82 



MANUAL OF BOTANY 



former is capable of bearing sexual cells as well, under appro- 

 priate conditions ; the latter is not. 



As mentioned above, the result of the coalescence of two 

 gametes ,is sometimes the production of a sporophyte. Very 

 often, however, the process only gives rise to another gameto- 

 phyte. 



The same plant body produces thus both sexual and asexual 

 cells. In this lowly class of plants the allotment of these two 

 varieties of reproductive structures each to its appropriate plant 

 body has not yet been reached. 



Above this group each kind is found developed upon its 

 special form, and the tw^o forms regularly alternate. 



Fig. 781. 



Fig. 782. 



Fig. 781. Vertical section of a female conceptacle of Fucus vesiculosus contaiiiiug 



oogonia and parapliyses. After Thuret. Fig. 782. Oogonium with the oo- 



spheres fully separated, and disengaging themselves from their coverings. 

 After Thuret. 



The gametophyte is the dominant form in the group ; the com- 

 mencement of the development of the sporophyte is indicated in 

 the group of Algse by the zygospores of many Chlorophyceae, which 

 produce two or four zoospores on germination, and by the forma- 

 tion of the cystocarps or fruits of the red seaweeds ; in the Fungi 

 by the behaviour of the zygote of the Myxochytridineae which gives 

 rise to zoospores and that of Mucor which produces a rudimentary 

 mycelium, giving rise at once to a sporangium with its spores. 



In the Thallophyta the differentiation of the sexual organs is 

 seldom very complex. The male cells or antherozoids, or sperma- 

 tozoids, are usually developed in antheridia, often consisting of 



