For microscopical study, fresh material may be used, or, if necessary, dried specimens. The latter, before mounting, should be soaked for a short time in water, to which has been added a few drops of caustic-potash solution. This will remove the brittleness, and swell up the dried filaments to their original proportions. A portion of the plant should be carefully scraped off the leaf on which it is growing, thoroughly washed in pure water, and transferred to a drop of water or very dilute glycerine, in which it should be carefully spread out with needles. If air bubbles interfere with the examination, they may be driven off with alcohol, and then the cover glass put on. If the specimen is mounted in glycerine, it will keep indefinitely, if care is taken to seal it up. The plant consists of much-interlaced filaments, divided at intervals by cross-walls.[6] They are nearly colorless, and the contents are not conspicuous. These filaments send up vertical branches ([Fig. 39], A), that become divided into a series of short cells by means of cross-walls. The cells thus formed are at first cylindrical, but later bulge out at the sides, becoming broadly oval, and finally become detached as spores (conidia). It is these spores that give the frosty appearance to the early stages of the fungus when seen with the naked eye. The spores fall off very easily when ripe, and germinate quickly in water, sending out two or more tubes that grow into filaments like those of the parent plant ([Fig. 39], B).
Fig. 40.—Chrysanthemum mildew (Erysiphe), showing the suckers (h) by which the filaments are attached to the leaf. A, surface view. B, vertical section of the leaf, × 300.
The spore fruits, as already observed, are formed toward the end of the season, and, in the species under consideration at least, appear to be the result of a sexual process. The sexual organs (if they are really such) are extremely simple, and, owing to their very small size, are not easily found. They arise as short branches at a point where two filaments cross; one of them ([Fig. 39], C, ar.), the female cell, or “archicarp,” is somewhat larger than the other and nearly oval in form, and soon becomes separated by a partition from the filament that bears it. The other branch (antheridium) grows up in close contact with the archicarp, and like it is shut off by a partition from its filament. It is more slender than the archicarp, but otherwise differs little from it. No actual communication can be shown to be present between the two cells, and it is therefore still doubtful whether fertilization really takes place. Shortly after these organs are full-grown, several short branches grow up about them, and soon completely envelop them (D, E). These branches soon grow together, and cross-walls are formed in them, so that the young spore fruit appears surrounded by a single layer of cells, sufficiently transparent, however, to allow a view of the interior.
The antheridium undergoes no further change, but the archicarp soon divides into two cells,—a small basal one and a larger upper cell. There next grow from the inner surface of the covering cells, short filaments, that almost completely fill the space between the archicarp and the wall. An optical section of such a stage ([Fig. 39], F) shows a double wall and the two cells of the archicarp. The spore fruit now enlarges rapidly, and the outer cells become first yellow and then dark brown, the walls becoming thicker and harder as they change color. Sometimes special filaments or appendages grow out from their outer surfaces, and these are also dark-colored. Shortly before the fruit is ripe, the upper cell of the archicarp, which has increased many times in size, shows a division of its contents into eight parts, each of which develops a wall and becomes an oval spore. By crushing the ripe spore fruit, these spores still enclosed in the mother cell (ascus) may be forced out ([Fig. 39], H). These spores do not germinate at once, but remain dormant until the next year.
Fig. 41.—Forms of mildews (Erysiphe). A, Microsphæra, a spore fruit, × 150. B, cluster of spore sacs of the same, × 150. C, a single appendage, × 300. D, end of an appendage of Uncinula, × 300. E, appendage of Phyllactinia, × 150.
Frequently other structures, resembling somewhat the spore fruits, are found associated with them ([Fig. 39], I, K), and were for a long time supposed to be a special form of reproductive organ; but they are now known to belong to another fungus (Cicinnobulus), parasitic upon the mildew. They usually appear at the base of the chains of conidia, causing the basal cell to enlarge to many times its original size, and finally kill the young conidia, which shrivel up. A careful examination reveals the presence of very fine filaments within those of the mildew, which may be traced up to the base of the conidial branch, where the receptacle of the parasite is forming. The spores contained in these receptacles are very small ([Fig. 39], K), and when ripe exude in long, worm-shaped masses, if the receptacle is placed in water.
The mildews may be divided into two genera: Podosphæra, with a single ascus in the spore fruit; and Erysiphe, with two or more. In the latter the archicarp branches, each branch bearing a spore sac ([Fig. 41], B).