THE THALLOPHYTES 351 
parent valves and a second, which it has formed itself, more or 
less parallel to the first. 
In a number of species, repeated fission results in the forma- 
tion of succeedingly smaller and weaker individuals. This 
process, however, goes on only for a certain number of genera- 
tions until the decrease of size has reached a limit for the species, 
when the plant is rejuvenated by the formation of an auxospore. 
‘This may be formed with or 
without the conjugation of two 
parent-protoplasts. In either 
case the auxospore resulting 
undergoes a resting stage after 
which it develops new valves. 
The newly formed diatom is 
then several times the size of the 
individual or individuals which 
contributed to itsformation and fees >< 
is endowed with renewed vigor = 
for growth and division. 
Cxiass [V.—PHAOPHYCE, 
‘THE Brown ALG 
Mostly marine forms show- hn \ 
ing great diversity in the form Fig. 255.—Licmophora flabellata, a dia- 
of their vegetative bodies. pa nga ee ee 
They occur for the most part eA (4 sehag ohne a 
in salt water between the high 
and low tide marks. Some of them are always submerged. 
Their bodies are usually fixed to some support in the water by 
means of a holdfast, and are often highly differentiated both as 
to form and tissues. Some reach a hundred feet or more in length 
as, for example, Macrocystis, a giant kelp, which grows in the 
Pacific Ocean off the coast of California. They all contain the 
brown pigment called phycophein or fucoxanthin and the green pig- 
ments, chlorophyll, both of which are present in their chromato- 
phores. A yellowish pigment called phycoxanthin has also been 
isolated from some of the species. Many of the kelps and rock- 
