CHAPTER III 



CHLOROPHYCEAE 



VOLVOCALES, CHLOROCOCCALES, ULOTRICHALES, 



OEDOGONIALES 



*INTRODUCTION 



The older botanists included in the term Chlorophyceae the forms 

 which are now placed in the Xanthophyceae (cf. p. 113), but in 

 1897 Bohlin pointed out that some of the green algae possessed 

 unequal cilia, and in 1899 Luther coined the term " Heterokontae " 

 for such forms. In 1902 Blackman and Tansley revised the classi- 

 fication of the green algae using the terms Isokontae, Akontae, 

 Stephanokontae and Heterokontae. These were adopted by most 

 workers and remained in use until 1927 when Fritsch included the 

 Akontae and Stephanokontae in the Isokontae. The term Isokontae 

 has thus ceased to be of significance and the group is now included 

 with the Akontae and Stephanokontae in the Chlorophyceae. A 

 division into two great groups, marine and fresh water, as suggested 

 by Tilden in 1935, is not at all feasible, because nearly all the 

 morphologically distinguishable families possess representatives in 

 both environments. 



The cell structure is fairly characteristic, the protoplast often 

 containing a large central vacuole, which in the simpler forms is 

 contractile and serves to remove surplus water and waste matter. 

 The green pigment, which is essentially identical with that of the 

 higher plants, is contained in plastids : there is usually only one of 

 these in a cell and its outline may be discoid, star-shaped, spiral, 

 plate-like or reticulate. There is some evidence to show that these 

 plastids are capable of movement in response to light stimuli. 

 Other colouring matter may also be present, e.g. haematochrome in 

 Sphaerella and phycoporphyrin in some of the Zygnemales, whilst 

 fucoxanthin (cf. p. 129) is found in Zygnema pectinatum. The cells 

 are commonly surrounded by a two-layered wall, the inner, which is 

 often lamellate, being of cellulose, and the outer of pectin, but in 

 some forms the outer surface of this pectin sheath is dissolved as 

 fast as it is formed on the inner side. In a few species there is a third 



