428 PHYSIOLOGY OF GROWTH. 



disappear. If a plate into which alga-containing water has been 

 poured is so placed {e.g. in front of a window) that one side 

 becomes somewhat warmer than the other, it will be found (even 

 when the light is excluded) that the swarmers collect according 

 to circumstances on one or the other side of the plate. According 

 to Sachs 3 all these phenomena are caused by currents in the water, 

 which group the swarmers in a definite manner, and are them- 

 selves dependent on the distribution of temperature. Sachs was 

 led to this conclusion by the study of emulsion figures. To 

 investigate emulsion figures, we pour pure olive oil over coarsely 

 ground alkanna root, and after twenty-four hours filter off the 

 intensely red coloured oil resulting. We further make in a glass 

 cylinder a mixture of water and alcohol, whose specific gravity 

 is exactly 0'920. This fluid has almost exactly the same specific 

 gravity as olive oil. If we pour a small quantity of it into a 

 beaker, and then add some of the red olive oil extract, large oil 

 drops will rise very slowly in the fluid, which is therefore of 

 somewhat higher specific gravity than the oil. 



These preliminaries having been arranged, we add 5 c.c. of the 

 red oil to every 500 c.c. of the alcohol mixture, and shake 

 violently, so as to break up the large oil drops into thousands of 

 minute droplets, and in this way the necessary emulsion fluid is 

 prepared. 



For use we pour it into a flat porcelain dish to a depth of about 

 10-15 mm. We cover the dish with a sheet of glass, or leave 

 it uncovered, and observe how the moving oil drops at first form 

 series of dots and networks, and after a time (one-quarter to half 

 an hour) group themselves into regular figures. If we have 

 covered the dish with a sheet of glass after pouring into it the 

 emulsion fluid, and now remove it after the formation of the figures, 

 they rapidly dissipate under the eye of the observer. 



The form of the emulsion figures is very varied. In Fig. 139 (J5) 

 is indicated one which frequently occurs. Such concentric figures, 

 however, only form when the dish containing the emulsion fluid is 

 placed in the middle of the room. Polar figures, such as the one 

 indicated in Fig. 139 (yl),"form when we place the fluid near a 

 window or a stove, so that one side of the dish is warmer than the 

 other. If we experiment, e.g., with the emulsion described, in 

 which the coloured oil is only very little lighter specifically than 

 the mixture of alcohol and water, the pole and marginal lines of 

 the polar figure are always directed towards the colder side of the 



