CYANOPHYCEAE 37 



the protista. Of these, two groups in particular deserve careful attention, 

 the blue-green algae or Cyanophyceae, and the Flagellata. 



The Cyanophyceae present in their outward configuration many points 

 of resemblance to the bacteria. We find spherical cells (Chroococcus), rods 

 (Aphanothece), long cell chains or filaments (Oscillaria), and spirally twisted 

 unbranched forms (Spirulina). As among the bacteria, too, colonies or 

 ' growth-forms ' occur ; some like sarcina (Gloeocapsa), others in flat plates 

 (Merismopoedia), while sheath-bearing bacterial species like Cladothrix find 

 their parallels among the Scytonemeae (Tolypothrix) with their dichotomous 

 false branching. But it must not be forgotten that these striking similarities 

 to the bacteria are not confined to the Cyanophyceae; the chlorophyll- 

 bearing algae offer a like series of parallels. The powdery green coating seen 

 on the north side of tree-trunks, &c, consists of minute, free-living spherical 

 cells (Pleurococcus), pools and ditches are often coloured green by myriads 

 of rod-shaped species (Stichococcus), the curved vibrios have their analogy 

 in the gracefully-formed Raphidia, and examples of sheathed filaments and 

 capsulation are not wanting. This is not surprising, for free-living cells 

 must have the form either of spheres or cylinders, and the simplest cell 

 unions must be filaments, flat plates, or packets. The similarity of external 

 configuration is only a superficial similarity and in no way justifies a 

 systematic approximation of the two groups. 



The cells of Cyanophyceae, whether free-living {Chroococcus, Aphano- 

 thece) or united to form filaments, multiply like all other cells by division, 

 just as do the bacteria. Just as these, too, do the isolated Cyanophyceae 

 ' split off ' from their sister cells. And for this reason, supported by the 

 not less superficial one of resemblance in shape, the bluish-green algae 

 (' Schizophyceae ') have been placed alongside the ' fission-fungi ' (' Schizo- 

 mycetes') to form a separate class of plants, the ' Schizophyta/ But the 

 ' fission ' of these organisms is not peculiar to them, for it must always occur 

 when isolated unicellular organisms divide ; and the opinion that the bacteria 

 are a series of colourless organisms parallel to the schizophyceae is not 

 well founded. 



The differences between the two groups are as numerous as the similari- 

 ties. The Cyanophyceae, if we except the gliding movements of the 

 Oscillariae, are non-motile, whereas a large proportion of the bacteria are 

 actively motile and possess special locomotive organs, the cilia. These are 

 moreover present not only at the time of reproduction, but persist throughout 

 the life of the cell. The sporulation of the two groups is also different. 

 The Cyanophyceae form not endospores but arthrospores, which arise by 

 the differentiation and enlargement of a whole cell. In the finer structure 

 of the cell again there is but one detail which the Cyanophyceae and bacteria 

 have in common, the absence of a nucleus. In other respects the cells of the 

 Cyanophyceae are much more highly differentiated than those of bacteria. 



