Cytology.—The study of the nucleus of yeast-cells is rendered difficult by the presence of other deeply staining granules termed by Guillermond metachromatic granules. These have often been mistaken for nuclei and have to be carefully distinguished by differential stains. In the process of budding the nucleus divides apparently by a process of direct division. In the formation of spores the nucleus of the cell divides, the protoplasm collects round the nuclei to form the spores by free-cell formation; the protoplasm (epiplasm) not used in this process becomes disorganized. A fusion of nuclei was originally described by Jansens and Leblanc, but it was observed neither by Wager nor Guillermond and is probably absent. In Schizosaccharomyces and Zygosaccharomyces, however, we have a fusion of nuclei in connexion with the conjugation of cells which precedes sporangium-formation. The theory may be put forward that the ordinary forms have been derived from sexual forms like Schizosaccharomyces and Zygosaccharomyces by a loss of sexuality, the sporangium being formed parthenogenetically without any nuclear fusion. This suggests a possible relationship to Eremascus, which can only doubtfully be placed in the Ascomycetes (vide supra).
Carpoascomycetes.—The other divisions of the Ascomycetes may be distinguished as Carpoascomycetes because they do not bear the asci free on the mycelium but enclosed in definite fruit bodies or ascocarps. The ascocarps can be distinguished into two portions, a mass of sterile or vegetative hyphae forming the main mass of the fruit body, and surrounding the fertile ascogenous hyphae which bear at their ends the asci. When the ascogonium (female organ) is present the ascogenous hyphae arise from it, with or without its previous fusion with an antheridium. In other cases the ascogenous hyphae arise directly from the vegetative hyphae. In connexion with this condition of reduction a fusion of nuclei has been observed in Humaria rutilans and is probably of frequent occurrence. The asci may be derived from the terminal cell of the branches of the ascogenous hyphae, but usually they are derived from the penultimate cell, the tip curving over to form the so-called crozier. By this means the ascus cell is brought uppermost, and after the fusion of the two nuclei it develops enormously and produces the ascospores. The ascospores escape from the asci in various ways, sometimes by a special ejaculation-mechanism. The Ascomycetes, at least the Carpoascomycetes, exhibit a well-marked alternation of sexual and asexual generations. The ordinary mycelium is the gametophyte since it bears the ascogonia and antheridia when present; the ascogenous hyphae with their asci represent the sporophyte since they are derived from the fertilized ascogonium. The matter is complicated by the apogamous transition from gametophyte to sporophyte in the absence of the ascogonium; also by the fact that there are normally two fusions in the life-history as mentioned earlier. If there are two fusions one would expect two reductions, and Harper has suggested that the division of the nuclei into eight in the ascus, instead of into four spores as in most reduction processes, is associated with a double reduction process in the ascus. Miss Fraser in Humaria rutilans finds two reductions: a normal synaptic reduction in the first nuclear division of the ascus, and a peculiar reduction division termed brachymeiosis in the third ascus division.
Various types of ascocarp are characteristic of the different divisions of the Carpoascomycetes: the cleistothecium, apothecium and perithecium.
Perisporineae.—This includes two chief families, Erysiphaceae and Perisporiaceae. They are characterized by an ascocarp without any opening to the exterior, the ascospores being set free by the decay or rupture of the ascocarp wall; such a fruit-body is termed a cleistothecium (cleistocarp). The Erysiphaceae are a sharply marked group of forms which live as parasites. They form a superficial mycelium on the surface of the plant, the hyphae not usually penetrating the tissues but merely sending haustoria into the epidermal cells. Only in rare cases is the mycelium intercellular. Owing to their appearance they go by the popular name of mildews. 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. Besides peritheca the members of the Erysiphaceae possess conidia borne in simple chains. De Bary brought forward very strong evidence for the origin of the ascocarp in Sphaerotheca and Erysiphe by a sexual process, but Harper in 1895 was the first to prove conclusively, by the observation of the nuclear fusion, that there was a definite fertilization in Sphaerotheca Humuli by the fusion of a male (antheridial) nucleus with a female, ascogonial (oogonial) nucleus. Since then Harper has shown that the same process occurs in Erysiphe and Phyllactinia.
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| Fig. 11.—Development of Eurotium repens. (After De Bary.) | |
A, Small portion of myceliumwith conidiophore (c), andarchicarp (as). B, The spiral archicarp (as),with the antheridium (p). D, The same, beginning to besurrounded by the hyphaeforming the perithecium wall. D, The perithecium. | E, F, Sections of young perithecia. w, Parietal cells. f, Pseudo-parenchyma. as, Ascogonium. G, An ascus. H, An ascospore. |
The Perisporiaceae are saprophytic forms, the two chief genera being Aspergillus and Penicillium. The blue-green mould P. crustaceum and the green mould A. herbariorium (= Eurotium herbariorum) are extraordinarily widely distributed, moulds being found on almost any food-material which is exposed to the air. They have characteristic conidiophores bearing numerous conidia, and also cleistothecia which are spherical in form and yellowish in colour. The latter arise from the crown of a spirally coiled archicarp (bearing an ascogonium at its end) and a straight antheridium. Vegetative hyphae then grow up and surround these and enclose them in a continuous sheath of plectenchyma (fig. 11). It has lately been shown by Fraser and Chambers that in Eurotium both ascogonium and antheridium contain a number of nuclei (i.e. are coenogametes), but that the antheridium disorganizes without passing its contents into the ascogonium. There is apparently a reduced sexual process by the fusion of the ascogonial (female) nuclei in pairs. Aspergillus Oryzae plays an important part in saccharifying the starch of rice, maize, &c., by means of the abundant diastase it secretes, and, in symbiosis with a yeast which ferments the sugar formed, has long been used by the Japanese for the preparation of the alcoholic liquor saké. The process has now been successfully introduced into European commerce.
| From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer. | Fig. 13.—Ascobolus furfuraceus. Diagrammatic section of the fructification. (After Janczewski.) |
| Fig. 12.—Peziza aurantiaca. (After Krombholz, nat. size.) | m, Mycelium. c, Archicarp. l, Pollinodium. s, Ascogenous filaments. a, Asri. r, p, The sterile tissue from which the paraphyses h spring. |
Discomycetes.—Used in its widest sense this includes the Hysteriaceae, Phacidiaceae, Helvellaceae, &c. The group is characterized in general by the possession of an ascocarp which, though usually a completely closed structure during the earlier stages of development, at maturity opens out to form a bowl or saucer-shaped organ, thus completely exposing the layer of asci which forms the hymenium. Such an ascocarp goes by the name of apothecium. Owing to the shape of the fruit-body many of these forms are known as cup-fungi, the cup or apothecium often attaining a large size, sometimes several inches across (fig. 12). Functional male and female organs have been shown to exist in Pyronema and Boudiera; in Lachnea stercorea both ascogonia and antheridia are present, but the antheridium is non-functional, the ascogonial (female) nuclei fusing in pairs; this is also the case in Humaria granulata and Ascobolus furfuraceus, where the antheridium is entirely absent. In H. rutilans, however, both sexual organs are absent and the ascogenous hyphae arise apogamously from the ordinary hyphae of the mycelim. In all these cases the ascogonium and antheridium contain numerous nuclei; they are to be looked upon as gametangia in which there is no differentiation of gametes, and since they act as single gametes they are termed coenogametes. In some forms as in Ascobolus the ascogonium is multicellular, the various cells communicating by pores in the transverse walls (fig. 13).
| From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer. |
| Fig. 14.—Perithecium of Podospora fimiseda in longitudinal section. After v. Tavel. |
| s, Asci. a, Paraphyses. e, Periphyses. m, Mycelial hyphae. |