The North American Slime-Moulds A Descriptive List of All Species of Myxomycetes Hitherto Reported from the Continent of North America, with Notes on Some Extra-Limital Species

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INTRODUCTORY

The Myxomycetes, or slime-moulds, include certain very delicate and extremely beautiful fungus-like organisms common in all the moist and wooded regions of the earth. Deriving sustenance, as they for the most part do, in connection with the decomposition of organic matter, they are usually to be found upon or near decaying logs, sticks, leaves, and other masses of vegetable detritus, wherever the quantity of such material is sufficient to insure continuous moisture. In fruit, however, as will appear hereafter, slime-moulds may occur on objects of any and every sort. Their minuteness retires them from ordinary ken; but such is the extreme beauty of their microscopic structure, such the exceeding interest of their life-history, that for many years enthusiastic students have found the group one of peculiar fascination, in some respects, at least, the most interesting and remarkable that falls beneath our lens.

The slime-mould presents in the course of its life-history two very distinct phases: the vegetative, or growing, assimilating phase, and the reproductive. The former is in many cases inconspicuous and therefore unobserved; the latter generally receives more or less attention at the hands of the collector of fungi. The vegetative phase differs from the corresponding phase of all other plants in that it exhibits extreme simplicity of structure, if structure that may be called which consists of a simple mass of protoplasm destitute of cell-walls, protean in form and amœboid in its movements. This phase of the slime-mould is described as plasmodial and it is proper to designate the vegetative phase in any species, as the plasmodium of the species. It was formerly taught that the plasmodium is unicellular, but more recent investigation has shown that the plasmodial protoplasm is not only multinuclear but karyokinetic; its cells divide and redivide, as do the reproductive cells of plants and animals generally. Nevertheless, in its plasmodial phase, the slime-mould is hardly to be distinguished from any other protoplasmic mass, may be compared to a giant amœba, and justifies in so far the views of those systematists who would remove the slime-moulds from the domain of the botanist altogether, and call them animals. The plasmodium is often quite large. It may frequently be found covering with manifold ramifications and net-like sheets the surface of some convenient substratum for the space of several square feet.

The substance of the plasmodium has about the consistency of the white of an egg; is slippery to the touch, tasteless, and odorless. Plasmodia vary in color in different species and at different times in the same species. The prevailing color is yellow, but may be brown, orange, red, ruby-red, violet, in fact any tint, even green. Young plasmodia in certain species are colorless (as in Diderma floriforme), while many have a peculiar écru-white or creamy tint difficult to define. Not only does the color change, sometimes more than once in the course of the life history of the same species, but it may be the same for several forms, which in fruit are singularly diverse indeed, so that the mere color of the plasmodium brings small assistance to the systematist. In fact, the color depends no doubt upon the presence in the plasmodium of various matters, more or less foreign, unassimilated, possibly some of them excretory, differing from day to day.

In its plasmodial state, as has been said, the slime-mould affects damp or moist situations, and during warm weather in such places spreads over all moist surfaces, creeps through the interstices of the rotting bark, spreads between the cells, between the growth-layers of the wood, runs in corded vein-like nets between the wood and bark, and finds in all these cases nutrition in the products of organic decomposition. Such a plasmodium may be divided, and so long as suitable surroundings are maintained, each part will manifest all the properties of the whole. Parts of the same plasmodium will even coalesce again. If a piece of plasmodium-bearing wood be brought indoors, be protected from desiccation by aid of a moist dark chamber, not too warm (70° F.), the organism seems to suffer little if any injury, but will continue for days or weeks to manifest all the phenomena of living matter. Thus, under such circumstances, the plasmodium will constantly change shape and position, can be induced to spread over a plate of moist glass, and so be transferred to the stage of a microscope, there to exhibit in the richest and most interesting and abundant fashion the streaming protoplasmic currents. As just indicated, the plasmodia follow moisture, creep from one moist substance to another, especially follow nutritive substrata. They seem also to secure in some way exclusive possession. I have never seen them interfered with by hyphæ or enemies of any sort, nor do they seem to interfere with one another. Plasmodia of two common species, Hemitrichia clavata and H. vesparium are often side by side on the same substratum, but do not mix, and their perfected fruits presently stand erect side by side, each with its own characteristics, entirely unaffected by the presence of the other....