Fig. 31.—To the right is a pair of leaves of the Scotch pine, with the blister-like Æcidia a. of Peridermium Pini (var. acicola) projecting from their tissues: these blisters are orange yellow in color, and contain spores, as shown in Fig. 33. Between the blisters are the minute spermogonia, b. To the left is a small branch, killed at a a a by Peridermium Pini (var. corticola), the blister-like yellow Æcidia of the fungus being very conspicuous. (Reduced, after Hartig.)
On the younger branches of the Scotch pine, the Weymouth pine, the Austrian pine, and some others, there may also be seen in May and June similar but larger bladder-like orange vesicles (Æcidia) bursting through the cortex (Fig. 31); and here, again, careful examination shows the darker smaller Spermogonia in patches between the Æcidia. These also arise from a fungus mycelium in the tissues of the cortex, whence the fungus was named Peridermium Pini (var. corticola). 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.
Fig. 32.—Blisters (Æcidia) of Peridermium Pini (var. corticola) on a branch of the Scotch pine: some of the Æcidia have already burst at the apex and scattered their spores, b, b; the others are still intact. (Natural size, after Hess).
If we cut thin vertical sections through a leaf and one of the smallest Æcidia, and examine the latter with the microscope, it will be found to consist of a mass of spores arranged in vertical rows, each row springing from a branch of the mycelium: the outermost of these spores—i.e., those which form a compact layer close beneath the epidermis—remain barren, and serve as a kind of membrane covering the rest (Fig. 33, p). It is this membrane which protrudes like a blister from the tissues. The hyphæ of the fungus are seen running in all directions between the cells of the leaf tissue, and as they rise up and form the vertical chains of spores, the pressure gradually forces up the epidermis of the leaf, bursts it, and the mass of orange yellow powdery spores protrude to the exterior enveloped in the aforesaid membrane of contiguous barren spores. If we examine older Æcidia, it will be found that this membrane bursts also at length, and the spores escape.
Similar sections across a Spermogonium exhibit a structure which differs slightly from the above. Here also the hyphæ in the leaf turn upward, and send delicate branches in a converging crowd beneath the epidermis; the latter gives way beneath the pressure, and the free tips of the hyphæ constrict off very minute spore-like bodies. These minute bodies are termed Spermatia, and I shall say no more about them after remarking that they are quite barren, and that similar sterile bodies are known to occur in very many of the fungi belonging to this and other groups.
Sections through the Æcidia and Spermogonia on the cortex present structures so similar, except in minute details which could only be explained by lengthy descriptions and many illustrations, that I shall not dwell upon them; simply reminding the reader that the resemblances are so striking that systematic mycologists have long referred them to a mere variety of the same fungus.
Now as to the kind and amount of damage caused by the ravages of these two forms of fungus.
In the leaves, the mycelium is found running between the cells (Fig. 33, h), and absorbing or destroying their contents: since the leaves do not fail the first season, and the mycelium remains living in their tissues well into the second year, it is generally accepted that it does very little harm. At the same time, it is evident that, if very many leaves are being thus taxed by the fungus, they cannot be supplying the tree with food materials in such quantities as if the leaves were intact. However, the fungus is remarkable in this respect—that it lives and grows for a year or two in the leaves, and does not (as so many of its allies do) kill them after a few weeks. It is also stated that only young pines are badly attacked by this form: it is rare to find Æcidia on trees more than twenty years or so old.