It is not merely that the potato-fields afford plenty of food for the fungus, and that the dull weather causes the tissues to be surcharged with moisture, owing to diminished transpiration, but the mists and dew--to say nothing of actual rain and the flapping of wet leaves--favour the germination and spread of the zoospores throughout the field. Whether the dull light also favours the accumulation of sugars in the tissues, and the partial etiolation of the latter implies less resistance to the entering hyphae, may be passed over here, but in any case it is clear that we have several factors of the non-living environment here favouring the parasite and not improving the chances of the host, even if they do not directly disfavour it. As another instance I will take the Larch-disease, which is due to the ravages of a Peziza (_Dasyscypha Willkommii_) the hyphae of which obtain access by wounds to the sieve-tubes and cambium of the stem, and gradually kill them over a larger and larger area and so ring the tree, with the symptoms of canker described below. Now the Larch fungus is also to be found on trees in their Alpine home, but there it does very little damage and never becomes epidemic except in certain sheltered regions near lakes and in other damp situations. How then are we to explain the extensive ravages of the Larch disease over the whole of Europe during the latter half of this century? The extensive planting, providing large supplies for the fungus, does not suffice to explain it, because there are large areas of pure Larch in the Alps which do not suffer. In its mountain home the Larch loses its leaves in September and remains quiescent through the intensely cold winter, until May. Then come the short spring and rapid passage to summer, and the Larch buds open with remarkable celerity when they do begin--_i.e._ when the roots are thoroughly awakened to activity. Hence the tender period of young foliage is reduced to a minimum, and any agencies which can only injure the young leaves and shoots in the tender stage must do their work in a few days, or the opportunity is gone, and the tree passes forthwith into its summer state. In the plains, on the contrary, the Larch begins to open at varying dates from March to May, and during the tardy spring encounters all kinds of vicissitudes in the way of frosts and cold winds following on warm days which have started the root-action--for we must bear in mind that the roots are more easily awakened after our warmer winters than is safe for the tree.