What we are here more directly interested in is the following. A few years ago Wolff showed that if the spores from the Æcidia Peridermium Pini (var. acicola) are sown on the leaf of Senecio, the germinal hyphæ which grow out from the spores enter the stomata of the Senecio leaf, and there develop into the fungus called Coleosporium Senecionis. In other words, the fungus growing in the cortex of the pine, and that parasitic on the leaves of the groundsel and its allies, are one and the same: it spends part of its life on the tree and the other part on the herb.
Fig. 35. A spore of Peridermium Pini germinating. It puts forth the long, branched germinal hyphæ on the damp surface of a leaf of Senecio, and one of the branches enters a stoma, and forms a mycelium in the leaf: after some time, the mycelium gives rise to the uredospores and teleutospores of Coleosporium Senecionis. (After Tulasne: highly magnified.)
If I left the matter stated only in this bald manner, it is probable that few of my readers would believe the wonder. But, as a matter of fact, this phenomenon, on the one hand, is by no means a solitary instance, for we know many of these fungi which require two host plants in order to complete their life history; and, on the other hand, several observers of the highest rank have repeated Wolff's experiment and found his results correct. Hartig, for instance, to whose indefatigable and ingenious researches we owe most that is known of the disease caused by the Peridermium, has confirmed Wolff's results.
It was to the brilliant researches of the late Prof. De Bary that we owe the first recognition of this remarkable phenomenon of heterœcism—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 Rœstelia), 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 heterœcism 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.
Returning once more to the particular case in question, it is obvious that our new knowledge points to the desirability of keeping the seed beds and nurseries especially clean from groundsel and weeds of that description: on the one hand, such weeds are noxious in themselves, and on the other they harbor the Coleosporium form of the fungus Peridermium under the best conditions for infection. It may be added that it is known that the fungus can go on being reproduced by the uredospores on the groundsel plants which live through the winter.
[1] Continued from Supplement, No. 661, page 10558.