D, Young ascus of _Boudiera_ with eight spores. (After Claussen.)]_Encyclopaedia Britannica, 11th Edition, "Frost" to "Fyzabad" Volume 11, Slice 3

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:

+---------------------+---------------------------------+----------------------------------+----------------------------------------+

| Species. | Temperature for +----------+-----------+-----------+ Sugars Fermented and |

+---------------------+----------------+----------------+----------+-----------+-----------+----------------------------------------+

|_S. ellipsoideus_ | 25 deg. | 33 deg.-34 deg.| Low | Rounded | Globoid | \ |

| | | | | | | |

| | | | | | |

| | | | | | |

+---------------------+----------------+----------------+----------+-----------+-----------+----------------------------------------+

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.

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