CRYPTOGAMS 



373 



divisions leading to the development of the sexual cells in the oogonium and 

 antheridium, in which the reduction of chromosomes takes place, as corresponding 

 to a reduced sexual generation. On this view the thallus of Fucus would corre- 

 spond to the sporophyte of Dictyota, and the young sexual organs would be 

 equivalent to the tetrasporangia of the latter ( 41 ). 



Economic Uses. The dried stalks of Laminaria digitata and L. Cloustoni are 

 used as dilating agents in surgery. IODINE is obtained from the ash (varec, kelp) 

 of various Laminariaceae and Fucaceae, and formerly soda was similarly obtained. 

 Many Laminarias are rich in MANNITE (e.g. Laminaria saccharina), and are used in 

 its production, and also as an article of food by the Chinese and Japanese. 



CLASS XII 

 Rhodophyceae (Red Algae) ( 6> 20> 40 ) 



The Rhodophyceae or Florideae constitute an independent group 

 of the higher Algae, the phylogenetic origin of which is perhaps to 



FIG. 310. Chondrus traits. (^ nat. siw.) 



be sought among the higher Green Algae. They are almost ex- 

 clusively marine, and specially characterise the lowest algal region 

 on the coasts of all oceans, especially in temperate and tropical 



2 B 1 



