CLASSIFICATION 9 



end. Sometimes non-motile aplanospores are produced instead of 

 zoospores. Sexual reproduction is rare and when present is iso- 

 gamous, though in one genus ( Vaucheria) there is a well-developed 

 oogamy. This group is commonly regarded as having distinct 

 afiinities with the preceding group. 



(7) Bacillariophyceae {Diatoms) 



One of the characteristic features of these plants is their cell wall 

 which is composed partiy of sihca and partly of pectin. The wall is 

 always in two halves and frequently ornamented with deHcate 

 markings, which are so fine that microscope manufacturers make 

 use of them in order to determine the resolving power of the lenses. 

 Each cell contains one to many, variously shaped chromatophores, 

 which are yellow or golden brown, containing accessory xantho- 

 phylls in addition to the usual chlorophyll a, and /S-carotene. They 

 are divided into two major groups, the representatives of one 

 group being radially synmietrical, those of the other being bilater- 

 ally symmetrical. The presence of flagellate stages is highly prob- 

 able in the former whilst there is a special type of sexual fusion in 

 the latter group (cf p. 272). The relationship of this group to the 

 two preceding groups is not as distinct as it is between the Chryso- 

 phyceae and Xanthophyceae. 



Pyrrophycophyta 



(8) Cryptophyceae 



Each cell commonly contains two large parietal chromatophores 

 of diverse colour, though frequendy of a brown shade, whilst the 

 product of photosynthesis is starch. The motile cells have two un- 

 equal ciHa and often possess a complex vacuolar system. Nearly 

 all the members have a *flagellate' organization and there are no 

 true filamentous algal forms. One type {Tetragonidium) (cf p. 275) 

 has been described as having a tendency towards the coccoid (non- 

 motile unicell with a cell wall) habit, but in general the class must 

 be regarded as the least *algal' like of all. Isogamy has been recorded 

 for one species. 



(9) Dinophyceae 



Most of the members of this class are motile unicells, but there 

 has been an evolutionary tendency towards a sedentary existence 



B 



