186 F. KREBLE AND F. W. GAMBLE. 



cells. It is probable, therefore, tliat the constituents of a 

 colony may, under certain conditions, all pass into the resting 

 stage. 



Another curious colonial form also occurs. Oval green 

 cells showing the chai'acters of the active cells, differentiated 

 chloroplast, eye-spot, and pyrenoid, are attached to one 

 another by sleeve-like branching columns of stratiGed 

 mucilage (fig. 8, PI. 13). 



One of the most striking points iu the life-history of this 

 alga is the occurrence in approximately equal numbers of 

 green and colourless resting-cells. These cells ma}^ be — as 

 already stated — isolated or in pairs. The single green cells 

 undergo, after a period of rest, division into four, green, 

 oval cells which resemble active cells except that their walls 

 are extremely thin and that flagella are lacking. The four 

 daughter-cells are extruded together from the envelope of 

 the mother-cell and undergo further division or become at 

 once active cells. 



Similarly the green paired cells divide to eight oval cells. 

 The colourless cells behave in a precisely similar way, giving 

 rise to four or eight daughter-cells which are extruded through 

 a circular opening in the wall of the mother-cell. The colour- 

 less daughter-cells consist each of a highly vacuolated foam- 

 like protoplast, including a large refractive mass which 

 appears to correspond with pyrenoid and chloroplast of the 

 green cell. These cells may either undergo further division 

 or assume gradually and without further division a green 

 colour, becoming first faintly yellow, then quite yellow, and 

 finally green. 



The simultaneous formation by the alga of green and 

 colourless cells is certainly a curious phenomenon. 



It is well known that the zygotes of various alga) lose their 

 chlorophyll, but we know of no case where resting-cells are 

 alternatively colourless or green. The division of the green 

 cell into a green group of daughter-cells and the division of 

 the colourless cell into a corresponding group of colourless 

 cells appears to indicate that a true dimorphism exists. 



