58o 



BOTANY OF CROP PLANTS 



This table shows that the dry matter decreases from the 

 outside to the inside of the tuber. The nitrogen content 

 shows an increase, on dry basis, from the outside to the 

 center, although on a fresh basis there seems to be no regu- 

 larity of percentage, probably due to variability in water 

 content. The inner cells of the cortex contain a much larger 

 amount of starch than those of the external medulla, which 

 in turn contain considerably more than the cells of the inter- 

 nal medulla. The outer cells of the cortex which are removed 

 with the skin (in peeling) are comparatively low in starch. 



Starch and Sugar. — Potato starch grains are egg-shaped 

 or nearly spherical with eccentric markings, and with the 

 hilum near the small end. Some varieties of potatoes are 

 abundantly suppKed with large starch grains with infre- 

 quent small ones, while in other varieties the reverse is the 

 case. No correlation has been found between the size of 

 the starch grains and the size of the tuber or its total starch 

 content. In general, the early varieties contain large starch 

 grains while the late varieties contain a larger proportion of 

 small grains. Starch-grain formation is very slow. At first, 

 many small grains are found, most of which later increase in 

 size. This increase in size begins much sooner in early 

 varieties than in late varieties. 



