The ascus is thus one of the most sharply characterized structures among the fungi.
In a few cases both among the higher and the lower plants, of which the formation of spores in the ascus is a typical example, new cells are formed by the aggregation of portions of the cytoplasm around the nuclei which become delimited from the rest of the cell iontents by a membrane.
Ascophorous, producing asci; ascospore, the spore (or sporule) developed in the ascus; ascogonium, the organ producing it, &c.
Ascus from perithecium containing six spores, X30o.
B, With ascus-fruit at the ends of the branches.
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.
Thallus septate; spores developed in special type of sporangium, the ascus, the number of spores being usually eight.
They are characterized especially by the zygospores, but the asexual organs (sporangia) exhibit interesting series of changes, beginning with the typical sporangium of Mucor containing numerous endospores, passing to cases where, as in Thamnidium, these are accompanied with more numerous small sporangia (sporangioles) containing few spores, and thence to Chaetocladium and Piptocephalis, where the sporangioles form but one spore and fall and germinate as a whole; that is to say, the monosporous sporangium has become a conidium, and Brefeld regarded these and similar series of changes as explaining the relation of ascus to conidium in higher fungi.
According to his view, the ascus is in effect the sporangium with several spores, the conidium the sporangiole with but one spore, and that not loose but fused with the sporangiole wall.
On this basis, with other interesting morphological comparisons, Brefeld erected his hypothesis, now untenable, that the Ascomycetes and Basidiomycetes diverge from the Zygomycetes, the former having particularly specialized the ascus (sporangial) mode of reproduction, the latter having specialized the conidial (indehiscent one-spored sporangiole) mode.
This mode of sporeformation is totally different from that in the ascus; hence one of the difficulties of the acceptance of Brefeld's view of the homology of ascus and sporangium.
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.
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 lifehistory.
The advantage or significance of the second (ascus) fusion is not clearly understood.
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.
(After Harper.) Young ascus of Boudiera with eight 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.
By this means the ascus cell is brought uppermost, and after the fusion of the two nuclei it develops enormously and produces the ascospores.
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.
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.
This very large group of plants is characterized by the possession of a special type of conidiophore - the basidium, which gives its name to the group. The basidium is a unicellular or multicellular structure from which four basidiospores arise as outgrowths; it starts asa binucleate structure, but soon, like the ascus, becomes uninucleate by the fusion of the two nuclei.
Single ascus, more enlarged, showing the eight contained spores.