ALTERNATION OF GENERATIONS IN THE THALLOPHYTES. 325 



the ascospores must necessarily be regarded as oosxDores. The 

 alternation of generations in the A.scomycetes and the Floridecc is 

 then that of independent sexual and asexual plants, and not that 

 of cystocarp and sexual thahus. He considers, partly on 

 account of their structure and partly on account of the products of 

 the germination of the stylosj)ores, that probably the pycnidia aro 

 the neutral fructifications. This view derives support from Bauke's 

 observations on Pleospora herbarum, -■' in which the perithecium and 

 pycnidia appear to represent the two generations, though their 

 succession is very irregular. 



It cannot be denied that Pringsheim's objections to the accepted 

 interx3retation of the life-histories of Thalloj)hytes, according to the 

 Moss-type of alternation of generations, are well-founded. The 

 fact that such groups as the CunjugaUc and the Fucacca; exist, 

 which exhibit no such alternation, is presumptive evidence against 

 it ; this is supported by the above-mentioned development of a 

 sexual mycelium from the zygospore of Mucor dichotomus : moreover 

 it must not be overlooked that in Ferunospora, where the oospore 

 produces an ordinary mycelium, there cannot be an alternation of 

 generations according to the Moss-type ; and finally, it will be 

 readily granted that a great part of the fructification of the sexual 

 Carposporem belongs, as Pringsheim states, to the parent-plant, so 

 that the "fruit" cannot be regarded as constituting a distinct 

 generation, and the view that the carpospores are really oospores 

 is at least not contrary to our knowledge of the mode of fertilisation 

 in these plants, f 



Admitting then that it is not possible to interpret the life- 

 history of Thallophytes in the same way as that of a Moss, the 

 question arises whether or not there is any ground for continuing 

 to use the expression "alternation of generations " with reference 

 to Thallophytes as Pringsheim does. Nearly all the arguments 

 brought by Pringsheim against the Moss-type theory are equally 

 valid against any theory of alternation of generations whatsoever ; 

 and when it is also borne in mind that in certain of these plants, 

 such as Ulothrix zonata \ for instance, the differentiation of sexual 

 and asexual reproductive cells is so slight, that if the former fail to 

 conjugate and to form zygospores they germinate like ordinary 

 zoogonidia, there seems to be good reason for answering this 

 question in the negative. It seems to be more in accordance with 

 fact to say simply that a sexual Thallophyte may reproduce itself 

 either sexually or asexually, the mode of reproduction depending 

 more esj)ecially upon the external conditions. Speaking generally, 

 it is not possible to say of -a spore (conidium, gonidium) of any 

 given Thallophyte that the product of its germination will 

 necessarily be a sexual plant, nor is it j)ossible to say of the 

 oospore (zygospore, carpospore) that the j)roduct of its germination 



* Zur Entwickelungsgeschichte der Ascom3ceten, 'Bot. Zeit.,' 1877. 



+ See Thuret and Bornet, on the fertilisation of the Floridece, 'Anu. d. Sci. 

 Nat.,' 1855 and 1867; also Stahl, Ueb. die geschlechtliche Fortpflanzung der 

 CoUemaceen, 1877. 



J Dodel, Ueb. Ulothrix zonata , Prings. ' Jahrb. f. wiss. Bot.,' X. 



