54 
MORPHOLOGY 
cases the protoplast divides into two daughter protoplasts that escape and function 
as auxospores, the process suggesting real spore formation. Both of these methods 
are also associated with sexual reproduction : in the one case the escaped proto- 
plasts of two contiguous cells conjugate to form the auxospore; and in the other 
case the four daughter protoplasts from two contiguous cells pair and conjugate. 
An auxospore, therefore, may be a naked protoplast, an asexual spore, or a 
zygospore, the feature in common being that there is a restoration of size before 
valve formation. 
Relationships. Such characters do not suggest any evident relationships 
for the diatoms, and they seem to stand apart from other groups of thallophytes, 
excepting perhaps the Peridineae (see 
below), and such a connection would 
not help the problem much. In certain 
particulars the cell structure suggests 
that of the desmids, and by some the 
diatoms are associated with them under 
Chlorophyceae (Conjugales). The 
brown pigment associated with the 
green suggests Phaeophyceae, and 
hence in some texts diatoms are found 
among the brown algae. It is perhaps 
best at present to keep the group apart 
from others, as one of the several un- 
related groups of thallophytes. 
Peridineae. These organisms are 
mostly marine and are associated with 
diatoms in forming much of the 
plankton of the ocean. They consist 
K 
145 
142 143 144 
FIGS. 142145. Diatoms: 142, free- 
swimming form, valve side; 143, same form, 
girdle side, showing the relation of the 
valves; 144, 145, girdle and valve views of 
a stalked form. 142, 143, after PFITZER; 
144, 145, after ENGLER and PRANTL. 
usually of single cells which are naked 
or have a cellulose wall, which is often 
sculptured. The naked forms are 
laterally biciliate, thus resembling the zoospores of the brown algae. The nucleus 
is distinct ; there is a complex system of vacuoles ; and there may be green, 
yellow, or brown chromoplasts, or none at all ; meaning that some forms have 
the food-manufacturing power of algae and others have not. Many of the forms 
multiply freely by ordinary cell-division, and sometimes there are regularly 
formed zoospores. No gametes are recorded, but it seems very probable that 
the free " zoospores " may sometimes conjugate. The characters given indicate 
a low synthetic group ; for the ciliated vegetative cells suggest the simpler 
Volvocales ; the sculptured walls, chromoplasts, and other features suggest diatoms ; 
the laterally biciliate motile cells and yellow or brown chromoplasts suggest brown 
algae ; and the forms without chromoplasts do not suggest algae at all. 
(3) RHODOPHYCEAE 
General character. This great group, containing only a few fresh- 
water forms, includes the majority of the marine algae. They are by 
