Fossil plants, Vol. 1: [A text-book] for students of botany and geology

by A. C. Seward

Myxomycetes Mangini Ren.[400] It is not uncommon to find distinct traces of original or secondary cell-contents in well preserved petrified plant-tissues. There is often a difficulty, however, in distinguishing between the true cell-contents and the cells of some parasitic or saprophytic intruder. In some petrified corky tissue in a silicified nodule from the Permo-Carboniferous beds of Autun, Renault has recently discovered what he believes to be traces of a Myxomycetous plasmodium. The cork-cells would be without protoplasmic contents of their own, and their cavities contain a number of fine strands stretching from the cell-walls in different directions and uniting in places as irregular or more or less spherical masses. The drawings given by Renault of these irregular reticulated structures with scattered patches of what may possibly be petrified plasmodial protoplasm bear a striking resemblance to the plasmodium of a Myxomycete. A figure of the capillitium of a species of Leocarpus figured by Schröter[401] in his account of the Myxomycetes in Engler and Prantl’s work is very similar to that of Renault’s ‘plasmodium.’

It is by no means inconceivable that the Myxomycetes Mangini may be correctly referred to this group, but the wisdom of assigning a name to such structures may well be questioned.

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The other two examples call for little notice. Messrs Cash and Hick[402] in a paper on fossil fungi from the Coal-Measures refer to some small spherical bodies as possibly the spores of a Myxomycete. They might be referred equally well to numerous other organisms.

Göppert and Menge[403] in their monograph on plants in the Baltic Tertiary Amber, express the opinion that an ill-defined tangle of threads which they figure may be a Myxomycete.

It would serve no useful purpose to quote other instances of possible representatives of fossil Mycetozoa; but the consideration of the above examples may serve to emphasize the desirability of refraining from converting a possibility into an apparently recognised fact by the application of definite generic and specific names.

VI. FUNGI.

The most striking difference between the fungi and algae is the absence of chlorophyll in the former, and the consequent inability of fungi to manufacture their organic compounds from inorganic material. Fungi live therefore either as parasites or saprophytes, and as the same species may pass part of its life in a living host to occur at another stage of its development as a saprophyte, it is impossible to distinguish definitely between parasitic and saprophytic forms. The vegetative body of a fungus, that is the portion which is concerned with providing nourishment and preparing the plastic food-substance for the reproductive organs, is known as the mycelium. It consists either of a single and branched tubular cell known as a hypha, or of several hyphae or thread-like elements (filamentous fungi). The hyphal filaments may be closely packed together and form a felted mass of compact tissue, which in cross section closely simulates the parenchyma of the higher plants. This pseudoparenchymatous form of thallus is particularly well illustrated by the so-called sclerotia; these are sharply defined and often tuberous masses of hyphal tissue covered by a firm rind and containing supplies of food in the inner hyphae. They are able to remain in a quiescent state for some time, and to resist unfavourable conditions until germination and the formation of a new individual take place. The reproductive structures assume various forms; in some of the simpler fungi (Phycomycetes) sexual organs occur, as in the parallel group of Siphoneae among the algae, but in the higher fungi the reproduction is usually entirely asexual. An interesting case has recently been recorded among the more highly differentiated fungi in which distinct sexuality has been established[404]. In addition to the reproductive organs, such as oogonia and antheridia, the asexual cells or spores are borne either in special sporangia, or they occur as exposed conidia on supporting hyphae or conidiophores. Thick-walled and resistant resting-spores of various forms are also met with.

Without going into further details we may very briefly refer to the larger subdivisions of this group of Thallophytes.

PHYCOMYCETES. ZYGOMYCETES, OOMYCETES, including Chytridiaceae, &c.	Mycelium usually consisting of a single cell. Reproduction by means of conidia, and in many cases also by the conjugation of two similar hyphae or by the fertilisation of an egg-cell contained in an oogonium.
MESOMYCETES, including the Sub-classes HEMIASCI and HEMIBASIDII.	Intermediate between the Phycomycetes and the higher fungi. Multicellular hyphae. No sexual organs.
MYCOMYCETES. including the Sub-classes ASCOMYCETES and BASIDIOMYCETES.	Septate vegetative mycelium. No sexual reproduction—as a general rule. Asexual conidia and other forms of spores. In the Ascomycetes the spores are found in characteristic club-shaped cases or asci; in the Basidiomycetes the spores are borne on special branches from swollen cells known as basidia. The sporophore or spore-bearing body in this group may attain a considerable size (e.g. Agaricus, Polyporus, &c.) and exhibit a distinct internal differentiation.