268 A TEXTBOOK OF THEORETICAL BOTANY 



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. 



We shall consider one example of the order, HelveUa elastica, though 

 in most features it differs but little, so far as is known, from a number of 

 allied species. 



Helvetia 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 Mord{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 

 (Fig. 259). 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 0-5 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 



