ALTERNATION OF GENERATIONS IN ALGiE. 



481 



such alternation; and in others again (such forms are numerous), an alternation 

 occurs, but of a character quite different from that of higher plants. 



First we will mention such as show an alternation of generations not unlike 

 that of the Fern. It will be remembered that in the Fern there is a simple pro- 

 thallium upon which the sexual organs arise, and from the fertilized egg-cell a new 

 generation, of considerable dimensions, is developed which produces asexual spores, 

 these in turn giving rise to prothallia. In the group of the Red Seaweeds or 

 Floridese (c/. pp. 61, 62, and 

 figs. 204 7 and 204 ^ p. 53), the 

 seaweed plant is the sexual 

 generation and bears the rudi- 

 mentary fruits with tricho- 

 gynes and the male spermatiti. 

 After fertilization, a consider- 

 able growth is initiated, which 

 results in a mass of spores 

 being abstricted, these spores 

 being in many cases inclosed 

 in a sort of capsule, which 

 develops concurrently with the 

 spores. This capsular struc- 

 ture with its spores we may 

 interpret as a very .simple 

 asexual generation comparaljle 

 to the sporogonium of a Moss 

 or to a Fern -plant with its 

 spores. Of course this asexual 

 generation is very ill-marked 

 in the Red Seaweed, and it 

 is difficult to quite draw the 

 line between it and the sexual 

 generation of which it forms 



a continuation. It has this in common with Mosses and Ferns; that from a single 

 process of fertilization a numerous progeny of spores is begotten— spores which 

 on germinating give rise to sexual plants again. 



The brown Wrack, Fucus, is an example of a Thallophyte in which alternation 

 of generations is not known to take place. In this seaweed every generation is a 

 sexual generation, and the fertilized egg-cells, so far as is known, give rise — not to 

 spores — but to new sexual generations. Its life-history is described and figured on 

 pp. 51, 52. 



And now we come to a type of alternation of generations, prevalent amongst 

 green Algae and some families of Fungi, which seems to be quite distinct from the 

 rhythmic alternation which obtains in the Mosses, Ferns, &c. The oft-mentioned 



Vol. II. 81 



Fig. 353. — Asexual and sexual reproduction in the Mucoiini. 



1 Mycelium producing asexual spores in stalked sporangia. 2 a single sporan- 

 gium in section, s Formation of a zygospore. 1 x 40 ; 2 x 260 ; 3 x 180. 



