8 THE ALGAE 



chlorophyll a, c-phycoerythrin and c-phycocyanin. One interesting 

 feature is the complete lack, so far as is known, of sterols in this 

 class. There is no known sexual reproduction, propagation taking 

 place by simple division, by non-motile gonidia, by spores or else 

 by vegetative fragmentation. Members of the group are widely 

 distributed in marine and fresh waters and also occur terrestrially. 

 Some have the capacity to fix nitrogen from the atmosphere and 

 others have the capacity to live in quite hot water. 



Chrysophycophyta 



(5) Chrysophyceae 



These form a primitive group, mostly of unicellular organisms, 

 in which the brown or orange colour of the chloroplasts is deter- 

 mined by excess of the carotinoid pigments. Most of the forms have 

 no cellulose cell wall and hence are *flagellates' in the old sense of 

 that term, but there are some members which do possess a cellulose 

 wall and hence are *algal' in the old sense of that term. Leucosin (a 

 proteinaceous substance) and oils are the usual forms of food 

 storage, whilst another marked feature is the silicified cysts or 

 statospores, which generally have a small aperture that is closed by 

 a special plug. The motile cells possess one, two or, more rarely, 

 three equal flagella attached at the front end, but in one sub- 

 section tiie two flagella are unequal in length. The most advanced 

 habit known is that of a branched algal filament, e.g. Phaeothamnion 

 (cf. p. 273), whilst the palmelloid types attain to a higher state of 

 differentiation, e.g. Hydrurus (cf. p. 273), than is commonly en- 

 coxmtered in either the majority of the palmelloid Chlorophyceae 

 or the Xanthophyceae. The occurrence of sexual reproduction is 

 uncertain and such records as there are point to simple isogamy. 



(6) Xanthophyceae 



The plants in this group are usually of a simple nature, but their 

 lines of morphological development frequently show an interesting 

 parallel or homoplasy with those observed in the Chlorophyceae 

 (cf. p. 325). The chloroplast is yellow-green in colour because of an 

 excess of /S-carotene, and starch is replaced by oil or leucosin as the 

 normal food storage material. The cell wall is frequently composed 

 of two equal or unequal halves overlapping one another. The 

 motile cells possess two unequal flagella arising from the anterior 



