M. D. Glynne and V. G. Jackson 255 



opposite sectors are taken this is reduced to less than 1 per cent., while 

 if four alternating sectors are taken the error becomes negligible. The 

 obvious causes of error in the sector method are the difficulty of cutting 

 a sector the apex of which is exactly in the middle of the tuber, and the 

 fact that the composition of the tuber is different at either end. Both 

 these errors maj^ be counterbalanced by taking radially opposite sectors. 

 Another method is to cut the tuber into eighths by two cuts at right 

 angles to each other along the long axis and one cut along the transverse 

 axis of the tuber. Two diagonally opposed eighths taken as a sample gave 

 the following result in three tubers. 



Table XVI. Percentage Dry Matter in, diagonally opposed ^ths. 



The greatest difference from the average percentage dry matter is 

 0-5 per cent. 



SUMMARY. 



1. The percentage of dry matter in the potato tuber is lowest in the 

 skin and increases to the inner cortical la3'er, the zone containing the 

 greater part of the vascular system, and decreases towards the centre of 

 the tuber. 



2. In each zone the proportion of dry matter is higher towards the 

 umbilical than the terminal end of the tuber. 



3. The percentage of nitrogen in the fresh material tends to decrease 

 from the skin to the inner cortical layer and to increase in the medullary 

 zone. Thus it increases from zone to zone in the opposite direction to 

 the dry matter. 



4. Nitrogen tends to increase with dry matter from the terminal to 

 the umbilical end. 



5. Microscopical examination shows the starch grains densest in the 

 region of the vascular system, and decreasing towards the centre and 

 surface of the tuber. 



6. A high degree of correlation is found between the specific gravity 

 and percentage dry matter of whole tubers. 



7. For purposes of sampling the method of taking two radially 

 opposed sectors, or two diagonally opposed eighths, was far more accurate 

 than the coring method. 



17—2 



