EUPHYCOPHYTA I7 



(Siphonales, Dasycladales, Siphonocladales) lime may be deposited 

 on the walls. In Oedogonium and Cladophora there is said to be a 

 layer of chitin on the very outside. A study of the submicroscopic 

 structure of filamentous species has shown (Nicolai and Preston, 

 1952) that there are at least three groups : 

 Group I: The principal substance in the wall is crystalline 



natural cellulose. The wall structure in these algae 



resembles that of the higher plants. 

 Group II: The principal wall material is similar to mercerized 



cellulose.^ 

 Group III: Neither of the above forms of cellulose. Later (see 



p. 308) the significance of these groups in relation to 



classification will need to be discussed. 

 The chloroplasts, irrespective of shape or number, normally 

 contain rounded bodies, or pyrenoids, which are composed of a 

 viscous mass of protein (the crystalloid) surrounded by a sheath of 

 starch. The number of pyrenoids in each cell is commonly a con- 

 stant specific character and can be used for taxonomic purposes. 

 Very few famiUes of Chlorophyceae lack this structure, those that 

 do being the more highly evolved. In Derbesia the pyrenoids dis- 

 appear when the alga is grown in diffused light, and in several 

 genera they may disappear during zoospore formation. In such 

 cases they must arise again de novo, but more commonly they are 

 perpetuated by simple division at the time of mitosis. In those 

 algae with pyrenoids the production of starch grains is associated 

 with their presence. 



Each cell usually contains one nucleus, but in certain orders a 

 multinucleate condition is to be found, and in the Siphonales cell 

 walls are only laid down at reproduction so that the vegetative plant 

 is multinucleate. Each nucleus possesses a deeply staining body, 

 the nucleolus, together with chromosomes which are usually small, 

 short and few in number. At cell division, in those cases where 

 there is but one plastid, the chloroplast divides into two daughter 

 plastids. In many species there is a distinct diurnal periodicity in 

 the time at which nuclear and cell division takes place. The 

 flagella of the motile bodies are composed of an axial cytoplasmic 

 filament or axoneme surrounded, except at the very apex, by a 

 sheath which probably has the power of contraction. In Chlamydo- 

 monas the electron microscope has revealed that the axoneme 



1 When cellulose is treated with strong alkali, changes take place, and it is then 

 referred to as mercerized cellulose. 



