290 STRUCTURAL BOTANY 



present. It is doubtful whether the Cyanophyceoe should 

 be classed with the Algae or not ; some botanists group 

 them with the Bacteria in a class by themselves the 

 Schizophyta. As already pointed out, however, this 

 relationship only seems to hold good for a portion of the 

 Bacteria, such as Cladothrix and its immediate allies. 

 Until the histology of the Cyanophycese is better under- 

 stood, it will remain impossible to assign them to their 

 true position. Their present isolation depends on 

 negative characters (absence of sexual reproduction and 

 of nuclei and chloroplastids), which may cease to hold 

 good when our knowledge is more complete. 



Leaving these dubious forms, we come to Pleurococcus, 

 a perfectly typical green Alga, the cells of which, though 

 leading a separate existence, possess all the histological 

 characters of the green cells of the higher plants. The 

 life-history of Pleurococcus is probably a somewhat com- 

 plex one, but, however that may be, its structure is a 

 sufficient indication of real affinity with more highly- 

 organised Algae. 



From unicellular green Algae quite a number of 

 distinct lines of affinity branch out. In one direction 

 we reach the Conjugates, some of which are themselves 

 unicellular, while in others, such as Spirogym, the cells 

 are usually united in filaments. This group always 

 remains at a low level anatomically, for no more com- 

 plex thallus than a simple filament is ever produced. 

 On the other hand, the histological structure shows a 

 great advance, as indicated especially by the highly- 

 differentiated chloroplastids, which not only assume 

 strange and varied forms, but are much specialised 

 internally, possessing proteid bodies (pyrenoids) around 

 which the formation of starch is localised. In many 



