244 Dry Matter and Nitrogen, in the Potato Tuber 



The decrease in dry matter begins immediately inside the inner 

 cortical layer and proceeds to the inner medullary region. 



Comparison of dry matter determinations in different layers of small, 

 medium and large tubers respectively shows in each zone a slight 

 tendency for the smaller tubers to be drier than the larger ones. This 

 is most marked in the inner medullary zone. Appendix Table I shows 

 that while this tendency is perceptible in the average results, it is not 

 found consistently in individual tubers. 



A higher percentage dry matter was found in the umbilical region 

 than in the terminal or sprout end. Six tubers divided into the two halves 

 each showed this (Appendix Table II) and gave an average dry matter 

 of 25-16 per cent, in the umbilical and only 22-51 per cent, in the ter- 

 minal half. Further determinations were made on three large tubers, 

 three medium and three small. Each was divided into four sections by 

 three cuts at right angles to the long axis and each section was zoned 

 (Fig. 1). A sample of 20 tubers was treated in the same way except that 

 a layer 2-5 mm. thick was separated from the outside of the outer 

 medullary zone (Fig. 2). 



The percentage of dry matter in individual tubers (Appendix 

 Tabic III) and in the average (Figs. 1 and 2) is higher in the umbilical 

 than in the terminal half in every zone except the skin, in which there 

 is no consistent variation. In the outer cortical zone dry matter rises 

 from the terminal section 1 to 3 and then either rises or falls to section 4. 

 The increase in proportion of dry matter from the terminal to the um- 

 bilical region is most marked in the inner cortical layer where it rises 

 steadily from section 1 to 4. It behaves in the same way in the layer 

 immediately beneath it (Fig. 2). In the medullary zones, especially the 

 inner, the most central region of the zone, that is the part contained in 

 the two intermediate sections, tends to be the wettest. The umbilical 

 medullary region in section 4 is drier than the terminal 1 , and section 3 

 is drier than 2. 



Again in each section of every tuber without exception dry matter 

 increases from the skin to the inner cortical layer and then decreases to 

 the inner medullary. 



