Class 2. Mesomycetes.
The Mesomycetes are intermediate forms between the Phycomycetes and the Higher Fungi. In the vegetative organs, and in the multicellular hyphæ, they resemble the Higher Fungi; the methods of reproduction, however, show the characters of the Phycomycetes, namely sporangia and conidiophores of varying size and with varying number of spores; definite and typically formed asci and basidia are not present. Sexual reproduction is wanting. The Hemiasci are transitional between the Phycomycetes and the Ascomycetes, the Hemibasidii (Brand-Fungi) form the transition to the Basidiomycetes.
Sub-Class 1. Hemiasci.
The Hemiasci are Fungi with sporangia which, although resembling asci, yet have not, however, a definite form and a definite number of spores. Besides endospores, conidia, chlamydospores and oidia are found.
Order 1. Ascoideaceæ. Ascoidea rubescens forms irregular, reddish-brown masses in the sap issuing from felled Beeches. It has free sporangia, which resemble asci in their structure, in the development and ejection, and in the definite shape and size of the spores. The formation of the sporangia takes place when the nutriment is nearly exhausted, and resembles that of the conidia, since they are developed from the end of a hypha which enlarges, and the swelling becomes separated by a transverse wall. Within the sporangia numerous spores of a cap-like form are developed, which are set free through an opening at the apex. Sporangia are formed successively at the apex of the same hypha, the second commencing to develope as the first is dehiscing. Conidia and sporangia are not formed simultaneously; the former may be considered as closed sporangia.
Order 2. Protomycetaceæ. Protomyces pachydermus causes hard swellings on the stems and leaf-stalks of the Cichorieæ (Taraxacum, etc.). These swellings consist of chlamydospores (resting-spores), which germinate and become free, ascus-like sporangia, with numerous small spores. In nutritive solutions the chlamydospores form conidia with yeast-like buddings. P. macrosporus on Ægopodium, and other Umbelliferæ.
Order 3. Thelebolaceæ. Thelebolus stercoreus, is found on the dung of deer, hares, and rabbits, and has closed sporangia, which resemble asci in their shape and regular construction, and in the ejection of spores. The covering encloses only one sporangium, even where the sporangia arise close together.
This order, by reason of the covering of the sporangia, forms the transition from the Hemiasci to the Carpoasci, while the two first supply an intermediate step to the Exoasci.
Sub-Class 2. Hemibasidii, Brand-Fungi.
The Brand-Fungi (also known as Ustilagineæ) are Fungi with basidia-like conidiophores, which, however, have not yet advanced to a definite form or number of conidia. They are true parasites, whose mycelium spreads itself in the intercellular spaces of Flowering plants. The mycelium is colourless, quickly perishable, has transverse walls at some distance from each other (Fig. [96]), and sends out haustoria into the cells of the host-plant.