292 STRUCTURAL BOTANY 



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 seems remote, except perhaps in the case of 

 Cladotlirix and its immediate allies. Until the histology 

 of the Cyanophycese and Bacteria is better understood, it 

 will remain impossible to assign them to their true 

 position. The absence of sexual reproduction and of 

 ciliated swarm-cells tends to keep the Cyanophyceae in an 

 isolated position, though it now appears probable that 

 they resemble the Algae in possessing nuclei and possibly 

 chromatophores. 



Leaving these dubious forms, we come to Pleurococcus, 

 a perfectly typical green Algae, 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 no doubt a somewhat com- 

 plex one, but, even apart from that, its structure is a 

 sufficient indication of real affinity with more highly- 

 organised Algae. 



From unicellular green Algae, or their ancestors 

 among the Flagellata, quite a number of distinct lines 

 of affinity branch out. In one direction we reach the 

 Conjugatae, some of which are themselves unicellular, 

 while in others, such as Spirogyra, the cells are usually 

 united in filaments. This group always remains at a 

 low level anatomically, for no more complex 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 chloro- 

 plastids, 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 members of the group, namely 



