50 R. A. GORTNER AND J. A. HARRIS 



differences between saps extracted by pressing after grinding and 

 by means of pressing after freezing need be as large as some 

 they have recorded. But the ease, rapidity (with consequently 

 a minimum concentration by evaporation, or contamination from 

 the air) and completeness with which the sap may be expressed 

 after freezing renders the method one of the greatest utility. 

 Furthermore, samples may be frozen and kept in this condition 

 for hours before analysis with little probability of change. 



The method of freezing suggested by Dixon and Atkins is that 

 of immersion in liquid air. For some purposes, for instance in the 

 study of the properties of the sap of small quantities of tissue as 

 soon as possible after their collection, the method is doubtless 

 very useful. But for the great bulk of the work it seems to us as 

 superfluous and undesirable as it is unavailable to the average 

 botanist. 



We have depended entirely upon the ordinary salt and ice 

 mixture. The tissues are placed in as large quantities as possible 

 in thick-walled test tubes. These are corked (preferably with 

 rubber stoppers) and to preclude every possibility of contamina- 

 tion are capped with oiled paper held firmly by a strong rubber 

 band wrapped several times around the tube. These tubes are 

 plunged into a slushy mixture of finely chopped ice and salt, 

 with which a temperature of -15° to -17° C. can be easily attained 

 and maintained for hours. Of course, any receptacle will serve 

 for the freezing mixture, but we have found a water tight wooden 

 box, fitting neatly in our general laboratory ice box to conserve 

 ice, advantageous. In any case, the cover should be an inch 

 board with holes closely fitting the body of the tubes. This holds 

 the tubes firmly, allows uniform space for freezing mixture, and 

 by keeping the mouths of the tubes well above the surface of the 

 mixture is a further precaution against contamination. 



Thus the liquid air method is expensive and cumbersome, 

 available to only the man working in the rarely equipped labora- 

 tories. The method which we have employed with complete 

 success is so simple that it could be used in the field for the col- 

 lection of samples under their peculiar environmental conditions. 

 A freezing box could easily be devised for a run of many miles 



