242 Dry Matter and Nitrogen In the Potato Tiiher 



Method. 



The tubers were weighed and their specific gravity determined. They 

 were then divided into zones. The sliin was cut off as thinly as possible 

 with a scalpel, but a little of the subcutaneous layer invariably adhered 

 to it. The outer and inner cortical layers each approximately 2-5 mm. 

 thick were removed with a potato peeler. The central area was divided 

 into outer and inner medullary zones. 



In some cases the tubers were divided by three parallel cuts at right 

 angles to the main axis into an umbilical, two intermediate, and a ter- 

 minal region. These were then zoned and put immediately into covered 

 petri dishes for weighing. The drying process was begun in the drying 

 room, average temperature about 30° C. and completed in a steam oven 

 running continuously. The complete process takes from 14-20 days. 



Nitrogen was determined by the Kjeldahl method. Preliminary 

 determinations were made on fresh and dried material, but as these 

 are necessarily made on diiierent tubers exact comparison of the results 

 is not possible. The following Table gives the percentage of nitrogen 

 determined in the wet material of the difierent zones of thi-ee separate 

 tubers. It gives for comparison the percentage of nitrogen determined 

 on a sample composed of the dried zones of six tubers, and calculated 

 to the fresh basis. 



Table VI. Comparison of Nitrogen determinations made on fresh and 

 dry material of different tubers. 



This indicates that there is no great difference in the results obtained 

 by either method. The large bulk of sample necessary when working 

 with fresh material gives much trouble as it tends to froth over. Deter- 

 minations on fresh material were therefore discontinued. 



Samples were prepared by thoroughly mixing together the dried 

 material of corresponding zones of three or six tubers of about the same 

 size. This composite sample was powdered with a pestle and mortar and 

 duplicate samples weighing about 1 gm. taken for nitrogen estimations. 



