Coelastrex 



205 



Sub-family CfELASTRE^E 1 . This is the only group of the Autosporacese in 

 which all the forms possess a definite and regular coenobium. In this case, 

 however, the coenobium is not flat (as in Scenedesmus among the Selenastrese) 

 but spherical or polyhedral. The three known genera are essentially different 

 in the grouping of the cells. In Coelastrum the cells are rounded or polygonal, 

 adhering closely by their margins to form a hollow sphere or cube (G. cubicmn). 

 In most species the outer surface of each cell is furnished with a short truncate 

 process, which attains its maximum development in G. cambricum var. 

 nasutum. In the African species, G. compositum, the normal single cell is 

 replaced by a tetrad of four (vide G. S. W., '07). In C. cambrictim the cells 

 are joined by lateral truncate processes (fig. 133 A) and in C. reticidatum, 

 which is mostly* a plankton-species, similar processes occur, but of a more 

 slender and elongate character. In BurJdllia, a genus at present only known 

 from Burma, the cells are more loosely coherent, being held in position 



Fig. 133. 



B 



A, Ccclastrum cambricum Archer. B D, Coelastrum sph&ricum Nag. Both C and D 

 show the formation of daughter-ccenobia (autocolonies). All x 475. 



mainly by a small amount of mucus resulting from the conversion into 

 mucilage of the wall of the original mother-cell. Each cell of the colony is 

 provided with a solid conical horn of large size (fig. 134 A). In Sorastrum 

 the cells are slightly compressed and lunate, each angle being furnished with 

 a pair of spines (fig. 1345 and C). From the back of each cell there projects 

 a colourless process, and all the processes meet in the central region of the 

 colony where they form the facets of a small sphere. 



The number of cells in a colony varies from 8 to 64, but 16 is the most 

 frequent number. There is a single massive chloroplast in each cell, practi- 

 cally filling up the whole cell-cavity, and in it there is usually a centrally 

 placed pyrenoid. Multiplication occurs by the formation of autocolonies 

 which are miniatures of the adult form when set free, their liberation being 



1 G. S. West, '04. The ' Ccelastraceae ' of Wille ('09) appears to be an unnatural group in 

 which are included a number of genera having little close affinity with each other. Genera such 

 as Ankistrodesmus, Selenastrum, Actinastrum, etc., are much less advanced types than Caelastrum 

 or Sorastrum. 



