In the higher Fungi nuclear fusions take place in basidia or asci which involve the union of two (fig.
Ascophorous, producing asci; ascospore, the spore (or sporule) developed in the ascus; ascogonium, the organ producing it, &c.
Fructification of the fungus, entire and in section; the latter shows the asci containing ascospores, much enlarged.
The hypothecium is the basal part of the apothecium on which the hymenium is borne; the latter consists of asci (thecae) with ascospores, and paraphyses.
As indicated above, the weight of evidence seems to favour what has been put forward in the case of the non-lichenforming fungi (see Fungi), that in some cases the ascogonia develop as a result of a previous fertilization by spermatia, in other cases the ascogonia develop without such a union, while in still other Epithecium Asci :Thecium (Hymenium) Paraphyses Parathecium, - Cortex gn,go mphitheciam - __ __MeddllzC-- _Hypoitienum==?=_` After Darbishire, from Berichte der deutschen botanischen Gesellschaft, by permission of Borntraeger & Co.
In these forms gonidia are found in connexion with the young fruit; such algal cells undergo numerous divisions becoming very small in size and penetrating into the hymenium among the asci and paraphyses.
B, Asci (thecae) with bilocular spores.
The paraphyses branch and form a network (capillitium) over the asci, the capillitium and ejected spores forming a long persistent powdery mass (mazaedium).
The reproductive spores are borne in sacs (asci) which form a dense layer on the surface, appearing like a bloom in July; they are scattered by the wind and propagate the disease.
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.
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.
Among the simplest cases are the sheet-like aggregates of sporogenous hyphae in Puccinia, Uromyces, &c., or of basidia in Exobasidium, Corticium, &c., or of asci in Exoascus, Ascocorticium, &c. In the former, where the layer is small, it is often termed a sorus, but where, as in the latter, the sporogenous layer is extensive, and spread out more or less sheet-like on the supporting tissues, it is more frequently termed a hymenium.
Other series of modifications arise in which the tissues corresponding to the stroma invest the sporogenous hyphal ends, and thus enclose the spores, asci, basidia, &c., in a cavity.
Another simple case is where the plane or slightly convex surface of the stroma rises at its margins and overgrows the sporogenous hyphal ends, so that the spores, asci, &c., come to lie in the depression of a cavity - e.g.
Solenia, Cyphella - and even simpler cases are met with in Mortierella, where the zygospore is invested by the overgrowth of a dense mat of closely branching hyphae, and in Gymnoascus, where a loose mat of similarly barren hyphae covers in the tufts of asci as they develop.
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.
Thus in Penicillium, Eurotium, Erysiphe, &c., hyphal ends which are the initials of ascogenous branches, are invested by closely packed branches at an early stage of development, and the asci develop inside what has by that time become a complete investment.
The asci are borne directly on the mycelium and are therefore fully exposed, being devoid from the beginning of any investment.
- 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 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.
The ascospores escape from the asci in various ways, sometimes by a special ejaculation-mechanism.
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 ascogenous hyphae with their asci represent the sporophyte since they are derived from the fertilized ascogonium.
Four ripe asci, a i, a2, with eight spores, a 3, a4, with yeast-like conidia abstricted from the spores.
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.
In the Helvellaceae there is no apothecium but a large irregular fruit body which at maturity bears the asci on its surface.
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.
The asci are developed in the large dense fruit bodies (cleistothecia) and the spores escape by the decay of the wall.
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.
Zygomycetes: Harper, "Cell-division in Sporangia and Asci," Ann.