Chapter IX 



CHRYSOPHYCOPHYTA 



XANTHOPHYCEAE, BACILLARIOPHYCEAE, 

 CHRYSOPHYCEAE 



PYRROPHYCOPHYTA 



CRYPTOPHYCEAE, DINOPHYCEAE 



* Xanthophyceae 



As a group the Xanthophyceae exhibit considerably less differen- 

 tiation than the Chlorophyceae. Two of the most characteristic 

 features are the replacement by oil of starch as a food reserve and a 

 greater quantity of B-carotene in the plastids, although the actual 

 amount of the latter is partially dependent upon the external con- 

 ditions. The pigment turns blue-green when the cells are heated in 

 concentrated hydrochloric acid and this forms a convenient test for 

 distinguishing them from the Chlorophyceae. The walls are fre- 

 quently in two equal or unequal portions which overlap, their com- 

 position being principally of a pectic substance although some 

 cellulose may occasionally be present. The motile bodies contain 

 more than one chloroplast and are further characterized by two 

 unequal flagella, the longer one often possessing dehcate ciHa. 

 The Xanthophyceae exhibit very Uttle regularity in the formation 

 of reproductive bodies. Sexual reproduction is rare and in most of 

 the known examples is isogamous. The outstanding exception is 

 Vaucherta, which in the past has been placed in the Siphonales. 

 Here there is a well-developed oogamy. The principal mode of 

 reproduction within the class is by means of zoospores and aplano- 

 spores. Simple division occurs in Chloramoeha and cyst formation 

 is known in Halosphaera and Botrydium. The majority of the 

 species are confined to fresh water. It would seem that they have a 

 motile imicell ancestry, the chief interest of the group being the 

 manner in which evolution has taken place along Hnes parallel to 

 those found in the Chlorophyceae. As a result there exists a set of 



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