424 BOTANY OF THE LIVING PLANT 



the ascus-walls, and of the nutritive tissues that embedded them 

 (Fig. 321, 2, 3, 4). On germination the ascospores form a new 

 mycelium like the original one. 



In Eurotium both the antheridial and oogonial cells are multinucleate, 

 and it appears that sometimes a normal sexual fusion takes place. But in 

 others the antheridium degenerates, and sexual fusion is replaced by a fusion 

 of oogonial nuclei in pairs. Thus Eurotium illustrates that degradation 

 of sexuality, evidence of which is common among the Fungi. 



The type of life-history seen in these simple Mildews and Moulds is common 

 for other Ascomycetes, though it may be worked out with greater complication. 

 But in many of them the sexual organs are so degraded that they, or their 

 equivalents, if present, have hitherto escaped observation. There is normally 

 an alternation of generations, the critical points of which are the sexual 

 fusion in the oogonium and the reduction which precedes the formation 

 of the ascospores. The stage of the ascogenous hyphae, which intervenes 

 between these events may be regarded as a diploid sporophyte. The rest is 

 held to be the haploid gametophyte, which is liable to indefinite repetition by 

 means of conidia. The fruit-body is then a composite structure, consisting 

 essentially of the ascogenous hyphae, constituting the sporophyte, which is 

 enveloped in a covering derived from the mycelial gametophyte. The nearest 

 analogies are with the fruiting bodies of the Red Seaweeds. 



On the fertilisation of the oogonium in some Ascomycetes the male and 

 female nuclei fuse in pairs and diploid nuclei result. These pass out into the 

 ascogenous hyphae which grow out from the oogonium. The young ascus 

 typically develops from the penultimate cell of an ascogenous hypha. This 

 cell always contains two nuclei. These fuse together and therefore produce a 

 nucleus which is tetraploid. Subsequently, as the ascus develops, three 

 successive nuclear divisions take place, thereby producing the eight nuclei 

 for the eight ascospores. The first division is a meiotic division, the second is 

 a mitotic division, and the third a second meiotic division. This remarkable 

 phenomenon of a second reduction division, on which a considerable amount 

 of investigation has been carried out and which is still a matter for controversy, 

 is known as br achy meio sis. Where no genuine sexual, or substitute sexual, 

 fusion has taken place in the oogonium, only a single reduction division takes 

 place in the young ascus. 



OTHER TYPES OF ASCOMYCETOUS FUNGI. 



Both the mycelial and the fruiting stages of Ascomycetous Fungi 

 are subject to modifications according to habit and circumstance, 

 and either may attain large size in some of the representatives of the 

 family. The mycelium may, by repeated branching and knotting 

 together of its hyphae, form dense masses stored with nutritive 

 material, hard and dark coloured, called sclerotia. When the rest of 

 the mycelium is killed off by dry or cold weather these remain un- 



