74 THE ALGAE 



means of quadriflagellate macrozoospores, one to four being pro- 

 duced in each cell. These, after swarming for a few minutes, settle, 

 and germinate into a short filament which already possesses a hair 

 at the four- or five-celled stage when it commences to put out 

 rhizoids. Sexual reproduction is secured by means of quadriflagel- 

 late microswarmers or isogametes which fuse whilst in an amoeboid 

 state, though these gametes may also develop parthenogenetically. 

 The behaviour of the microswarmers demands further investiga- 

 tion as it does not seem to be clearly understood, nor has it been 

 determined whether the plants are haploid or diploid. In Drapar- 

 naldia glomerata the nature of the swarmer is controlled by the 

 pH of the medium, microswarmers being formed under alkaline 

 conditions and macrozoospores under neutral or acid conditions. 



Among the species and genera placed in the sub-family Erectae 

 the species Fritschiella tuherosa is of more than passing interest. 

 The plants possess four different vegetative systems: there is a 

 prostrate system attached by a system of rhizoidal threads and 

 giving rise to a primary erect system from which a secondary erect 

 system arises. The prostrate system is perennial and also gives rise 

 to the reproductive bodies. 



Other genera exhibit signs of reduction, being represented by 

 only the basal portion of the thaUus. Such genera are the epiphytic 

 disc-like Chaetopeltis (with emergent setae), the plate-like Pseuden- 

 doclonium and Ulvella, and the endophytic and endozoic Entocladia 

 {Endoderma) consisting of branched threads ramifying in the host. 

 This process of reduction has continued and reaches its climax in 

 the next example. 



* Pleurococcaceae : Pleurococcus (pleuro, box; coccus, berry). 



Fig- 39 

 The systematic position of this alga has varied considerably. By 

 some authors it has been placed in the Chlorococcales whilst others 

 have placed it in a special group, the Pleurococcales, but as the alga 

 can occasionally develop branched threads there would seem to be 

 evidence for regarding it as a much reduced member of the Chaeto- 

 phorales. There are, it is true, almost equally sound arguments for 

 the other systematic treatments of the genus, and its place at 

 present must be largely a matter of opinion. Pleurococcus is terrest- 

 rial and forms a green coat on trees, rocks and soil, growing in 

 situations where it may have to tolerate prolonged desiccation. 



