PRINCIPAL GROUPS OF PLANTS. 75 



aration of orchil, a purplish-red pasty mass, sulphuric acid and 

 salt are subsequently added. A number of species contain a yel- 

 low coloring principle, as Zeora sulphurea, Zeora sordida, Lecidca 

 geographica and Opegrapha cpigcva. 



ARCHEGONIATES. 



The two main features which distinguish the Archegoniates 

 from the Thallophytes are the structure of the sexual organs and 

 the distinct manner in which the peculiar phases known as alter- 

 nation of generations are shown. The antheridium or male sexual 

 organ is a well differentiated multicellular body which is either 

 sunk in the adjacent tissues of the plant or is provided with a 

 stalk. Within it are organized the sperms or spermatozoids, which 

 are ciliate and swim freely in water. Corresponding to the oogo- 

 nium of the Thallophytes is the archegonium or female sexual 

 organ which gives name to the group. The archegonium is a 

 flask-shaped cellular body consisting of a basal portion of venter, 

 which contains a single ^gg, and a neck through which the sperms 

 enter (Figs. 49 and 51). 



In the life history of this group of plants there are two gen- 

 erations or phases of development. During one stage the arche- 

 gonium and antheridium are developed, and this is known as the 

 sexual generation, and as these organs give rise to gametes or 

 sexual cells it is also spoken of as the gametophyte. By the union 

 of the sex cells (sperm and ^gg) an oospore is formed which 

 germinates at once within the archegonium. That portion of the 

 plant which develops from the oospore gives rise to asexual 

 spores, and hence this phase is called the asexual generation. 

 It is also spoken of as the sporophyte from the fact that it gives 

 rise to spores. These spores are in the nature of resting spores 

 and do not germinate on the plant as does the oospore. They are 

 distributed and on germination give rise to the gametophyte stage. 



In some of the Archegoniates these two phases are combined 

 in one plant, as in the Bryophytes, whereas in other members of 

 the group the two phases are represented by two distinct plants ; 

 that is, the gametophyte and sporophyte become independent of 

 each other, as in the Ferns. 



