potato plant, and afterwards attacks the stem, and ultimately the tuber. For examination it is better to select the leaves soon after the fungus makes its appearance.