As a type of this group we will select a very common one (_Cystopus bliti_), that is always to be found in late summer and autumn growing on pig weed (_Amarantus_). It forms whitish, blister-like blotches about the size of a pin head on the leaves and stems, being commonest on the under side of the leaves (Fig. 33, _A_). In the earlier stages the leaf does not appear much affected, but later becomes brown and withered about the blotches caused by the fungus. If a thin vertical section of the leaf is made through one of these blotches, and mounted as described for _Mucor_, the latter is found to be composed of a mass of spores that have been produced below the epidermis of the leaf, and have pushed it up by their growth. If the section is a very thin one, we may be able to make out the structure of the fungus, and then find it to be composed of irregular, tubular, much-branched filaments, which, however, are not divided by cross-walls. These filaments run through the intercellular spaces of the leaf, and send into the cells little globular suckers, by means of which the fungus feeds. The spores already mentioned are formed at the ends of crowded filaments, that push up, and finally rupture the epidermis (Fig. 33, _B_). They are formed by the ends of the filaments swelling up and becoming constricted, so as to form an oval spore, which is then cut off by a wall. The portion of the filament immediately below acts in the same way, and the process is repeated until a chain of half a dozen or more may be produced, the lowest one being always the last formed. When ripe, the spores are separated by a thin neck, and become very easily broken off. In order to follow their germination it is only necessary to place a few leaves with fresh patches of the fungus under a bell jar or tumbler, inverted over a dish full of water, so as to keep the air within saturated with moisture, but taking care to keep the leaves out of the water. After about twenty-four hours, if some of the spores are scraped off and mounted in water, they will germinate in the course of an hour or so. The contents divide into about eight parts, which escape from the top of the spore, which at this time projects as a little papilla. On escaping, each mass of protoplasm swims away as a zoöspore, with two extremely delicate cilia. After a short time it comes to rest, and, after developing a thin cell wall, germinates by sending out one or two filaments (Fig. 33, _C_, _E_).