Harry Marshall Ward and Vernon Herbert Blackman

“ Physiologically, any cell or group of cells separated off from a hypha or unicellular fungus, and capable of itself growing out—germinating—to reproduce the fungus, is a spore; but it is evident that so wide a definition does not exclude the ordinary vegetative cells of sprouting fungi, such as yeasts, or small sclerotium like cell-aggregates of forms like Coniothecium. ”

“ The most reasonable interpretation of the spermatia is that they are abortive male cells. They have never been found to cause infection, and they have not the characters of conidia; the large size of their nuclei, the reduction of their cytoplasm and the absence of reserve material and their thin cell wall all point to their being male gametes. Although in the forms without aecidia the two generations are not sharply marked off from one another, we may look up the generation with single nuclei in the cells as the gametophyte and that with conjugate nuclei as the sporophyte. ”

“ Many fungi, however, cannot complete their life-history apart from the host-plant. Such obligate parasites may be epiphytic (Erysipheae) , the mycelium remaining on the outside and at most merely sending haustoria into the epidermal cells, or endophytic (Uredineae, Ustilagineae, &c.) , when the mycelium is entirely inside the organs of the host. An epiphytic fungus is not necessarily a parasite, however, as many saprophytes (moulds, &c.) germinate and develop a loose mycelium on living leaves, but only enter and destroy the tissues after the leaf has fallen ”

“ Even the isolated cells of the yeast plant have each one nucleus. As a rule the nuclei of the mycelium are very minute (1.5–2 μ in Phycomyces) , but those of many asci and spores are large and easily rendered visible. As with other plants, so in fungi the essential process of fertilization consists in the fusion of two nuclei, but owing to the absence of well-marked sexual organs from many fungi, a peculiar interest attaches to certain nuclear fusions in the vegetative cells or in young spores of many forms. ”

“ II. Ascomycetes.—This, except in the case of a few of the simpler forms, is a very sharply marked group characterized by a special type of sporangium, the ascus. In the development of the ascus we find two nuclei at the base which fuse together to form the single nucleus of the young ascus. The single nucleus divides by three successive divisions to form eight nuclei lying free in the protoplasm of the ascus. Then by a special method, described first by Harper, a mass of protoplasm is cut out round each nucleus; thus eight uninucleate ascospores are formed by free-cell formation. ”

“ The real difficulty of classification of the fungi lies in the polyphyletic nature of the group. There is very little doubt that the primitive fungi have been derived by degradation from the lower algae. It appears, however, that such a degradation has occurred not only once in evolution but on several occasions, so that we have in the Phycomycetes not a series of naturally related forms, but groups which have arisen perfectly independently of one another from various groups of the algae. ”

“ The Basidiales are further characterized by the complete loss of normal sexuality, but at some time or other in the life-history there takes place an association of two nuclei in a cell; the two nuclei are derived from separate cells or possibly in some cases are sister nuclei of the same cell. The two nuclei when once associated are termed “conjugate” nuclei, and they always divide at the same time, a half of each passing into each cell. This conjugate condition is finally brought to a close by the nuclear fusion in the basidium. ”

“ The outer parts of the mass then differentiate as a wall or investment, and the interior becomes a hollow, into which hyphal ends grow and abstrict the spores. Much more complicated are the processes in a large series of “fructifications,” where the mycelium first develops a densely packed mass of hyphae, all alike, in which labyrinths of cavities subsequently form by separation of hyphae in the previously homogeneous mass, and the hymenium covers the walls of these cavities and passages as with a lining layer. ”

“ Saccharomycetaceae include the well-known yeasts which belong mainly to the genus Saccharomyces. They are characterized by their unicellular nature, their power of rapid budding, their capacity for fermenting various sugars, and their power of forming endogenous spores. The sporangium with its endogenous spores has been compared with an ascus, and on these grounds the group is placed among the Ascomycetes—a very doubtful association. ”

“ Brefeld regards the budding process as the formation of conidia. Under other conditions, of which the temperature is an important one, the nucleus in the yeast-cell divides, and each daughter-nucleus again, and four spores are formed in the mother cell, a process obviously comparable to the typical development of ascospores in an ascus. Under yet other conditions the quiescent yeast-cells floating on the surface of the fermented liquor grow out into elongated sausage-shaped or cylindrical cells and branching cell-series, which mat together into mycelium-like veils. ”

“ The ascus is thus one of the most sharply characterized structures among the fungi.
In some forms we find definite male and female sexual organs (Sphaerotheca, Pyronema, &c.) , in others the antheridium is abortive or absent, but the ascogonium (oogonium) is still present and the female nuclei fuse in pairs (Lachnea stercorea, Humaria granulata, Ascobolus furfuraceus) ; while in other forms ascogonium and antheridium are both absent and fusion occurs between vegetative nuclei (Humaria rutilans, and probably the majority of other forms) . ”

“ Later, however, these Uredineae were placed as a mere subdivision of the Basidiomycetes. Although the Uredineae clearly lead on to the Basidiomycetes, yet owing to their retaining in many cases definite traces of sexual organs they are clearly a more primitive group. Their marked parasitic habit also separates them off, so that they are best included with the Basidiomycetes in a larger cohort which may be called Basidiales. Most of Basidiomycetes are characterized by the large sporophore on which the basidia with its basidiospores are borne. ”

“ The oogonia, unlike the Peronosporaceae, contain more than one oosphere. Klebs has shown that the development of zoosporangia or of oogonia and pollinodia respectively in Saprolegnia is dependent on the external conditions; so long as a continued stream of suitable food-material is ensured the mycelium grows on without forming reproductive organs, but directly the supplies of nitrogenous and carbonaceous food fall below a certain degree of concentration sporangia are developed. Further reduction of the supplies of food effects the formation of oogonia. ”

“ That such enzymes are formed in the protoplasm is evident from the behaviour of hyphae, which have been observed to pierce cell-membranes, the chitinous coats of insects, artificial collodion films and layers of wax, &c. That a fungus can secrete more than one enzyme, according to the materials its hyphae have to attack, has been shown by the extraction of diastase, inulase, trehalase, invertase, maltase, raffinase, malizitase, emulsin, trypsin and lipase from Aspergillus by Bourquelot, and similar events occur in other fungi. ”

“ When we remember that many parts of the world are practically unexplored as regards fungi, and that new species are constantly being discovered in the United States, Australia and northern Europe—the best explored of all—it is clear that no very accurate census of fungi can as yet be made, and no generalizations of value as to their geographical distribution are possible. ”

“ Associated with these relations are the specializations which parasites show in regard to the age of the host. Many parasites can enter a seedling, but are unable to attack the same host when older—e.g. Pythium, Phytophthora omnivora.
Chemotropism.—Taken in conjunction with Pfeffer’s beautiful discovery that certain chemicals exert a distinct attractive influence on fungus hyphae (chemotropism) , and the results of Miyoshi’s experimental application of it, the phenomena of enzyme-secretion throw considerable light on the processes of infection and parasitism of fungi. ”

“ Classification.—It has been accepted for some time now that the majority of the fungi proper fall into three main groups, the Phycomycetes, Ascomycetes and Basidiomycetes, the Schizomycetes and Myxomycetes (Mycetozoa) being considered as independent groups not coming under the true fungi. ”

“ Sporophores.—In some of the simpler fungi the spores are not borne on or in hyphae which can be distinguished from the vegetative parts or mycelium, but in the vast majority of cases the sporogenous hyphae either ascend free into the air or radiate into the surrounding water as distinct branches, or are grouped into special columns, cushions, layers or complex masses obviously different in colour, consistency, shape and other characters from the parts which gather up and assimilate the food-materials. ”

“ Thus in Ustilagineae the chlamydospores, and in Uredineae the teleutospores, each contain two nuclei when young, which fuse as the spores mature. In young asci a similar fusion of two nuclei occurs, and also in basidia, in each case the nucleus of the ascus or of the basidium resulting from the fusion subsequently giving rise by division to the nuclei of the ascospores and basidiospores respectively. ”

“ As regards their geographical distribution, fungi, like flowering plants, have no doubt their centres of origin and of dispersal; but we must not forget that every exchange of wood, wheat, fruits, plants, animals, or other commodities involves transmission of fungi from one country to another; while the migrations of birds and other animals, currents of air and water, and so forth, are particularly efficacious in transmitting these minute organisms. ”

“ Against this, of course, it may be argued that parasitic forms can only go where their hosts grow, as is proved to be the case by records concerning the introduction of Puccinia malvacearum, Peronospora viticola, Hemileia vastatrix, &c. Some fungi—e.g. moulds and yeasts—appear to be distributed all over the earth. That the north temperate regions appear richest in fungi may be due only to the fact that North America and Europe have been much more thoroughly investigated than other countries; it is certain that the tropics are the home of very numerous species. ”

“ Sphaerotheca Humuli is the well known hop-mildew, Sphaerotheca Mors-Uvae is the gooseberry mildew, the recent advent of which has led to special legislation in Great Britain to prevent its spreading, as when rampant it makes the culture of gooseberries impossible. Erysiphe, Uncinula and Phyllactinia are other well-known genera. The form of the fruit body, the difference and the nature of special outgrowths upon it—the appendages—are characteristic of the various genera. ”

“ The resemblance between this genus and Oedogonium among the algae is very striking, as is also that of Myrioblepharis and Vaucheria.
Peronosporaceae are a group of endophytic parasites—about 100 species—of great importance as comprising the agents of “damping off” disease (Pythium) , vine-mildew (Plasmopara) , potato disease (Phytophthora) , onion-mildew (Peronospora) . Pythium is a semi-aquatic form attacking seedlings which are too plentifully supplied with water; its hyphae penetrate the cell-walls and rapidly destroy the watery tissues of the living plant ”

“ Recent experiments have shown that the difficulties of getting orchid seeds to germinate are due to the absence of the necessary fungus, which must be in readiness to infect the young seedling immediately it emerges from the seed. ”

“ When the free ends of the hyphae emerge again into the air they swell up into spherical bodies which may either fall off and behave as conidia, each putting out a germ-tube and infecting the host; or the germ-tube itself swells up into a zoosporangium which develops a number of zoospores. ”

“ The mucilage surrounding endospores of Mucor, conidia of Empusa, &c., serves to gum the spore to animals. Such gums are formed abundantly in pycnidia, and, absorbing water, swell and carry out the spores in long tendrils, which emerge for days and dry as they reach the air, the glued spores gradually being set free by rain, wind, &c. In oidial chains (Sclerotinia) a minute double wedge of wall-substance arises in the middle lamella between each pair of contiguous oidia, and by its enlargement splits the separating lamella. ”

“ Specialization of Parasitism.—The careful investigations of recent years have shown that in several groups of fungi we cannot be content to distinguish as units morphologically different species, but we are compelled to go deeper and analyse further the species. ”

“ In such examples as the above we may regard the hymenium (Solenia, Cyphella) , zygospores, or asci as truly invested by later growth, but in the vast majority of cases the processes which result in the enclosure of the spores, asci, &c., in a “fructification” are much more involved, inasmuch as the latter is developed in the interior of hyphal tissues, which are by no means obviously homologous with a stroma. ”

“ If the membrane is of some impermeable substance, like gold leaf, the hyphae cannot dissolve its way through, but the tip finds the most minute pore and traverses the barrier by means of it, as it does a stoma on a leaf We may hence conclude that a parasitic hyphae pierces some plants or their stomata and refuses to enter others, because in the former case there are chemotropically attractive substances present which are absent from the latter, or are there replaced by repellent poisonous or protective substances such as enzymes or antitoxins. ”