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 accumulate 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. 137-140. Fucus: 137, an egg freed , ,, 
. , ill OF 3.S aigcLc tit all* 
from the oogonium; 138, an egg surrounded by . 
a swarm of sperms; 139, a fertilized egg begin- Diatoms. - 
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 skeletoa 
