576 BOTANY OF CROP PLANTS 



The internal medulla is usually more or less star-shaped. 

 Many of the radiating areas of internal medulla penetrate 

 deeply the outer medulla, some of them extending to the eyes. 

 In some tubers, these two zones are more or less intermixed, 

 with no definite zone boundaries. As a rule, in long potatoes, 

 the central area is very much elongated and with lateral 

 radiations, while in many round potatoes it is typically star- 

 shaped. The greater the size of the internal medulla and the 

 more its ramifications into the outer area, the poorer the 

 quality of the tuber, since it means a larger area poor in starch 

 and hence less mealy on cooking. 



Shape.' — The common tuber shapes are round, oblong, and 

 elongated, in outline. One dominant form is found in each 

 variety but never one exclusive form. New varieties based 

 on tuber form are produced by a selection of tubers and are 

 maintained only by continued selection. 



In tuber propagation, there appear among the normal- 

 shaped tubers a number of aberrant (diverging) forms which 

 are usually in the minority. 



Color .^ — The common tuber colors are yellow, red, violet of 

 different shades, and variegated. Bluish forms are also 

 known. Color variation has been found in a number of 

 cases. In propagation by cuttings, yellow and streaked 

 tubers have appeared from colored ones (red and violet). 

 Yellow tubers have given red and violet ones, a white tuber 

 has given two red and two white tubers, and .one with a 

 bluish color has given a series white in color. The Pearl 

 with a brownish-white or a well-russeted skin is from the 

 Blue Victor which has a purple color often streaked with 

 white. When the white streaks cover an eye, the tubers 

 from the eye usually come true (white) in following genera- 

 tions. The People's variety, also from the Blue Victor, is a 

 deeper brown color than the Pearl. At the present time, 



