The mildews grow on the surface of fruits, and injure them more by choking up their pores and mechanically confining them with their dense, felty growth, than by abstracting their juices. The potato rot is accompanied by a rapid growth of the mycelium of botrytis infestans, which penetrates the leaves, stems, and tubers, inducing rapid decay. It appears on the surface in the form of a minute white mould. Many other plants are similarly affected. Boleti are sometimes traversed by a minute mould, sepedonium chrysospermum, which gives a golden-yellow hue to the flesh. Dry rot in timber is caused by the penetrating mycelium of merulius lacrymans and polyporus destructor. The black excrescent growth on plum trees is occasioned by the sphoeria morbosa, which covers the warts its mycelium has made with its minute black, compacted perithecia. The fairy rings which in olden times were thought to be the scenes of midnight fairy revels, are produced by the growth of different species of agaricus. As they exhaust the soil by one year's growth, their mycelium pushes into the richer portions around; and thus they extend the circle of their growth, furnishing by their decay a manure for the next year's grass, which is darker and denser in consequence.-Fungi have been classified in various ways by different mycologists.

By the early writers they were arranged according to their external appearances; but as more exact means of observation multiplied, their microscopic structure became better known, and a nearer approach was made to a classification in consonance with their true affinities. From Caesalpinus in 1583 to Nees von Esenbeck in 1817, the progress of knowledge was comparatively small for a period of nearly 250 years. But in 1821 appeared the Systema Mycologicum of Elias Fries, a work of the most learned and profound character, evincing a comprehensiveness and thoroughness far surpassing all that bad preceded it. It is even now the great work to which all students refer, though since that time a host of observers have been exploring this obscure field, and collecting a vast array of facts concerning the laws which govern these minute organisms. Montagne, Leveille, Tulasne, Berkeley, Desmazieres, and many others have of late years been engaged in the elucidation of their structure. The latest system given to the world is that in Berkeley's Introduction to Cryptogamic Botany," which is essentially similar to that of Fries. The two principal divisions are: sporidiiferi, spores contained in special sacs called asci; and sporiferi, spores naked, not enclosed.

These are again subdivided into six principal orders, all formed on the mode in which the spores are borne, viz.: 1. Ascomycetes (Berk.), spores produced in little sacs (asci), and formed out of the protoplasm they contain. This order comprises a vast number of the black, pustular growths, abundant on dead wood, bark, twigs, leaves, etc. They are generally formed of a mass of carbonized cells arranged in the form of hollow spheres or cups called perithecia. Within these grow the asci containing the spores, which escape either from a pore in the peritbecium or by its breaking up irregularly. The basal cells bearing the asci are collectively termed the hymenium. Among these are the mildews (erysiphe) and the black mildews (cap-noclhim), and the whole great tribe of sphaerice. The truffles (tuber) also belong here. They are subterraneous, fleshy forms, whose substance is intersected by veins which are inward folds of the hymenium, covered by the expanding growth of the fleshy receptacle. The morels (morchella) and the helvelloe are carnose, bulky forms, which have their asci on the outer surface of a variously folded, wrinkled, and pitted hymenium. The cyttaria is akin to these, of a sub-gelatinous consistence.

These are all made up of compacted cells, forming horny, carbonized, or heavy, fleshy masses. 2. Physomycetcs (Berk.), spores growing in bladder-shaped cells on the end of delicate, individual, scattered fibres, composed of cells applied to each other in a linear series. A small group comprising the true moulds (mucor). 3. Hyphomycetes (Fr.), spores naked, simple, or aggregated on the ends of fertile threads. These differ from the last in the naked growth of the spores. Here belongs the great host of minute moulds which cover almost every substance exposed to dampness with their floccose fibres. Nothing organic is free from their attacks. Their colors are sometimes extremely beautiful. To this order belong the mould of the potato rot (bo-trytis infestans), and many which induce decay in fruit (oidium), the bread and cheese moulds (penicillium, aspergillus), the rigid black moulds (cladosporium, helminthosporium), and the yeast and vinegar plants, which are submerged mycelia of pencillium. (See Fermentation.) 4. Coniomycetes (Fr.), spores naked on the ends of filaments or vesicles; hymenium sometimes obsolete, sometimes contained in a perithecium.

This order differs from the last in having scarcely any filamentous growth, and in having the spores produced in the utmost profusion, greatly disproportionate to the rest of the plant. It comprises an infinity of minute pustular forms, which infest the tissues of every variety of plant, many presenting to the eye but a mere speck on their surface. Here belong the whole family of rusts, smuts, and bunt (puccinea, uredo, ustilago, tilletia, acidium, &c), which creep through the tissues of living plants, and finally burst forth on the exterior and fructify in dense, dusty masses, which cover their whole surfaces. Different species affect different organs, some being on stems and leaves, others on flowers and fruit. They are the scourge of the farmer, whose fields they devastate. The savin trees (juniperus) are attacked by a peculiar genus (podisoma), which bursts from their bark and swells under the influence of moisture to a gelatinous mass. It also occasions the globular excrescent growth called cedar apples, from orifices in which it protrudes in long orange-colored spurs, formed by the spores, tipping the aggregated mass of filaments. The black, irregular scars on apples are caused by the spilocaea fructigena.