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. 

 142 143 144 Peridineae. — These organisms are 



Figs. 142-145. — Diatoms: 142, free- mostly marine and are associated with 

 swimming form, valve side; 143, same form, diatoms in forming much of the 

 girdle side, showing the relation of the plankton of the ocean. They consist 

 valves; 144, 145. girdle and valve views of usually of single cells which are naked 

 a stalked form. - 142, 143, after Pfitzer; ^^ ^^^^ ^ cellulose wall, which is often 

 144, 145, after Englee and Prantl. , 1 j r„, , , ^ 



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 greeii, 

 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 





