64 THE SALTON SEA. 



The division of the cells of both species of Dunaliella, brown and green, as they come 

 to rest on agar-agar impregnated with brine, is the most striking first change which takes 

 place, a change which, however, is directly proportioned to the change in density of the 

 brine'. Thus figures 2 and 3 show the divisions in the two species of Dunaliella at the end 

 of one week when agar-agar impregnated with brine of slightly less than 1.190 specific 

 gravity is inoculated from a brine of exactly this specific gravity. Figure 4 indicates the 

 rate of division in a similar agar-agar plate of 1.050 specific gravity inoculated a week 

 earlier from a brine of 1.200 specific gravity. The same result is shown by the growths 

 in test-tubes containing brines of different concentrations inoculated from the same source, 

 thus: December 4, test-tubes containing brines of 1.195, 1.105, 1.050, 1.025, and 1.0125 

 specific gravity were sterilized, cooled, and inoculated by a "loopfull" of brine from a 

 stock bottle containing brine of 1.200 specific gravity. These tubes were placed in a 

 test-tube rack on a shelf in a window. Direct access of sunlight to the cultures was pre- 

 vented by a white holland shade constantly drawn over the window, but the illumination 

 was ample, as the success of the cultures proved. On February 12, almost ten weeks 

 later, I examined the tubes and found very little growth in those containing brine of 1.195 

 specific gravity, some in the tubes of 1.105, very abundant in the 1.050 tubes, fair in 1.025 

 brine, very slight in 1.0125. From these plate and tube cultures, therefore, it is evident 

 that an increase in water (a decrease in concentration) in a brine is advantageous, provided 

 it is not excessive. This corresponds exactly with the observations in the salterns them- 

 selves, as previously recorded (see table 22, pp. 56-00). 



Some idea of the rate of division is indicated by the series of drawings in figures 4-8. 

 Thus figure 4 represents a colony a of eight cells on a 1 per cent agar-agar plate of brine 

 of 1.050 specific gravity. This plate was inoculated on January 15. The figure was drawn, 

 under camera lucida, at 11 a.m. on January 22. Presumably this colony of eight cells 

 started from a single cell, as evidently all the others in the figure did; for, with the single 

 exception noted in table 22, tank No. 1, column G, for December 14, I do not recall colonies 

 being formed out of doors or in the stock bottles. The same colony is shown in figure 5a at 

 4 h 20™ p.m., January 22, with 9 cells; in figure 56 at ll h 45 m a.m., January 23, with 15 cells; 

 in figure 8a at 3 h 50 m p.m., on January 23 with 15 cells; in figure 86 at 3 1 ' 50 m p.m., Jan- 

 uary 24, with 1G cells; in figure 8c at 10 h 55 m a.m., January 25, with 1G cells. But this 

 plate had been continuously in the dark, except for the short times required to make the 

 camera drawings, since the first drawing was made on January 22 till January 25 at the 

 same hour. On the other hand, figure 66' shows a week-old colony of 4 cells at 4 h 40 m p.m., 

 on January 22, on another similar plate inoculated at the same time and from the same 

 source. The same colony, now 5-celled, is shown in figure 66," which was ^drawn at 

 ll h 30 m a m., on the day following, namely, January 23; again in figure 7a at 3 h 45 m p.m. 

 of the same day, which had been dull; in figure 76 at 3'' 40 m p.m. of the following day 

 (January 24), which had been showery and dark; and in figure 7c at 10 h 40 ra a.m. on 

 January 25, on a day dark and showery, but still light enough to bring about an increase 

 in the starch-content of the cells. Comparing these two colonies, the one in the dark, the 

 other in all the light there was, we see, in addition to the differences in starch-contents 

 disclosed by the figures, that a colony of 8 cells formed 8 new cells in three days in the dark 

 (a 100 per cent gain), while a colony of 4 cells formed 3 new cells in the same time in the 

 light of showery or rainy days alternating with complete darkness during the long January 

 nights (a 75 per cent gain). These are average figures for other colonies on these two 

 plates. Later in the year, on brighter days, the difference is greater. 



Thus we see that the effect (on a small scale and under conditions deliberately con- 

 trolled, in cultures) of slight increase in the water-content of a brine is the same as on a 

 large scale out of doors, namely, an increase in the number of individuals by vegetative 

 means, by division, and that division is favored by darkness, other things being equal. 



