4 6 



early winter. It is known that from the time the leaves fall for at 

 least one or one and one-half months, the cortex as well as the winter 

 buds and the sap wood of deciduous trees (at least of peaches) con- 

 tinue to increase in hardiness. This Station made an effort to de- 

 termine whether this increase in hardiness might not be due to such a 

 change in the composition of proteids that they would tend to remain 

 in solution at lower temperatures. Consequently twigs were gath- 

 ered on the dates shown in the following table, beginning before leaf 

 fall. Large quantities of cortex sap were expressed. After pouring 

 back and forth to insure a uniform solution, one-half of this sap was 

 frozen at the temperatures shown in the table, a temperature that 

 would at that time kill the cortex and practically all other tissue of 

 the twig. The other half was kept in an ice box until the freezing of 

 the other sap was complete, then both were taken to the Department 

 of Agricultural Chemistry for proteid analysis, the frozen sap being 

 thawed on the filter except when otherwise stated in the tables. The 

 analysis was by the following plan, kindly furnished by the Depart- 

 ment of Agricultural Chemistry: 



KJELDAHL-GUMMING METHOD FOR ESTIMATION OF 



NITROGEN. 



"Weigh out accurately 4-6 grams of the sap by difference, 

 place in 500 c. c. Kjeldahl flask, add 0.7 gram of mercury and 25 c. c. 

 of sulphuric acid, heat for a few minutes until frothing ceases, add 

 7 grams of potassium sulphate and digest over a flame until clear, 

 cool ; wash down the neck of the flask with distilled water, heat again 

 one hour until water is expelled. Cool and dilute with water until 

 flask is 2 / 3 full, add a piece of zinc and a piece of paraffin and 80 c. c. 

 of alkaline solution containing 500 grams potassium sulphide and 18 

 kilos sodium hydroxide in 40 liters of the solution. Distil into standard 

 hydrochloric acid and titrate back with standard ammonium hydrox- 

 ide, using cochineal as indicator". The factor 6.25 was used in re- 

 ducing nitrogen reading to proteid readings. 



Twigs of apple, peach, pear and plum were used with results 

 as shown in the following table: 



