362 ALG^. [chap. IV. 



one or more individuals, to conditions presenting considerable 

 diversity between the sexual cells, more especially manifest 

 in the male- cell {a?itheridium), which when diverse in form 

 from the female- cell {pogoiiiiivi) usually liberates motile 

 zoospore-like antherozoids, which become merged in the 

 contents of the oogonium and so determine the development, 

 either immediately or after a pause, of a germ-spore. 



Forms of a higher grade of structure are represented by 

 the fine hair-like filaments which we find floating in rivulets 

 and tanks, rooted at one extremi'y to stems or to larger 

 water-plants. Many of these filamentous species ( Confef- 

 voidece) multiply themselves by the contents of the cells 

 which form their filaments being resolved into innumerable 

 minute moving bodies, called zoospores, which break out of 

 the cells and rush about in the water until they finally setUe 

 down and grow. 



The higher species, such as the Olive-coloured Sea-Weeds 

 (Fucus), v/hich clothe the rocks between tide marks upon 

 the shores of northern countries, possess a complicated re- 

 productive system of spores and antheridia, contained in con- 

 ceptacles embedded in the thickened extremities of the divided 

 fronds. Their mode of reproduction, adapted to the medium 

 in which the species grow, agrees in essentials with that which 

 is characteristic of Ferns and Mosses ; with this difference, 

 however, that the spores themselves are directly fertilized 

 and rendered capable of independent growth by the contact 

 of the minute spermatozoids contained in the antheridia. 



The marine species vary in colour, some being usually 

 olive, others red or green. The colour is employed as an 

 aid in their classification. 



Many species are used for food. Some of the larger 

 marine species used to be burnt for the sake of their 

 alkaline ash (kelp) and for the iodine which they contain. 



