ALTERNATION OF GENERATIONS IN THE THALLOPHYTES. 



323 



actually or potentially an oophore ; the cells of the coenobmm 

 divide and give rise to motile cells which, according to circum- 

 stances, either conjugate in pairs and form zygospores, when they 

 are termed gametes,* or simply divide and form new ccenobia, 

 when they are termed zoogonidia. 



The oospore of the (Jusjwreoi, like the zygospore of the iDrecediug 

 Class, is regarded as the sporophore. Its development may 

 resemble that of the zygospore of Fandorina, swarm-spores bemg 

 formed within it which either give rise to potentially sexual 

 individuals reproducing by means of spores (conidia, gonidia), or 

 at ^ once to sexual individuals; or it may resemble that of the 

 zygosj)ore of the Mucorini, and form a mycelium (e.g., Peronospora 

 ValerianeU(E)\ which, in this case, does not differ materially from 

 the mycelium produced by a germinating conidium. It is of 

 interest to note that both modes of development are exhibited by 

 closely allied i3lants in the Feronosporecr, and by the same plant X 

 in the Saprolet/iiiea. 



In the Carposporea it is the cystocarp which is regarded as the 

 sporophore, and the plant developed from the carpospore is the 

 actual or potential oophore. The cystocarp varies much in 

 structure ; in Coleochcete it is not very different from an oospore or 

 a zygospore, but in the Ascoimjcetes and in the Floridem it is an 

 organ of considerable complexity. 



These views may be conveniently tabulated as follows : — 



Moss . 



Fern 



Zygosporece. 

 Mucor . . 

 Pandorina . 



Oosporece. 

 Peronospora. 

 CEdo{?onium . 



Carposporece. 

 Ascomjcetes. 

 Flo ride 36 . . 



Sporophore. 



Sporogonium 



Plant 



Zygospore and rudimentary mycelium 

 Zygospore 



Oospore 



Oospore . 



Cystocarp (apothecium) 



Cystocarp 



Oophore. 



Plant. 



Pi-othallium 



Mycelium. 

 Coenobium. 



Mycelium. 

 Thallus. 



Mycelium. 

 Thallus. 



This interpretation of the life-history of ThaUophytes has been 

 recently criticised by Pringsheim. § He lays it down as a funda- 

 mental axiom, that the generations of ThaUophytes, like those of 

 Cormophytes, begin in all cases with a free cell, the spore, but 

 that, unlike those of Cormophytes, the generations are distinct and 



* See Strasburger, ' Befruchtung und Zelltbeiluug,' p. 9. 

 + De Bary, ' Beitr. z. Morphol. u. I'hysiol. d. Pilze,' Heft II., p. 40. 

 X Pringsheim, ' Jahrb. f. vviss. Bot.,' Bd. IX. 



§ Ueb. d. Generationswechsel der Thallophyten, &c., 'Jahrb. f. wiss. 

 Bot.,' XL 



