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 _Æcidium_, 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). [Illustration: 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_. [Illustration: 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).]