52 



MORPHOLOGY 



sporophyte generation (the one with 2X chromosomes), and the gameto- 

 phyte (the x generation) is represented only by the gametes. 



Sargassum. — The gulf weeds are well known on account of their con- 

 nection with the so-called Sargasso Sea. In that great ocean eddy, 

 these gulfweeds accumidate in vast quantities, and the impression has 

 been that they have been torn from the coast and swept out to sea. In 



any event, they continue grow- 

 ing, and perhaps pass through 

 their whole life history in this 

 floating condition. They are 

 remarkable for the differentia- 

 tion of the body into regions 

 which may well be called 

 leaves and branching stems, 

 and they also produce short 

 branches that develop the 

 bladder-like floats which re- 

 semble small berries (fig. 

 141). So far as is known, 

 the reproduction resembles 

 that of Fucus. 



In connection with the brown 

 algae, it is convenient to consider 

 two groups of thallophytes whose 

 connections are entirely uncertain. 

 It must be understood that they 



are not presented as brown algae, 

 Figs. 137-1*0— Fucus: 137, an egg freed „, ^^ ^1 ^ ^,1 



from the oogonium; 138, an egg surrounded by ° _, . . 



, , ,.,• J . Diatoms. — Ihis is a vast as- 



a swarm of sperms; 139, a fertilized egg begin- 

 ning to germinate; 140, a young plant. — After semblage of one-celled plants that 

 Thuret. occur in profusion in fresh and 



salt water and damp soil. They 

 exist in such tremendous numbers in the ocean as to form a large part of the floating 

 plankton, that free-swimming and free-floating world of minute organisms. Many 

 diatoms occur as fossils, forming large deposits, as the so-called siliceous earths, 

 etc. They are solitary and free-swimming forms, or are attached by gelatinous 

 stalks excreted by the cells, the stalks often profusely branching. The forms of 

 the cells are too numerous for description, a common free-swimming form being 

 boat-shape {Navicula), but there are rods, wedges, disks, etc. (figs. 142-145). 



Cell wall. — The cell wall is a special feature, for it consists of two siliceous 

 valves, one overlapping the other, like the two parts of a pill box. The wall is so 

 impregnated with silica that it forms a complete and resistant siliceous skeleton. 



