EEPKODUCTIVE CELLS. 145 



(sexual generation) or prothallia, and the process occurring in 

 the Homosporous type — viz., formation of the prothallus from the 

 one spore — might be looked upon as the fusion of two prothallia, 

 produced by the germination of a potentially double (male and 

 female) spore. 



The further history of the cycle in Marsilea consists in the 

 freeing of the motile antherozooids, and the fusion of one of 

 these with the oosphere nucleus, the oosphere being then known 

 as the oospore. From the oospore the embryo-sporophyte 

 (asexual generation) is again produced. Other Heterosporous 

 Pteridophyta are the SelaginelleEe and Isoetese. Equisetum gives 

 rise to spores all of the same size, but the sexual organs arise on 

 separate prothallia (dioecism). The determination of the sex of 

 the prothallium in this case is largely a question of nutrition. 



Reproduction in the Bryophyta, Fungi, and Algae. 



In order to complete the survey of the reproductive 

 processes occurring in plants it is necessary to examine briefly 

 the main variations occurring in the reproduction of Mosses, 

 Liverworts, Fungi, and Algse. It is not intended here to give 

 an exhaustive account of these, as this would involve the con- 

 sideration of many subsidiary groups, and would, moreover, lead 

 to inevitable confusion. For a full account of many of these 

 the student may be referred to Goebel's Outlines of Classification 

 and Special Mwpliology, or any of the larger text-books. 



1. Reproduction in the Musci (Bryophyta). — In these plants 

 an alternation of generations exists, the plant arising from the 

 oospore — viz., the sporophyte — nevertheless, remaining in a special 

 organ found in connection with the fructification of the moss- 

 plant. Thus, antheridia and oogonia arise on the moss-plant, 

 which is here the gametophyte, or sexual generation, in special 

 fertile shoots, and the result of fusion of an antherozooid with 

 the oosphere (the product being the oospore) is the forma- 

 tion of a mass of cells known as sporogenous cells, inside a 

 special organ, the sporogonium. The sporogonium and sporo- 

 genous cells are thus comparable to the sporophyte, or asexual 

 generation, and the ripe spore, on germination, gives rise to a 

 rudimentary cellular structure, the protonema, from which the 

 moss-plant or gametophyte, is produced. The moss-plant proper 



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