V. MYXOMYCETES (MYCETOZOA).
This class of organisms affords an interesting example of the impossibility of maintaining a hard and fast line between the animal and plant kingdom. Zoologists and Botanists usually include the Myxomycetes[399] in the text-books of their respective subjects, and the name Animal-fungi which has been applied to these organisms expresses their dual relationship. They constitute one of three groups which we may include in that intermediate zone or ‘buffer-state’ between the two kingdoms. From a palaeobotanical point of view the Myxomycetes are of little interest, but a very brief reference may be made to them rather for the sake of avoiding unnecessary incompleteness in our classification than from their importance as possible fossils.
They are organisms without chlorophyll, consisting of a naked mass of protoplasm, known as the plasmodium, which may attain a size of several inches. Such plasmodia creep over the surface of decaying organic substrata, and in forming their asexual reproductive cells they are converted into somewhat complex fruits containing spores. The spores produce motile swarm-cells, which eventually coalesce together to form a new plasmodium.
A few examples of fossil Myxomycetes have been recorded from the Palaeozoic and more recent formations, but none of them are entirely beyond suspicion. We may mention three examples of fossils referred to this group, but only one of these is entitled to serious consideration.
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.