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 amoeboid 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 amoeba, 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 ecru-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.