Chapter III 



EUPHYCOPHYTA 



CHLOROPHYCEAE 



volvocales3 chlorococcales5 ulotrichales 



* Introduction 



Cell structure within the Chlorophyceae is fairly characteristic, the 

 protoplast commonly surrounding a large central vacuole, whilst in 

 some of the simpler unicellular forms there may also be two or 

 more small, contractile vacuoles, the fimction of which is the re- 

 moval of surplus water and waste matter. The pigments, which are 

 essentially identical with those of the higher plants, are contained 

 in plastids : these vary from one to many per cell and in outline may 

 be discoid, star-shaped, spiral, plate-like or reticulate. In certain 

 primitive members plastids are lacking. There is some evidence to 

 show that these plastids are capable of movement in response to 

 light stimuh. The proportions of the pigments may change at the 

 time of gamete formation, i.e. in Ulva lohata fertile female thaUi 

 are oHve green and fertile male thalH brownish. Cells of fertile 

 thalU contain much more carotene than those of vegetative thaUi 

 and a large proportion is the y isomer.^ Other colouring matter 

 may also be present, e.g. euglenarhodone in Sphaerella and phyco- 

 porphyrin in some of the Zygnemales, whilst fucoxanthin (cf. 

 p. 2) has been reported in Zygnema pectinatum. 



The cells are commonly surrounded by a two-layered wall, the 

 inner, which is often lamellate, being of cellulose (callose in certain 

 Siphonales), and the outer of pectin, but in some forms the outer 

 surface of thds pectin sheath is dissolved as fast as it is being formed 

 on the inner side. Some of the very primitive forms lack a cell wall 

 but in such cases the exterior portion of the protoplasm still re- 

 mains rigid. In a few species the outer layer of pectose becomes 

 impregnated with an insoluble substance whilst in others the pectin 

 layer gradually increases in thickness, and in at least three orders 



1 Only a trace of this occurs normally. 



16 



