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 direction of growth which these fungi take forms an interesting question for study. The common mushroom, the Agaricus, the amani-tas, and other central stemmed species grow usually in an upright fashion; that is, the stem is erect. The cap then, when it expands, stands so that it is parallel with the surface of the earth. Where the cap does not fully expand, as in the campanulate forms, the pileus is still oriented horizontally, that is, with the gills downward. Even in such species, where the stems are ascending, the upper end of the stem curves so that the cap occupies the usual position with reference to the surface of the earth. This is beautifully shown in the case of those plants which grow on the side of trunks or stumps, where the stems could not well grow directly upward without hugging close to the side of the trunk, and then there would not be room for the expansion of the cap. This is well shown in a number of species of Mycena.
In those species where the stem is subcentral, i. e., set toward one side of the pileus, or where it is definitely lateral, the pileus is also expanded in a horizontal direction. From these lateral stemmed species there is an easy transition to the stemless forms which are sessile, that is, the shelving forms where the pileus is itself attached to the trunk, or other object of support on which it grows.
Agaricus campestris. Spore print. (Natural size.)
Where there is such uniformity in the position of a member or part of a plant under a variety of conditions, it is an indication that there is some underlying cause, and also, what is more important, that this position serves some useful purpose in the life and well being of the plant. We may cut the stem of a mushroom, say of the Agaricus campestris, close to the cap, and place the latter, gills downward, on a piece of white paper. It should now be covered securely with a small bell jar, or other vessel, so that no currents of air can get underneath. In the course of a few hours myriads of the brown spores will have fallen from the surface of the gills, where they are borne. They will pile up in long lines along on either side of all the gills and so give us an impression, or spore print, of the arrangement of the gills on the under side of the cap as shown in Fig. 12. A white spore print from the smooth lepiota (L. naucina) is shown in Fig. 13. This horizontal position of the cap then favors the falling of the spores, so that currents of air can scatter them and aid in the distribution of the fungus.
But some may enquire how we know that there is any design in the horizontal position of the cap, and that there is some cause which brings about this uniformity of position with such entire harmony among such dissimilar forms. When a mushroom with a comparatively long stem, not quite fully matured or expanded, is pulled and laid on its side, or held in a horizontal position for a time, the upper part of the stem where growth is still taking place will curve upward so that the pileus is again brought more or less in a horizontal position.
Lepiota naucina. Spore print. (Natural size.)
In collecting these plants they are often placed on their side in the collecting basket, or on a table when in the study. In a few hours the younger, long stemmed ones have turned upward again. The plant shown in Fig. 14 (Amanita phalloides) was placed on its side in a basket for about an hour. At the end of the hour it had not turned. It was then stood upright in a glass, and in the course of a few hours had turned nearly at right angles. The stimulus it received while lying in a horizontal position for only an hour was sufficient to produce the change in direction of growth even after the upright position had been restored. This is often the case. Some of the more sensitive of the slender species are disturbed if they lie for only ten or fifteen minutes on the side. It is necessary, therefore, when collecting, if one wishes to keep the plants in the natural position for photographing, to support them in an upright position when they are being carried home from the woods.
Amanita phalloides. Plant turned to one side by directive force of gravity, after having been placed in a horizontal position. (Natural size.)
The cause of this turning of the stem from the horizontal position, so that the pileus will be brought parallel with the surface of the earth, is the stimulus from the force of gravity, which has been well demonstrated in the case of the higher plants. That is, the force which causes the stems of the higher plants to grow upward also regulates the position of the cap of the pileated fungi. The reason for this is to be seen in the perfection with which the spores are shed from the surfaces of the gills by falling downward and out from the crevices between. The same is true with the shelving fungi on trees, etc., where the spores readily fall out from the pores of the honeycombed surface or from between the teeth of those sorts with a spiny under surface. If the caps were so arranged that the fruiting surface came to be on the upper side, the larger number of the spores would lodge in the crevices between the extensions of the fruiting surface. Singularly, this position of the fruiting surface does occur in the case of one genus with a few small species.
Interesting examples of the operation of this law are sometimes met with in abandoned coal mines, or more frequently in the woods. In abandoned mines the mushrooms sometimes grow from the mycelium which spreads out on the rock roof overhead. The rock roof prevents the plant from growing upright, and in growing laterally the weight of the plant together with the slight hold it can obtain on the solid rock causes it to hang downward. The end of the stem then curves upward so that the pileus is brought in a horizontal position. I have seen this in the case of Coprinus micaceus several times. In the woods, especially in the case of the perennial shelving fungi, interesting cases are met with. Figure 15 illustrates one of these peculiar forms of Polyporus (Fomes) applanatus. This is the species so often collected as a "curio," and on account of its very white under surface is much used for etching various figures. In the figure the larger cap which is horizontal represents the position of the plant when on the standing maple trunk. When the tree fell the shelf was brought into a perpendicular position. The fungus continued to grow, but its substance being hard and woody it cannot turn as the mushroom can. Instead, it now grows in such a way as to form several new caps, all horizontal, i. e., parallel with the surface of the earth, but perpendicular to the old shelf. If the page is turned one-fourth way round the figure will be brought in the position of the plant when it was growing on the fallen log.
Polyporus applanatus. From this view the larger cap is in the normal position in which it grew on the standing tree. Turn one-fourth way round to the right for position of the plant after the tree fell. (1/6 natural size.)
Plate 3, Figure 16
Daedalea ambigua. Upper right-hand shows normal plant in normal position when on tree. Upper left-hand shows abnormal plant with the large cap in normal position when growing on standing tree. Lower plant shows same plant in position after the tree fell, with new caps growing out in horizontal direction. (Lower plant 1/2 natural size.)
Another very interesting case is shown in the ambiguous trametes (Trametes ambigua), a white shelving fungus which occurs in the Southern States. It is shown in Fig. 16. At the upper right hand is shown the normal plant in the normal position. At the upper left hand is shown an abnormal one with the large and first formed cap also in the normal position as it grew when the tree was standing. When the tree fell the shelf was on the upper side of the log. Now numerous new caps grew out from the edge as shown in the lower figure, forming a series of steps, as it were, up one side and down the other.