120 A TEXT-BOOK OF BOTANY 



of vegetation floating in mid-ocean continue to grow luxu- 

 riantly, especially in warmer parts of the Atlantic. 



From the ashes of kelps and rockweeds the chief supply 

 of iodine is obtained; and these great masses of vegetation, 



FIG. 112. Fragment of Sargassum, showing differentiation of the thallus into 

 stem-like and leaf-like portions, and also the bladder-like floats. After BENNETT 

 and MURRAY. 



thrown up or left exposed by the tides, are used for enriching 

 farm lands. 



72. Ectocarpus. The two principal groups of brown 

 Algae are distinguished from each other by their re- 

 production. By far the larger group includes the kelps, 

 whose method of reproduction is very simple, although many 



