120 A TEXT-BOOK OF BOTANY 
of vegetation floating in mid-occan 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, 
Fic, 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 hy the tides, are used for enriching 
farm lands. 
72. Ectocarpus.—The two principal groups of brown 
Alge 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 
