CLASSIFICATION 3 



(5) Bacillariophyceae (Diatoms). One of the characteristics of 

 these plants is their cell walls which are composed partly of 

 silica and partly of pectic material. The wall is always in two halves 

 and frequently ornamented with delicate markings, which are so 

 fine that microscope manufacturers make use of them in order to 

 determine the resolving power of their lenses. The chromato- 

 phores are yellow or golden brown containing, in addition to the 

 usual pigments, accessory brown colouring materials whose nature 

 is only just being established. One set of forms is radially sym- 

 metrical, the other bilaterally so. The presence of flagellate stages is 

 highly probable in the former whilst there is a special type of 

 sexual fusion in the latter group (cf. p. 122). 



(6) Cryptophyceae. There are usually two large parietal chloro- 

 plasts with diverse colours, though frequently of a brown shade, 

 whilst the product of photosynthesis is starch or a closely related 

 compound. The motile cells have two unequal flagellae and often 

 possess a complex vacuolar system. Nearly all the members have a 

 "flagellate " organization and there is no example of the filamentous 

 habit. One type, however, has been described with a tendency 

 towards the coccoid (non-motile unicell with a cell wall) habit, and 

 so this must be regarded as the least "algal "-like class. Isogamy 

 has been recorded for one species. 



(7) DiNOPHYCEAE. Most of the members of this class are motile 

 unicells, but there has been an evolutionary tendency towards a 

 sedentary existence and the development of short algal filaments, 

 e.g. Dinothrix (cf. p. 126). Many are surrounded by an elaborate 

 cellulose wall bearing sculptured plates and inside there are discoid 

 chromatophores, dark yellow or brown in colour and containing a 

 number of special pigments. The products of photosynthesis are 

 starch and fat. The motile cells normally possess two furrows, one 

 transverse and one longitudinal, although they may be absent in 

 some of the more primitive members. The transverse flagellum lies 

 in the former, and the latter is the starting point for the other 

 flagellum which points backwards. Sexual reproduction, if it 

 occurs, is isogamous, and it has not been clearly established in the 

 few cases reported. Characteristic resting cysts are also produced by 

 many of the forms. 



(8) Phaeophyceae. This group comprises the common brown 

 algae of the seashore and it is worth noting that the majority are 



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