The appearance of the walls of apical and sub-apical regions of hyphae of predominantly 5 day cultures of Neurospora crassa, Schizophyllum commune and Phytophthora parasitica as seen with the electron microscope, employing shadowed or sectioned material, is illustrated and desCribed in detail. The appearance of untreated or control material in buffer is compared with that exposed to various single and sequential treatments with enzymes, including laminarinase, Pronase, cellulase and chitinase, as well as various chemical treatments.From these observations is inferred the co-axial distribution of polymers such as PI ,3-, PI ,6-glucans, glycoproteins, proteins, chitin and cellulose in the walls of each species. The distributions have both similarities and differences between the species. The significance of all these features for the growth, mechanical rigidity and integrity of a hypha is briefly discussed. I N T R O D U C T I O NThe growth of fungal hyphae is not well understood. One reason for this is that the molecular architecture of hyphal walls has not received sufficient attention. Chemical studies are beginning to reveal the complexity of hyphal wall components (e.g. review, Bartnicki-Garcia, I 968), while studies with enzymes have indicated that they are arranged in definite layers, e.g. in Neurospora crassa . Only a little information is available concerning wall structure as revealed by electron microscopy (e.g. review, Aronson, 1965) and this is either based mostly on residual shadowed material after drastic chemical treatments, or from thin sections which alone are not very informative.In this paper we describe the consequences of treating hyphae with enzymes, singly or successively, and then examining both shadow-cast and sectioned material with an electron microscope. The three species studied were selected because they differ in the chemistry of their cell walls, which was relatively well known from studies by other investigators. This new approach provides information on a comparative basis, concerning the distribution and arrangement of certain kinds of polymers within hyphal walls. M E T H O D SOrganisms. The organisms used were as follows : an ascornycete, Neurospora crassa, wild type, Emerson A from the Neurospora stock cultures; Schizophyllum commune, a basidiomycete, was isolated from damp plywood in Newcastle upon

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Isolations of Mycocalia denudata and M. diiriaeana were grown in plate culture and their mating systems studied. The former species is heteromictic and shows both bipolar diaphoromixis and facultative homo-heteromixis, the latter species is almost certainly homomictic. None of these mating systems have been found before in Nidulariales. It is suggested that the abundance, morphological variation and ecological tolerance of these species may be related to their kinds of mating system. Facultative homo-heteromixis in M. denudata results in spores being produced in a single peridiolum which are either homocaryotic or heterocaryotic in respect of mating-type factors. The condition is due to a dominant gene Pd which determines a precocious division of the four nuclei, formed by a normal meiosis, in each basidium. The eight nuclei so formed appear to migrate at random, in respect of the mating-type factors they carry, into the basidiospores which are each ultimately binucleate. This mechanism is not yet known in other Basidiomycetes.Isolations of M. denudata fell into two groups (I and II) between which there was complete sterility but no other constant distinguishing features could be observed between them. The occurrence of such groups is thought to be of significance in fungi and at present it seems best to regard them as comparable to sibling species. Four mating-type factors were identified amongst the six isolations of group I, seven amongst the sixteen isolations of group II. Estimates of the total numbers of mating-type factors in group I and group II are seven to eight and twelve to thirteen respectively. Thus, like other Nidulariales, M. denudata appears to have only a relatively small number of mating-type factors and the relation of this to inbreeding and dispersal mechanisms is discussed.