The Cystopus has female reproductive cells, which had escaped notice from being hid in the plastic matter which nourishes them. They appear before the spore dust bearing cells, and are formed by terminal or interstitial swellings in the tubes of the mycelium, which become large oval cells, separated ultimately by a closure from the rest of the tube that bears them; they are filled with a granular liquid mixed with large granules of a coloured fatty matter. The tips of some branches of the mycelium swell into oval or club-shaped cells containing spermatozoids, which fertilize the female cells; then the matter within the latter assumes a globular form, gets a coat of cellulose, and becomes the true fruit of the Cystopus.
As early as the year 1807, M. B. Prévost had seen that the sporangia, or spore cells of the entophytes produced zoospores, and recently M. de Bary has seen them produced, during the germination of the spores, collected within a sporangium of the Cystopus. When they came into the water they had two cilia, one of which was short and went first, the other was long and trailed after the zoospore. Neither M. de Bary nor M. Tulasne have ever seen zoospores in the fungus itself, but if the drops of rain or dew round the white spot on the leaf of a plant be examined, empty sporangia are generally found, and spores in different states of development.
The Puccinia Fabæ, an entophyte on the common bean, has but one spore in its cylindrical case, and is considered identical with the Uromyces appendiculatus. Besides male organs like those of the Æcidium, M. de Bary found that the bean entophyte has four kinds of reproductive organs, of which one alone reproduces the original form, while the others present a well-marked alternation of generations. The Puccinia forms a prothallus on which conidia, or secondary spore dust cells, arise; these secondary spores form a mycelium, on which an Æcidium appears, whose orange-coloured fruit gives rise to a Uredo, and the dust spores of the Uredo enter the leaves of beans or peas, and grow into a Puccinia. All the species of Æcidium are similar to one another, and M. Tulasne is of opinion that they do not constitute a distinct genus, but that, like many of the Uredines, they are merely a secondary form of some other fungus, and inhabit the same cavity, as in the case of Æcidium cyparissiæ and Uromyces scutellatus, Æcidium leucospermum and Puccinia Anemones, and others.