or fruit, and the proportion of the size of the fruit to the mycelum or threads, have served as the basis of a classification of the Fungi. The mushroom, the puff-ball, the smut, the mildew, the truffle, and mould, are familiar examples of the different orders of Fungi. This classification, like all similar attempts, suffices, so long as types so different as a mushroom and mildew are compared. What are commonly collected as mushrooms are only the fruit of the fungus Agaricus: the greater part of the mildew examined show only the mycelium or threads of the fungus Botrytis. The mushrooms belong to the order Hymenomycetes, so called from the hymenium. Or part supporting the fruit, being so prominent, the threads being inconspicuous. The mildew (Fig. 105) illustrates the Hyphomycetes, which derive their name from the Hyphi, or threads, being so much developed, the fruit dropping off
The difficulty of classification arises from the fact that from time to time individuals are discovered which do not present such striking contrasts as the mushroom and mildew, their characters being so little defined as to make it impossible to say to what groups they belong. All such intermediate forms, the source of so much trouble in the arrangement of an herbarium, are most important proofs of the truth of the theory of the gradual transformation of plants. Not only is it true that, in the Fungi the orders pass insensibly into each other, but there are also forms of which it is doubtful whether they are Fungi, some botanists still regarding them rather as Algae. Thus, the Peronospora (Fig. 106), differing from Botrytis (mildew) in its continuous cells, the partitions of Botrytis (Fig. 105) being absent, is, according to Prof. Haeckel, a transitional form which links the Algse through Vaucheria (Fig. 107) with the Fungi, though the Peronospora is usually regarded as a Fungus, it having no chlorophyll. The Achlya, sometimes called Saprolegnia, formerly considered an Alga, seems to be only
