ORIGIN OP ASCOMYCETEAE 643 



sidered to be the primitive Ascomyceteae, if their Phycomycetous origin 

 is accepted, we have this same general method of sexual reproduction, but 

 in the forms considered under that hypothesis to be the furthest advanced 

 we find separate nonmotile sperms uniting with special receptive organs 

 (trichogynes). Whence have these been derived? They represent an 

 entirely new development. Attempts have been made to consider the 

 spermatia to be specialized conidia that have assumed the sexual function. 

 That might possibly be so but it is far easier to surmise that they are the 

 reproductive structures inherited from the Floridean ancestors where 

 that is the standard mode of sexual reproduction. 



The case of Liagora tetrasporifera Borgesen (1927) indicates how a 

 short-cycled red seaweed has developed a life history very closely parallel 

 to that of such an Ascomycete as some species of Mycosphaerella. In this 

 representative of the Order Nemalionales, after spermatization of the 

 trichogyne there soon grow out from the oogone a number of gonimo- 

 blasts whose terminal cells instead of becoming carpospores, as is the 

 usual case in red seaweeds, become tetrasporangia which produce four 

 tetraspores each. Although in this particular case the cytological behavior 

 of the nuclei in the oogone after fertilization and in the tetrasporangium 

 has not been studied it may well be assumed, from the analogy of other 

 Nemalionales and other orders of the Florideae, that the gamete nuclei 

 united normally in the oogone but did not undergo immediate meiosis, the 

 result being that the nuclei in the gonimoblasts remained diploid. The 

 terminal cells instead of becoming carpospores became tetrasporangia in 

 which meiosis occurred and tetraspores with haploid nuclei were pro- 

 duced. Compare this with Mycosphaerella. A uninucleate sperm cell 

 spermatizes a one-celled unbranched trichogyne and passes down into the 

 oogone where it associates itself with, but does not unite with, the egg 

 nucleus. The two nuclei then divide simultaneously by "conjugate divi- 

 sion" until many pairs of nuclei are produced which pass out into the 

 ascogenous hyphae. At or near the tip a pair of nuclei passes into a ter- 

 minal or subterminal cell where they unite. Then this diploid nucleus 

 undergoes meiosis to produce four haploid nuclei and, after the fashion of 

 the majority of the Ascomyceteae, a further mitotic division occurs, so 

 that eight ascospores arise in each ascus. It should be remembered that in 

 many Ascomyceteae where typical sexuality is known only four nuclei 

 and therefore not over four ascospores are produced in each ascus. Al- 

 though one cannot assume that Liagora is a direct ancestor of the Asco- 

 myceteae this shows that in the red seaweeds the conditions exist now 

 that might have given rise ages ago to this group of fungi. 



Possibly the acquisition of the land habit might have led to the pro- 

 duction of ascospores in an ascus where the epiplasm is of importance in 

 the scattering of the spores, whereas the tetraspores which depend upon 



