ALTERNATION OF GENERATIONS IN ALG^:. 



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 altei-nation 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 

 Floridea3 (c/. pp. 61, G2, and 

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

 seaweed plant is the sexual 

 generation and bears the rudi- 

 mentary fruits with tricho- 

 gynes and the male spermatia. 

 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 comparable 

 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 pi'ogeny 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 Algaj 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 



Asexual and sexual reprotluction in the "Mucorini. 



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

 gium in section, s Formation of a zygospore, i sTiO ; 2 x 260 ; a x 180. 



Vol. II. 



81 



