By H. Marshall Ward.

In the months of April and May, the younger needle-like leaves of the Scotch pine are occasionally seen to have assumed a yellow tinge, and on closer examination this change in color, from green to yellow, is seen to be due to the development of what look like small orange colored vesicles standing off from the surface of the epidermis, and which have in fact burst through from the interior of the leaf (Fig. 31). Between these larger orange yellow vesicles the lens shows certain smaller brownish or almost black specks. Each of the vesicular swellings is a form of fungus fructification known as an aecidium, and each of the smaller specks is a fungus structure called a Spermogonium, and both of these bodies are developed from a mycelium in the tissues of the leaf. I must employ these technical terms, but will explain them more in detail shortly: the point to be attended to for the moment is that this fungus in the leaf has long been known under the name of Peridermium Pini (var. acicola, i.e., the variety which lives upon the needle-like leaves).

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Fig. 31. - To the right is a pair of leaves of the Scotch pine, with the blister-like aecidia 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 aecidia 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 (aecidia) bursting through the cortex (Fig. 31); and here, again, careful examination shows the darker smaller Spermogonia in patches between the aecidia. 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 aecidia and Spermogonia.

Fig. 32.   Blisters (aecidia) of Peridermium Pini (var. corticola)
Fig. 32. - Blisters (aecidia) of Peridermium Pini (var. corticola) on a branch of the Scotch pine: some of the aecidia 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 aecidia, 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 hyphae 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 aecidia, 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 hyphae 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 hyphae 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 aecidia 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 aecidia on trees more than twenty years or so old.

Fig. 33.   Vertical section through a very young aecidium of Peridermium Pini (var. acicola)