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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. Many other well established cases of similar heteroecism could be quoted. But we must return to the Peridermium Pini.
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 heteroecism i.e., the inhabiting more than one host of the fungi.
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. A spore of Peridermium Pini germinating.
It has long been known that the leaves of these plants, and of several allied species, are attacked by a fungus, the mycelium of which spreads in the leaf passages, and gives rise to powdery masses of orange yellow spores, arranged in vertical rows beneath the stomata: these powdery masses of spores burst forth through the epidermis, but are not clothed by any covering, such as the Æcidia of Peridermium Pini, for instance.
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.
Between the blisters are the minute spermogonia, b. It is thus seen that the fungus Peridermium Pini was regarded as a parasite of pines, and that it possessed two varieties, one inhabiting the leaves and the other the cortex: the "varieties" were so considered, because certain trivial differences were found in the minute structure of the Æcidia and Spermogonia.
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