124 PLANT LIFE. 



contracts and forms a single oosphere containing a nucleus. 

 In monoecious species the antheridia are formed from cells 

 ' near to the oogonia. Two or three flask-shaped protuber- 

 ances spring from a cell and become cut off by septa, each 

 protuberance being an antheridium, the entire contents of 

 which form a single antherozoid furnished with two cilia. 

 It is presumed that the oospheres are fertilized by the 

 antherozoids passing down the tubular trichogyne, but this 

 has not yet been actually observed. After fertilization the 

 oosphere becomes clothed with a cell-wall, and forms an 

 oospore. At the same time the oogonium becomes sur- 

 rounded by a layer of cells, formed from branches that 

 spring from the cell supporting the oogonium. This cover- 

 ing is called the pericarp, and the whole structure is known 

 as a sforocarp, which assumes a dark brown colour, becomes 

 liberated by the disappearance of the vegetative parts of the 

 thallus, and enters the resting-stage. The trichogyne is not 

 enclosed in the pericarp, and soon disappears. 



During the resting-stage the oosphere remains unicellular, 

 but in the following spring the cortical layer is ruptured, 

 and the oosphere divides into several cells by repeated 

 bipartition. Each cell produces a single zoospore, which 

 gives origin to several asexual generations. Eventually a 

 sexual generation arises, which may be monoecious or 

 dioecious, according to the species. 



rioridesB. 



The most pronounced characteristics of the present order 

 are, asexual reproduction by perfectly motionless spores not 

 furnished with cilia ; being usually produced in fours in a 

 mother-cell or sporangium they are called tetraspores, or 

 sometimes tetragonidia. The oosphere is fertilized by 



