Phylum Thallophyta - The Algae - Ascomycetes - Helvellales
The Helvellales are large Ascomycetes usually growing saprophytically on dead wood, in which the fruiting body is fleshy and in which the hymenium is spread over the external surface of the greater part of hte fruiting body and covered at first by a membrane or veil. It seems probable that they have
originated by the eversion of an apothecium which was already provided with a stalk. By development on a convex surface the hymenial layer is increased in area, and this is further increased by folding of the surface which may in some genera result in the formation of a very complex, convoluted head supported on a sterile stalk. Stages in this evolution can be traced in this order and, to a lesser extent, in certain Pezizales, with the result that it is not easy to draw a sharp line between them, and some authorities prefer to merge the Helvellales into the Pezizales. This view has certainly much to recommend it.
Consider one example of the order, Helvella elastica, though in most features it differs but little, so far as is known, from a number of allied species.
Helvella elastica
This Fungus occurs quite commonly in woods throughout Great Britain in company with a number of other species which differ mainly in colour and in the degree of fragmentation of the cap. Most of the species are edible and constitute the Morels of the mycophagist, though the true Morel (Morchella esculenta) belongs to, a distinct genus.
Helvella elastica is usually white, though sometimes tinted with pale pink or grey. It is clearly divisible into a stalk or stipe and a cap or pileus. The former is very firm, somewhat swollen below and deeply grooved. The cap is thin, somewhat lobed, with the lobes at first more or less attached to the stem, later becoming free. The Fungus occurs in moist places and is not uncommon in summer and autumn.
The mycelium consists of long, sparsely branched, interwoven hyphae, which show numerous anastomoses and may contain from two to many nuclei in each cell.
DEVELOPMENT OF THE FRUITING BODY
Here and there, by more vigorous growth, little knots of hyphae appear the cells of which are thicker and shorter and the filaments more branched. These represent early stages in the formation of fruiting bodies.
Slightly later the main tissues of the fruiting body can be recognized, and in a knot of hyphae about o· 5 mm. in diameter it is possible to recognize a short thick stem passing insensibly into the mycelium below and surmounted by a bulbous cap, slightly larger in diameter than the stem. The whole structure is covered by a membrane. The membrane consists of two layers, an inner tissue termed the palisade layer, which is made up of the club-shaped ends of hyphae arranged close together to form an even surface. Outside this is an envelope formed from certain hyphae of the palisade layer which grow out beyond its surface, turn at right angles and grow over it. This envelope is transitory, breaking up at an early stage and then degenerating.
In the region of the cap paraphyses appear at quite an early stage. They force their way between the cells of the palisade layer, breaking the envelope as they do so. In this vvay a hymeniallayer becomes differentiated on the surface of the cap.
THE ASCOGENOUS HYPHAE
So far as is known no sex organs are produced. At a fairly early stage, however, some of the hyphal threads form a mat of tissue a short way below the paraphyses. These cells are multinucleate and considerably thicker than the ordinary hyphae. It is from these cells that asci are produced by
the vertical growth of branches from this sub-hymeniallayer. The branches so formed divide repeatedly and penetrate between the paraphyses. The end cells of these branches contain deeply staining protoplasm and usually only two nuclei. These end cells curve over and form hooks, and in each hook the two nuclei divide into four. This is followed by the septation of the hook into a row of three cells. One nucleus enters the terminal cell, one the basal, and two remain in the middle cell. This middle cell enlarges and becomes the ascus, in which the two nuclei fuse. Meiosis of the fusion nucleus follows and by a further division eight nuclei are formed, around each of which an ascospore is organized.
These ascospores are oval in shape and brovvn in colour. They are ejected forcibly from the apex of the ascus, and occasionally a tiny cloud of spores may be observed around a fruiting body; when large numbers of ascospores are being expelled together.
It will be noted that the two nuclei which fuse in the ascus have been produced apogamously from vegetative hyphae and there is no evidence that any sexual apparatus is developed. By analogy with other types it may well be supposed that a hyphal fusion occurs somewhere prior to the production of a fruiting body, but work along this line has not yet been done.
