FRESHWATER ALG^. 15 



In view of the fact that this characteristic arrangement ol the oil-drops was 

 noticed in all the cells of certain groups, it appears to be typical of a definite 

 form of the species. 



9. Pleurococctjs koettlitzi* sp. n. 

 (PI, I., figs. 36-42). 



Cellulis globosis vel saepe plus minusve ellipsoideis, plerumque 11-16 yu latis, in 

 familiis parvis irregularibus vel coelastriformibus vel quadricellularibus et tetraedricis 

 aggregatis ad superficiem stratuum Phormidii affixis ; membrana cellularum tenui, 

 sed distincta ; contentu homogeneo, obscurato accumulatione magna olei, quod 

 semper adesse videtur' et facit in familiis cellularum aspectum valde refringentem et 

 saepe nigrescentem ; contentu interdum rubescente ; chromatophora singula probabilis 

 est, cum pyrenoide. Propagatio divisione contentus cellulge in quatuor cellulas filiales, 

 quae sunt tetraedrice dispositse. 



Diam. cell. = 11-16 /a. ; cell, ovales plerumque 15 /a long, et 12-13 ju. lat. 



Hob. — Widely distributed on the Phormidium-sh.eets, especially from the Gap 

 pond. Winter Harbour. 



The most prominent characteristics of this species are the often slightly oval 

 shape of the cells, the very prominent storage of fatty oil and the tetrad-like 

 grouping of the cells in young and often in older families. Young colonies are not 

 uncommonly to be found in which the four cells form an obvious tetrad (figs. 37, 

 39, 41) ; in the older colonies the grouping of the cells is frequently very similar to 

 that of such a form as Coelastrum microporu7n, but, when the colonies consist of very 

 large numbers of cells, their grouping becomes irregular (fig. 36). Even in these 

 irregular colonies, however, individual groups of cells are often in tetrads. This 

 prevalent tetrad-like grouping is undoubtedly due to the method of reproduction, four 

 daughter-cells arising within the membrane of the mother-cell (fig. 40, only three 

 cells are visible) ; it appears that some of the cells of the tetrad may occasionally 

 abort (cf. fig. 42). As detached pieces of cell-membrane are sometimes found lying 

 near the colonies of this species it seems probable that the membrane of the 

 mother-cell is ruptured and does not undergo gelatinisation. 



The fatty oil generally obscures the whole of the cell-contents and gives the 

 colonies, under the low power of the microscope, a dark highly refractive appearance 

 which is very characteristic. Osmic acid gives a deep brownish-black colouration. 

 In some of the larger colonies the contents of occasional cells may be tinged 

 yellowish-red, without these cells presenting any other point of difi"erence to the 

 normal ones. A pyrenoid and a limited number of starch-grains are generally present, 

 but the pyrenoid does not appear to be quite constant. The cell-wall is 

 generally thin. 



* Named after the collector, Dr. Koettlitz, surgeon to the ' Discovery.' 

 VOIi. VI. I 



