DICTYOTACE^. 109 



alone germinating after a period of rest, the others being 

 sterile. This peculiarity must be considered as the shadow- 

 ing in of Alternation of Generations, which becomes such a 

 marked feature in the Mosses and Ferns. 



In the present case, the first or sexual generation 

 terminates with the formation of the zygospore, which is 

 the direct and immediate product of fertilization. The 

 zygospore however does not germinate directly, but at once 

 produces two, three, or more cells by division, which con- 

 stitute the second or asexual stage, the one cell that 

 eventually germinates not being the immediate result of 

 sexual fertilization, hence a spore ; and the sterile cells must 

 be considered as an incipient pericarp. 



In the present group the sexual mode of reproduction by 

 conjugation reaches its maximum of development, and does 

 not recur higher in the scale of plant-development, the two 

 sexes being in all the ascending groups morphologically 

 differentiated. 



Dictyotacese. 



The species included in the present group are olive-brown 

 in colour, of a membranaceous texture, usually growing erect, 

 and often beautifully iridescent. Asexual reproduction takes 

 place by the formation of tetraspores, that is, the contents 

 of special cells called tetrasporangia break up into four 

 spores, which on their escape enclose themselves in a cel- 

 lulose cell-wall and germinate at once. Occasionally the 

 contents of a tetrasporangium divides into two portions ; or, 

 rarely, the contents escape as a single large spore. Tetra- 

 spores are perfectly motionless, and ciliated motile zoospores 

 are unknown in the group. The tetrasporangia are formed 

 by outgrowths of the superficial cells of the thallus, and may 



