The tubes are small, and the walls thin and delicate, and are sometimes much torn, lacerated, and irregular. When the mycelium has grown in the interior of a log for a number of years it tends to grow in sheets along the line of the medullary rays of the wood or across in concentric layers corresponding to the summer wood. Also as the wood becomes more decomposed, cracks and rifts appear along these same lines. The mycelium then grows in abundance in these rifts and forms broad and extensive sheets which resemble somewhat chamois skin and is called "punk." Similar punk is sometimes formed in conifers from the mycelium of _Fomes pinicola_. [Illustration: PLATE 70, FIGURE 185.--Polyporus sulphureus. Caps joined in a massive tubercle (1/2 natural size).] _Polyporus sulphureus_ has long been known as an edible fungus, but from its rather firm and fibrous texture it requires a different preparation from the fleshy fungi to prepare it for the table, and this may be one reason why it is not employed more frequently as an article of food. It is common enough during the summer and especially during the autumn to provide this kind of food in considerable quantities. [Illustration: PLATE 71, FIGURE 186.--Polyporus brumalis. Cap and stem brown, tubes white. Lower three plants natural size, upper one enlarged twice natural size. Copyright.] =Polyporus brumalis= (Pers.) Fr.--This pretty plant is found at all seasons of the year, and from its frequency during the winter was named _brumalis_, from _bruma_, which means winter. It grows on sticks and branches, or on trunks. It usually occurs singly, sometimes two or three close together. The plants are 3--6 cm. high, the cap 2--6 cm. in diameter, and the stem is 3--6 mm. in thickness. The =cap= is convex, then plane, and sometimes depressed at the center or umbilicate. When young it is somewhat fleshy and pliant, then it becomes tough, coriaceous, and hard when dry. During wet weather it becomes pliant again. Being hard and firm, and tough, it preserves long after mature, so that it may be found at any season of the year. The cap is smoky in color, varying in shade, sometimes very dark, almost black, and other specimens being quite light in color. The surface is hairy and the margin is often fimbriate with coarse hairs. The =stem= is lighter, hairy or strigose. The =tubes= are first white, then become yellowish. The tubes are very regular in arrangement. Figure 186 represents well this species, three plants being grouped rather closely on the same stick; two show the under surface and one gives a side view. The upper portion of the plate represents two of the plants enlarged, the three lower ones being natural size. The plant is very common and widely distributed over the world. Those illustrated in the plate were collected at Ithaca. This species is too tough for food.