Mould
This term is frequently used, very vaguely, to denote a large number of microscopic Fungi that agree in being filamentous in structure, and in having the spores usually borne on the tips of certain branches, sometimes scattered, at other times crowded in masses, the arrangements of which are of importance in affording means of distinguishing the various kinds of Moulds. The Fungi included under the name are exceedingly numerous, and belong to widely different groups. Many of them live on decaying remains of animals, or of plants that remain damp; indeed, it is scarcely possible to prevent their growth in food, clothes, and other household articles in damp houses. But, in addition, many kinds also grow upon living plants, and are most destructive at times in their attacks upon certain of them; e.g., Peronospora infestans, on Potatoes; P. gangliformis, on Lettuce, &c. Only these latter kinds very greatly concern gardeners. The injurious kinds will be found treated of under the headings of the plants injured by them.
Moulds are often divided into two groups, viz., the Black (Dematiei), having the threads dark because of thickened cell-membranes; and the White (Mucedines), which have no thick cell-membranes, and cannot always be clearly distinguished from the Mildews (see Mildew). It is to the White Moulds that the more harmful kinds belong, particularly to the genus Peronospora. A good many of the Moulds, we have reason to believe, are only earlier stages of Fungi that belong to groups more complex in structure when mature; whilst a considerable number have the power of producing spores that, when lying in fluid which contains sufficient food, resemble the Yeast Fungus in appearance, in the mode of forming new cells, and in causing fermentation in the fluid if it contains fermentable substance. Good ventilation, and prevention of overcrowding, are essential in diminishing the risk of injury from Moulds to green-house subjects. Direct applications to plants containing parasites inside their tissues are useless, since the parasite cannot be reached, and the host-plant itself suffers. It is advisable to destroy diseased plants by fire, if practicable; and seed should not be taken from these. Plants of dry soils are less liable to damage from injury by Fungi than are those in damp situations. Lastly, the healthier the subject, as distinct from mere luxuriance, the less likely will it be to suffer injury from the growth on it of Moulds.