How such tender structures as the Myxomycetes, which are destitute of every kind of external protection, are enabled to carry on their existence, the knowledge of the remarkably delicate reaction of their plasmodia under external influences prepares us to understand. Plasmodia, which are not yet ripe for reproduction, are kept in the moist substratum by their peculiar affection for moisture and utter dislike of the light. But within the darkness and moisture of the substratum the plasmodia do not necessarily remain in one place, for the differences in the chemical composition of the substratum cause continual migrations. Nothing more remarkable can be said of the plasmodia than that they have a wonderful faculty of avoiding harmful substances, and, traversing the substratum in all directions, of taking up the materials they require for food and growth. When, however, their internal changes have advanced so far that the plasmodia are approaching the fructifying condition, they are brought by their dislike for moisture, which now sets in, from the moist ground of forest or wood which they affect to the surface, where they creep up various upright objects, frequently not doing more than forming rigid reproductive capsules at some height from the ground. If, however, the substratum becomes gradually colder, as is the case in autumn, a change which sets in at the surface moving downwards, then the plasmodia migrate into deeper regions still having a higher temperature; but when the cooling proceeds very gradually, which especially happens in large tan-heaps, the plasmodia may in their migration attain considerable depths, where they then change into sclerotia, which are hard tuberous substances, resembling the tubers and bulbs of flowering plants. If, however, the temperature begins to ascend, the sclerotia again germinate, and movement takes place from the deeper and cooler parts to the upper already named.