This section is from the book "Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc.", by George Francis Atkinson. Also available from Amazon: Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc..
The genus Lactarius is easily distinguished from nearly all the other agarics by the presence of a milky or colored juice which exudes from wounded, cut, or broken places on the fresh plant. There are a few of the species of the genus Mycena which exude a watery or colored juice where wounded, but these are easily told from Lactarius because of their small size, more slender habit, and bell-shaped cap. By careful observation of these characters it is quite an easy matter to tell whether or not the plant at hand is a Lactarius. In addition to the presence of this juice or milk as it is commonly termed, the entire plant while firm is quite brittle, especially the gills. There are groups of rounded or vesiculose cells intermingled with thread-like cells in the substance of the cap. This latter character can only be seen on examination with the microscope. The brittleness of the plant as well as the presence of these groups of vesiculose cells is shared by the genus Russula, which is at once separated from Lactarius by the absence of a juice which exudes in drops.
In determining the species it is a very important thing to know the taste of the juice or of the fresh plant, whether it is peppery, or bitter, or mild, that is, tasteless. If one is careful not to swallow any of the juice or flesh of the plant no harm results from tasting any of the plants, provided they are not tasted too often during a short time, beyond the unpleasant sensation resulting from tasting some of the very "hot" kinds. It is important also to know the color of the milk when it first exudes from wounds and if it changes color on exposure to the air. These tests of the plant should be made of course while it is fresh. The spores are white, globose or nearly so in all species, and usually covered with minute spiny processes. There are a large number of species. Peck, 38th Report, N. Y. State Mus., pp. 111-133, describes 40 American species.
Lactarius volemus Fr. Edible. - This species is by some termed the orange brown lactarius because of its usual color. It was probably termed Lactarius volemus because of the voluminous quantity of milk which exudes where the plant is broken or bruised, though it is not the only species having this character. In fresh, young plants, a mere crack or bruise will set loose quantities of the milky juice which drops rapidly from the plant. The plant is about the size of Lactarius delicio-sus and occurs in damp woods, where it grows in considerable abundance from July to September, several usually growing near each other. The pileus is convex, then expanded, often with a small elevation (umbo) at the center, or sometimes plane, and when old a little depressed in the center, smooth or somewhat wrinkled. The cap is dull orange or tawny, the shade of color being lighter in some plants and darker in others. The flesh is white and quite firm. The gills are white, often tinged with the same color as the pileus, but much lighter; they are adnate or slightly decurrent. The stem is usually short, but varies from 3-10x 1-2 cm. It is colored like the pileus, but a lighter shade.
Lactarius corrugis. Showing corrugated cap, and white milk exuding. Dark tawny brown, gills orange brown (natural size, often larger). Copyright.
The milk is white, abundant, mild, not unpleasant to the taste, but sticky as it dries. This plant has also long been known as one of the excellent mushrooms for food both in Europe and America. Peck states that there are several plants which resemble Lactarius volemus in color and in the milk, but that no harm could come from eating them. There is one with a more reddish brown pileus, Lac-tarius rufus, found sparingly in the woods, but which has a very peppery taste. It is said by some to be poisonous.
Lactarius corrugis Pk. Edible. - This species occurs with Lactarius volemus and very closely resembles it, but it is of a darker color, and the pileus is more often marked by prominent wrinkles, from which character the plant has derived its specific name. It is perhaps a little stouter plant than L. volemus, and with a thicker cap. The surface of the pileus seems to be covered with a very fine velvety tomentum which glistens as the cap is turned in the light. The gills are much darker than in L. volemus. The plants are usually clearly separated on account of these characters, yet there are occasionally light colored forms of L. corrugis which are difficult to distinguish from dark forms of L. volemus, and this fact has aroused the suspicion that corrugis is only a form of volemus.
The milk is very abundant and in every respect agrees with that of L. volemus. I do not know that any one has tested L. corrugis for food. But since it is so closely related to L. volemus I tested it during the summer of 1899 in the North Carolina mountains. I consider it excellent. The methods of cooking there were rather primitive. It was sliced and fried with butter and salt. It should be well cooked, for when not well done the partially raw taste is not pleasant. The plant was very abundant in the woods, and for three weeks an abundance was served twice a day for a table of twelve persons. The only disagreeable feature about it is the sticky character of the milk, which adheres in quantity to the hands and becomes black. This makes the preparation of the plant for the broiler a rather unpleasant task.
Figure 118 is from plants (No. 3910, C. U. herbarium) collected in the woods at Blowing Rock, during September, 1899. Just before the exposure was made to get the photograph several of the plants were wounded with a pin to cause the drops of milk to exude, as is well shown in the illustration.
The dark color of the lamellae in L. corrugis is due to the number of brown cystidia or setae, in the hymenium, which project above the surface of the gills, and they are especially abundant on the edge of the gills. These setae are longfusoid, 80-120 x 10-12 µ. The variations in the color of the gills, in some plants the gills being much darker than in others, is due to the variations either in the number of these setae or to the variation in their color. Where the cystidia are fewer in number or are lighter in color the lamellae are lighter colored. Typical forms of Lactarius volemus have similar setae, but they are very pale in color and not so abundant over the surface of the gills. In the darker forms of L. volemus the setae are more abundant and darker in color, approaching those found in L. corrugis. These facts, supported by the variation in the color of the pileus in the two species and the variations in the rugosities of the pileus, seem to indicate that the two species are very closely related.