2 CLASSIFICATION 



synthesis is starch together with oil, and a starch sheath can often 

 be demonstrated around the pyrenoids. In the bulk of the members 

 of this class the motile cells are very similar and commonly possess 

 either two or four flagellae, but in the Oedogoniales (Stephano- 

 kontae) there is a ring of flagellae whilst in the Conjugales (Akontae) 

 there are no organs of propulsion. Sexual reproduction is of 

 common occurrence and ranges from isogamy to anisogamy and 

 oogamy. The colour of the cells is usually a grass green because the 

 pigments are the same as those present in the higher plants and, 

 furthermore, they are present in much the same proportions. 



(3) Xanthophyceae (Heterokontae). The plants in this group 

 are usually of a simple nature, but their lines of development 

 frequently show an interesting parallel or homoplasy with those 

 observed in the preceding group (cf. p. 264). The chloroplast is 

 yellow-green owing to an excess of xanthophyll, one of the four 

 normal constituents of chlorophyll. Oil replaces starch as the 

 normal storage material, the lack of starch being correlated with the 

 absence or paucity of pyrenoids. The motile cells possess two 

 unequal flagellae (occasionally only one) arising from the anterior 

 end. Sexual reproduction is rare and when present is isogamous. 

 The cell wall is frequently composed of two equal or unequal halves 

 overlapping one another. 



(4) Chrysophyceae. These form another very primitive group 

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

 termined by the presence of accessory pigments such as phyco- 

 chrysin. Most of the forms have no cell wall and hence are 

 *' flagellates" in the old sense of that term, although there are some 

 members which do possess a cell wall and hence are "algal" in the 

 old sense of the term. Fat and leucosin (a protein-like substance) 

 are the usual forms of food storage, whilst another marked feature 

 is the silicified cysts which generally have a small aperture that is 

 closed by a special plug. The motile cells possess one, two or, 

 more rarely, three equal flagellae attached at the front end, but in 

 one subsection the paired flagellae are unequal in length. The most 

 advanced habit known is that of a branched filament, e.g. Phaeo- 

 thamnion (cf. p. 123), whilst the palmelloid types attain to a much 

 higher state of differentiation, e.g. Hydrurus (cf . p. 1 23), than in either 

 the Chlorophyceae or the Xanthophyceae. Sexual reproduction is not 

 certain, and such records as there are point simply to isogamy. 



