Chapter 38 of 52 · 3788 words · ~19 min read

Part 38

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). In other cases the sexual fusion is apparently absent altogether, as in _Exoascus_. In the first case (fig. 9) we have a true sexual process, while in the second and third cases we have a _reduced_ sexual process in which the fusion of other nuclei has replaced the fusion of the normal male and female nuclei. It is to be noted that all the forms exhibit the fusion of nuclei in the ascus, so that those with the normal or reduced sexual process described above have two nuclear fusions in their life-history. The advantage or significance of the second (ascus) fusion is not clearly understood.

[Illustration: From Strasburger's _Lehrbuch der Botanik_, by permission of Gustav Fischer.

FIG. 8.--Development of the Ascus.

A-C, _Pyronema confluens_. (After Harper.)

D, Young ascus of _Boudiera_ with eight spores. (After Claussen.)]

The group of the Hemiasci was founded by Brefeld to include forms which were supposed to be a connecting link between Phycomycetes and Ascomycetes. As mentioned before, the connexion between these two groups is very doubtful, and the derivation of the ascus from an ordinary sporangium of the Zygomycetes cannot be accepted. The majority of the forms which were formerly included in this group have been shown to be either true Phycomycetes (like _Ascoidea_) or true Ascomycetes (like _Thelebolus_). _Eremascus_ and _Dipodascus_, which are often placed among the Hemiasci, possibly do not belong to the Ascomycetes series at all.

[Illustration: From Strasburger's _Lehrbuch der Botanik_, by permission of Gustav Fischer.

Fig. 9.--_Sphaerotheca Castagnei_. Fertilization and Development of the Perithecium. (After Harper.)

1, Oogonium (og) with the antheridial branch (az) applied to its surface

2, Separation of antheridium (an).

3, Passage of the antheridial nucleus towards that of the oogonium.

4, Union of the nuclei.

5, Fertilized oogonium surrounded by two layers of hyphae derived from the stalk-cell (st).

6, The multicellular ascogonium derived by division from the oogonium; the terminal cell with the two nuclei (as) gives rise to the ascus.]

_Exoascaceae_ are a small group of doubtful extent here used to include _Exoascus_, _Taphrina_, _Ascorticium_ and _Endomyces_. The mycelium is very much reduced in extent. The asci are borne directly on the mycelium and are therefore fully exposed, being devoid from the beginning of any investment. The _Taphrineae_, which include _Exoascus_ and _Taphrina_, are important parasites--e.g. pocket-plums and witches' brooms on birches, &c., are due to their action (fig. 10). _Exoascus_ and _Ascorticium_ present interesting parallels to _Exobasidium_ and _Corticium_ among the Basidiomycetes.

_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. The group has attained an importance of late even beyond that to which it was brought by Pasteur's researches on alcoholic fermentation, chiefly owing to the exact results of the investigations of Hansen, who first applied the methods of pure cultures to the study of these organisms, and showed that many of the inconsistencies hitherto existing in the literature were due to the coexistence in the cultures of several species or races of yeasts morphologically almost indistinguishable, but physiologically very different. About fifty species of _Saccharomyces_ are described more or less completely, but since many of these cannot be distinguished by the microscope, and some have been found to develop physiological races or varieties under special conditions of growth, the limits are still far too ill-defined for complete botanical treatment of the genus. A typical yeast is able to develop new cells by budding when submerged in a saccharine solution, and to ferment the sugar--i.e. so to break up its molecules that, apart from small quantities used for its own substance, masses of it out of all proportion to the mass of yeast used become resolved into other bodies, such as carbon dioxide and alcohol, the process requiring little or no oxygen. 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. At the bottom of the fermented liquor the cells often obtain fatty contents and thick walls, and behave as resting cells (chlamydospores). The characters employed by experts for determining a species of yeast are the sum of its peculiarities as regards form and size: the shapes, colours, consistency, &c., of the colonies grown on certain definite media; the optimum temperature for spore-formation, and for the development of the "veils"; and the behaviour as regards the various sugars.

[Illustration: From Strasburger's _Lehrbuch der Botanik_, by permission of Gustav Fischer.

FIG. 10.--_Taphrina Pruni._ Transverse section through the epidermis of an infected plum. Four ripe asci, a1, a2, with eight spores, a3, a4, with yeast-like conidia abstricted from the spores. After Sadebeck.

st, Stalk-cells of the asci.

m, Filaments of the mycelium cut transversely.

cut, Cuticle.

sp, Epidermis.]

The following summary of some of the principal characteristics of half-a-dozen species will serve to show how such peculiarities can be utilized for systematic purposes:

Two questions of great theoretical importance have been raised over and over again in connexion with yeasts, namely, (1) the morphological one as to whether yeasts are merely degraded forms of higher fungi, as would seem implied by their tendency to form elongated, hypha-like cells in the veils, and their development of "ascospores" as well as by the wide occurrence of yeast-like "sprouting forms" in other fungi (e.g. _Mucor_, Exoasci, Ustilagineae, higher Ascomycetes and Basidiomycetes); and (2) the question as to the physiological nature and meaning of fermentation. With regard to the first question no satisfactory proof has as yet been given that Saccharomycetes are derivable by culture from any higher form, the recent statements to that effect not having been confirmed. At the same time there are strong grounds for insisting on the resemblances between _Endomyces_, a hyphal fungus bearing yeast-like asci, and such a form as _Saccharomyces anomalus_. Concerning the second question, the recent investigations of Buchner and others have shown that a ferment (zymase) can be extracted from yeast-cells which causes sugar to break up into carbon dioxide and alcohol. It has since been shown by Buchner and Albert that yeast-cells which have been killed by alcohol and ether, or with acetone, still retain the enzyme. Such material is far more active than the zymase obtained originally by Buchner from the expressed juice of yeast-cells. Thus alcoholic fermentation is brought into line with the other fermentations.

_Schizosaccharomyces_ includes a few species in which the cells do not "bud" but become elongated and then divide transversely. In the formation of sporangia two cells fuse together by means of outgrowths, in a manner very similar to that of _Spirogyra_; sometimes, however, the wall between two cells merely breaks down. The fused cell becomes a sporangium, and in it eight spores are developed. In certain cases single cells develop parthenogenetically, without fusion, each cell producing, however, only four spores. In _Zygosaccharomyces_ described by Barker (1901) we have a form of the usual sprouting type, but here again there is a fusion of two cells to form a sporangium.

_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_.

[Illustration: FIG. 11.--Development of _Eurotium repens_. (After De Bary.)

A, Small portion of mycelium with conidiophore (c), and archicarp (as).

B, The spiral archicarp (as), with the antheridium (p).

D, The same, beginning to be surrounded by the hyphae forming 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 sake. The process has now been successfully introduced into European commerce.

[Illustration: From Strasburger's _Lehrbuch der Botanik_, by permission of Gustav Fischer.

FIG. 12.--_Peziza aurantiaca._ (After Krombholz, nat. size.)]

[Illustration: FIG. 13.--_Ascobolus furfuraceus._ Diagrammatic section of the fructification. (After Janczewski.)

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).

In the Helvellaceae there is no apothecium but a large irregular fruit body which at maturity bears the asci on its surface. The development is only slightly known, but there is some evidence for believing that the fruit-body is closed in its very early stages.

[Illustration: 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.]

The genus _Peziza_ (in its widest sense) may be taken as the type of the group. Most of them grow on living plants or on dead vegetable remains, very often on fallen wood; a number, however, are found growing on earth which is rich in humus. The genus _Sclerotinia_ may be mentioned here; a number of forms have been investigated by Woronin. The conidia are fragrant and are carried by bees to the stigma of the bilberry; here they germinate with the pollen and the hyphae pass with the pollen tubes down the style; the former infect the ovules and produce sclerotia, therein reducing the fruits to a mummified condition. From the sclerotia later the apothecium develops. One species, _S. heteroica_, is _heteroecious_; the ascospores infecting the leaves of _Vaccinium uliginosum_, while the conidia which then arise infect only _Ledum palustre_. This is the only case of heteroecism known in the vegetable kingdom outside the Uredineae.

_Pyrenomycetes._--This is an extraordinarily large and varied group of forms which mostly live parasitically or saprophytically on vegetable tissue, but a few are parasitic on insect-larvae. The group is characterized by a special type of ascocarp, the _perithecium_. This is typically of a flask-shaped form opening with a small pore at the top. The asci live at the bottom often mixed with paraphyses, while the upper "neck" of the flask is lined with special hyphae, the periphyses, which aid in the ejection of the spores (fig. 14). The simpler forms bear the perithecia directly on the mycelium, but the more highly developed forms often bear them on a special mycelial development--the stroma, which is often of large size and special shape and colour, and of dense consistence. The cytological details of development of the perithecia are not well known; most of them appear to develop their ascogenous hyphae in an apogamous way without any connexion with an ascogonium. Besides the special ascocarps, accessory reproductive organs are known in the majority of cases in the form of conidia.

_Tuberineae._--These are a small group of fungi including the well-known truffles. They are found living saprophytically (in part parasitically) underground in forests. The asci are developed in the large dense fruit bodies (cleistothecia) and the spores escape by the decay of the wall. The fruit-body is of complicated structure, but its early stages of development are not known. Many of the fruit-bodies have a pleasant flavour and are eaten under the name of truffles (_Tuber brumale_ and other species). The exact life-history of the truffle is not known.

_Laboulbeniineae_ are a group of about 150 species of fungi found on insects, especially beetles, and principally known from the researches of Thaxter in America. The plant is a small, dark brown, erect structure (receptacle) of a few cells, and 1-10 mm. high, attached to the insect by the lowermost end (foot), and easily mistaken for a hair or similar appendage of the insect. The receptacle ends above in appendages, each consisting of one or a few cells, some of which are the male organs, others the female organs, and others again may be barren hairs. The male organ (antheridium) consists of a few cells, the terminal one of which either abstricts from its end, or emits from its interior the non-motile spermatia, reminding us of those of the Florideae. The female organ is essentially a flask-shaped structure; the neck of the flask growing out as the trichogyne, and the belly composed of an axial carpogenic cell surrounded by investing cells, and with one cell (trichophoric) between it and the trichogyne. These three elements--trichogyne, trichophoric cell, and carpogenic cell--are regarded as the procarp. The spermatia have been shown by Thaxter to fuse with the trichogyne, after which the axial cell below (carpogenic cell) undergoes divisions, and ultimately forms asci containing ascospores, while cells investing this form a perithecium, the whole structure reminding us essentially of the fructification of a Pyrenomycete. Many modifications in details occur, and the plants may be dioecious. No injury is done to the infested insects. It has lately been shown that there is a fusion of nuclei in connexion with ascus formation, so that there can be no doubt of the position of this extraordinary group of plants among the Ascomycetes. The various cells of these organisms are connected by large pits which are traversed by thick protoplasmic threads connecting one cell with the next. In this point and in their method of fertilization the Laboulbeniineae suggest a possible relationship of Ascomycetes and the Red Algae.

[Illustration: From Strasburger's _Lehrbuch der Botanik_, by permission of Gustav Fischer.

FIG. 15.--_Armillaria mellea._ (After Ruhland.)

A, Young basidium with the two primary nuclei.

B, After fusion of the two nuclei. _Hypholoma appendiculatum_.

C, A basidium before the four nuclei derived from the secondary nucleus of the basidium have passed into the four basidiospores.

D, Passage of a nucleus through the sterigma into the basidiospore.]