250 Dry Matter and Nitrogen in, the Potato Tnlier 



Fig. 8 shows the strucfcure of a few cells typical of each layer. The starch 

 grains are easily seen without staining, but iodine may be used to show 

 them up yet more clearly. 



The skin consists of layers of cork cells and probably includes some 

 of the large cells just underneath. These contain a little chlorophil but 

 very little if any starch. 



In the outer cortical layer there is a fair amount of starch which 

 increases to a maximum in the inner cortical and then decreases towards 

 the central part of the tuber so that in the inner medullary layer there is 

 very little starch present. The inner cortical layer not only contains the 

 maximum amount of starch, but also the greater part of the vascular 

 system. 



The starch grains are densest in the region of the vascular bundles 

 and decrease towards the centre and towards the surface of the tuber. 

 The distribution of dry matter has been shown to be very similar, as it 

 attains its maximum in the inner cortical layer and decreases from this 

 zone towards the skin and towards the inner medullary zone. Thus there 

 appears a very close relation between dry matter and starch content. 



In this connection the results which Dr W. E. Brenchley{9) obtained 

 in her study of the wheat grain are of interest. As the grain develops 

 instead of the soluble carbohydrate being carried from the vascular 

 bundle across the width of the endosperm and deposited at the outer 

 edge of the grain, thus leaving a clear passage behind it, the cells nearest 

 the bundle are the first in which starch is deposited. This makes the 

 percolation of reserves more difficult, so that the tendency in the earlier 

 stages is for starch to be densest in the region of the conducting strand 

 and to decrease in the cells further from it in the same way as has been 

 shown in the potato tuber. Later the whole endosperm becomes packed 

 with starch but no comparable stage of maturity is ever reached in the 

 potato tuber. 



Dry Matter and Specific Gravity. 



The total percentages of dry matter and the specific gravity of 27 

 individual tubers are given in Table XII. 



The specific gravity increases almost invariably with the percentage 

 of dry matter. This consistent relationship is clearly brought out in 

 Fig. 9 in which the specific gravity of each tuber is plotted against the 

 corresponding percentage dry matter. The correlation coefficient is 

 0-972 ± 0-007. 



A further illustration of this close relationship was obtained from 



