286 THE ALGAE 



endospores which are abstricted successively from the apex of the 

 surface cells by transverse division, and as these bodies have been 

 regarded as one-celled hormogones, the genus thus forms a link 

 between the Chroococcales and Nostocales. There has been much 

 confusion about this order, but Drouet and Daily consider that 

 species previously placed in different genera, e.g. Chamaesiphon, 

 Dermocarpa, Xenococcus, simply represent growth stages of one 

 and the same species. The few species recognized are now placed 

 in the single genus Entophysalis. 



Pleurocapsales 



This is another small order about which very Httle is known. It 

 consists primarily of cells which divide in one or two planes to give 

 a heterotrichous thallus forming crusts or cushions that are epi- 

 phytic, epiHthic or endohthic. They may play a considerable part in 

 rock formation in some parts of the world. 



Pleurocapsaceae : P/^wrocap5a (Pleuro, bag; capsa, box). Fig. 165 



This is the only genus that has really been studied adequately. 

 The primary thread creeps over the substratum and later gives rise 

 to a pseudo-parenchymatous prostrate system. Still later densely 

 packed erect threads arise from this basal system so that the final 

 plant thallus is a crust. The erect threads may branch, or segment 

 along three planes or divide obHquely, whilst gelatinization giving 

 Chroococcoid groupings can also occur. Reproduction takes place 

 by means of endospores that are usually produced in the terminal 

 cells. 



Fig. 165 Pleurocapsa minor; left, adult; right, juvenile creeping 

 filament ( x 800). (After Geitler.) 



