CHAPTER V 



CHLOROPHYCEAE (cont.) (CONJUGALES, CHAR- 

 ALES), XANTH OP HYCEAE(HETE ROKO NT AE), 

 BACILLARIOPHYCEAE, CHRYSOPHYCEAE, 

 CRYPTOPHYCEAE, DINOPHYCEAE 



CHLOROPHYCEAE 



*CONyUGALES 



The members of this group are somewhat distinct from the other 

 groups of the Chlorophyceae that have already been described and 

 at one time they were classed in a separate division, the Akontae. 

 As their pigmentation and metabolism are fundamentally the same, 

 however, it would seem desirable to abandon this arrangement. 

 Their peculiar reproduction suggests that they were evolved at a 

 very early stage from one of the simpler orders of the Chlorophyceae. 

 The order is subdivided into two distinct divisions, the Zygne- 

 maceae which are filamentous and the Desmidiaceae most of which 

 are not, although recently some desmids have been classed with 

 the Zygnemaceae. 



*Zygnemaceae : Spirogyra {spiro, co\\\ gyra, curved). Figs. 67, 68. 



The unbranched filaments are normally free-living although 

 attached forms are known, e.g. S. adnata, and they form slimy 

 threads which are known as ''Water-silk" or "Mermaid's tresses". 

 These grow in stagnant water and are most abundant in either the 

 spring or autumn, the latter phase being due to the germination of 

 a percentage of the spring zygospores. Each cell contains one or 

 more chloroplasts possessing either a smooth or serrate margin and 

 arranged in a characteristic parietal spiral band. The single nucleus 

 is suspended in the middle of the large central vacuole by means of 

 protoplasmic threads that radiate out to the parietal protoplasm. 

 The chloroplasts, which may occasionally be branched, are T- or 

 U-shaped in cross-section and contain numerous pyrenoids which 

 project into the vacuole on the inner side, the majority of the 

 pyrenoids arising de novo at cell division. The cell wall is thin and 

 composed, according to some investigators, of two cellulose layers. 



