386 



SCIENCE 



[N. S. VOD. XLVI. No. 1190 



Eobert Philip has been appointed as the first 

 professor of the subject. 



DISCUSSION AND CORRESPONDENCE 



ISOLATION CULTURES WITH SMALL AQUARIA 



When raising small forms of vegetable or 

 animal aquatics, it is sometimes desirable to 

 follow the development of several individuals 

 simultaneously, and for some considerable 

 time. This can done of course by removing 

 the specimens to separate small aquaria, but 

 by so doing the temperature and other condi- 

 tions are likely to offer a considerable range 

 of variation among the different specimens. 

 This invites uncertainty as to the natural rate 

 of development, or in response to any intended 

 variable introduced by the experimenter. The 

 desirable condition is to combine a consider- 

 able volume of water with isolation of individ- 

 uals so that each specimen may have essen- 

 tially identical conditions of temperature, and 

 concentration as each other, in groups of 

 eight to twelve individuals. 



During a study of Lemna carried on for 

 several months, it was desired to isolate in- 

 dividual plants in order to watch the rate of 

 growth. As the frond floats freely, some method 

 by which the surface of the water could be en- 

 closed in distinct areas seemed likely to meet 

 conditions. It was found that common cotton 

 cord, waxed with parafine, and tied into loops 

 two inches in diameter, were excellent for this 

 work so long as the water was undisturbed. 

 Any disturbance, however, either accidental 

 or in course of the work, by which the upper 

 surface of the string loops become wet, made 

 these sink quickly after they had been in use 

 two or three days, and the enclosed specimens 

 would then be confused with any others which 

 might be near. Small snails developing in 

 pond water used were quite a source of loss of 

 specimens by their destructive habits, as well 

 as factors of uncertainty, through the dis- 

 placement of the string loops, drawn below 

 the surface by the movement of the snails in 

 case these crawled across the strands. The 

 vessels used at this time were common glass 

 battery jars, and served very well in keeping 



the plants in good condition, but they were un- 

 necessarily deep. 



Later work was done with large crystallizing 

 dishes, and the separation of individuals was 

 secured by the use of glass dehydrating dishes 

 with short legs and perforated bottoms, for 

 inside dishes. This was found very satisfac- 

 tory. The volume of water in the crystallizing 

 dish was large enough to retain a much more 

 steady temperature than did the small sep- 

 arate dishes tried for a time, and the perfo- 

 rated walls of the enclosing inner dishes 

 permitted the movement of the water with 

 sufficient freedom to eliminate any variable 

 concentration or composition. 



In securing single specimens for the isola- 

 tion work, some interesting conditions were en- 

 countered on account of the toughness of the 

 water film. It was found difficult, for ex- 

 ample, to lift a single specimen of Lemna or 

 Wolffia because the surface film would drag 

 several additional specimens along with the 

 desired individual. This trouble was largely 

 eliminated by giving a smart puff of breath 

 close to the desired specimen, which would 

 cause a general scattering of all the floating 

 particles from that point. As the elasticity 

 of the film was released upon the cessation of 

 the blowing, the dispersed specimens were 

 drawn inward toward the center of the cleared 

 area. On account of size, root development 

 or other causes, the different specimens did 

 not move with equal speed, and any one of the 

 specimens first entering the cleared space 

 could be lifted and removed with ease. It was 

 found that a lance-head needle was an excel- 

 lent lifter for the specimens. 



Of three species under observation, Wolffia 

 was the easiest to thus isolate, Lemna pauci- 

 costata next, and Spirodella the most difficult 

 to lift with certainty. This is because Wolffia 

 is completely immersed in the slight amount 

 of water adhering to the needle, and sticks 

 closely as this is raised from the dish. The 

 single root of Lemna, and the many roots of 

 Spirodella, prevent the fronds of these plants 

 from so closely adliering to the flat needle, 

 and their added weight also is adverse. It 

 was found further that a dry needle was far 



