CLASSIFICATION 5 



phycophyta, Phaeophycophyta, Rhodophycophyta), and a few 

 would also treat the Charophyceae in the same way. The present 

 writer considers that phylogenetically these four classes are dis- 

 tantly related (see p. 309) and therefore groups them together. 

 Ultimately the classification adopted must depend upon the view 

 taken of the inter-relationships of these groups. A class of wholly 

 fossil organisms, the Nematophyceae, is also included among the 

 algae, but this is somewhat tentative as we have no means of know- 

 ing whether the plants, which only existed in the very earliest 

 times, really were algae. 



A brief summary of the principal characteristics of the different 

 classes follows. 



EUPHYCOPHYTA 



(i) Chlorophyceae 



This group used to comprise four great subclasses, the Isokontae 

 (equal cilia), Stephanokontae (ringed cilia), Akontae (no cilia) and 

 Heterokontae (imUke cilia). These subdivisions, as their names 

 imply, were based upon the organs of locomotion of the motile re- 

 productive bodies. The last subdivision has since been renamed 

 the Xanthophyceae and, because of its relationships with the 

 Bacillariophyceae and Chrysophyceae, has been transferred to the 

 Chrysophycophyta (see below). The structure of the Chloro- 

 phyceae covers a great range from simple unicells to multicellular 

 plants that are quite complex, though not as complex as those of 

 the Phaeophyceae and Rhodophyceae. In the Chlorophyceae the 

 chloroplasts vary considerably in shape and size whilst the final 

 product of photosynthesis is mostly starch together with fat. 

 Structures known as pyrenoids are often present and a starch 

 sheath can generally be demonstrated around them. The motile 

 cells of most members of the class are very similar and commonly 

 possess either two or four equal fiagellae, but in the Oedogoniales 

 (the former Stephanokontae) there is a ring of cilia whilst in the 

 Conjugales (the former Akontae) organs of locomotion are absent. 

 Sexual reproduction is of common occurrence and ranges from iso- 

 gamy to anisogamy and oogamy. Asexual reproduction normally 

 takes place by means of motile zoospores, but a variety of non- 

 motile spores (p. 19) may also be produced. The colour of the 

 cells is usually grass green because the pigments are the same as 



