It was to the brilliant researches of the late Prof. De Bary that we owe the first recognition of this remarkable phenomenon of _heteroecism_--i.e., the inhabiting more than one host--of the fungi. De Bary proved that the old idea of the farmer, that the rust is very apt to appear on wheat growing in the neighborhood of berberry bushes, was no fable; but on the contrary, that the yellow _Æcidium_ on the berberry is a phase in the life history of the fungus causing the wheat rust. Many other cases are now known, e.g.., the _Æcidium abietinum_, on the spruce firs in the Alps, passes the other part of its life on the rhododendrons of the same region. Another well known example is that of the fungus _Gymnosporangium_, which injures the wood of junipers. Oersted first proved that the other part of its life is spent on the leaves of certain Rosaceæ, and his discovery has been repeatedly confirmed. I have myself observed the following confirmation of this. The stems of the junipers so common in the neighborhood of Silverdale (near Morecambe Bay) used to be distorted with _Gymnosporangium_, and covered with the _teleutospores_ of this fungus every spring: in July all the hawthorn hedges in the neighborhood had their leaves covered with the Æcidium form (formerly called Roestelia), and it was quite easy to show that the fungus on the hawthorn leaves was produced by sowing the _Gymnosporangium_ spores on them. Many other well established cases of similar heteroecism could be quoted. But we must return to the _Peridermium Pini_. It will be remembered that I expressed myself somewhat cautiously regarding the _Peridermium_ on the leaves (var. _acicola_). It appears that there is need for further investigations into the life history of this form, for it has been thought more than probable that it is not a mere variety of the other, but a totally different species. Only so lately as 1883, however, Wolff succeeded in infecting the leaves of _Senecio_ with the spores of _Peridermium Pini_ (_acicola_), and developing the _Coleosporium_, thus showing that both the varieties belong to the same fungus. It will be seen from the foregoing that in the study of the biological relationships between any one plant which we happen to value because it produces timber and any other which grows in the neighborhood there may be (and there usually is) a series of problems fraught with interest so deep scientifically, and so important economically, that one would suppose no efforts would be spared to investigate them: no doubt it will be seen as time progresses that what occasionally looks like apathy with regard to these matters is in reality only apparent indifference due to want of information.