An evolutionary study in Agarics: Collybia apalosarca and the veils

Abstract

The structure and development of the fruit body of Collybia apalosarca B. & Br. is described in detail as a multifilamentous soma formed by the apical growth and ramification of hyphae with modification of the cells on cessation of growth. A specific description is given, based on Malayan material, and a new variety perstipitata and a new form radicans are proposed. In the mature fruit body all external and internal surfaces between tissues are bounded by palisades of free hyphal ends the terminal cells of which have characteristic shapes. On the pileus there is a thick gelatinous layer derived from the blematogen and bounded by an external and an internal palisade. Development is indirect and hemiangiocarpic. A short, loosely corticated primordial shaft is first formed. At the apex of the shaft the limb arises slightly endogenously. There is a rudimentary blematogen with a rudimentary marginal veil reinforced by the outgrowth and anastomosis of hyphae from the margin of the pileus and the surface of the stem, but there is no ring or cortina. The gill cavity is more or less closed at first. Development is strictly acropetal with apical growth of the parts of the fruit body followed by intercalary growth at the surfaces by sympodial branching, which multiplies the tissue-elements and compacts the palisades, and finally with inflation of the medullary hyphae causing expansion of the primordium. The base of the primordial shaft becomes the base of the stem; the body of the shaft becomes the shank of the stem; the apex becomes the centre of the pileus while the limb is formed by marginal growth from the apex of the shaft and the gills by marginal growth in centrifugal lines on the proximal side of the limb. Inflation of the hyphae proceeds acropetally from the base of the stem and reaches a maximum in the supra-annular portion of the stem and the central portion of the limb on the night of expansion. In the typical form the infra-annular portion of the stem is enlarged into a supporting disc which does not elongate; in var. perstipitata the stem elongates throughout and there is no disc. Biologically, the fruit bodies belong to Buller's Armillaria subtype By correlating in different fruit bodies the average elongation of the cells in the mature stem with the length of the mature stem, it is shown that the great variations in size of the fruit body (from 3 mm. to 9 cm. high) are due either to juvenescence or to differences in the inflation of the cells. The arrangement and number of the gills are also correlated with the size of the fruit body. Those with a thick stem apex generally have more primaries than those with a thin stem apex, and those with a wide limb more ranks than those with a narrow limb, from which conclusion additional proof is given that small fruit bodies are generally juvenescent. C. apalosarca is intermediate in many respects between C. radicata and Armillaria mucida . The three species are compared as fully as possible. The structure of the mature fruit body of Collybia radicata , as it occurs in Malaya, is described and the relevant facts about Armillaria mucida are summarised. In all three species the construction of the fruit body is essentially the same. But Collybia radicata has a single palisade on the pileus, without a gelatinous layer, and is evidently gymnocarpic. Armillaria mucida has evidently the same structure of the pileus as Collybia apalosarca but is angiocarpic with an inferior annulus. These and other minor differences are evaluated by certain general propositions concerning the evolution of agarics. The three species are considered specialised offshoots of two hypothetical ancestral species: while the first hypothetical is probably extinct, the second may be found in tropical America or the south temperate zone. The morphology of the marginal veil is reconsidered. The investigation of C. apalosarca shows that there is really no antagonism between de Bary's early account and that of the modern American school, since each is based on a common developmental process in agarics and is partly correct. The formation of the marginal veil may depend on the outgrowth and anastomosis of hyphae from the margin of the pileus and the surface of the stem, as de Bary held, as well as on the presence of a blematogen, as the American school maintains: both factors may be equally important, or the one or the other ascendant. It is shown further how the presence of the blematogen depends on the endogenous origin of the limb and how the marginal veil is related to the universal and partial veils.