SOLANACE^ 575 



Vascular Ring. — The vascular ring consists of a discon- 

 tinuous circle of vascular bundles. It is located between the 

 cortex and the medulla. At the eyes, the vascular tissue ap- 

 proaches the surface of the tuber. It maintains, however, its 

 proper relationship with the other tissue, i.e., between cortex 

 and medulla. The cortical layer gradually becomes thinner 

 as the vascular bundles approach the eyes. The vascular 

 tissue is poor in starch. The vascular ring is easily recog- 

 nized as a very narrow darkened ring near the edge of the 

 exposed surface of a cut. 



Parenchyma. — Almost the entire mass of tuber tissue in- 

 side of the periderm (skin), except the vascular tissue, is 

 parenchyma, and will be referred to as such in this discussion. 



The parenchyma is divided into two principal parts: the 

 cortex, and the medulla. 



Cortex. — The cortical layer of the parenchyma is just 

 within the periderm. It is separated from the medulla by the 

 vascular ring. The outer cortex is made up of smaller cells 

 than the inner cortex. The cells of the cortex are consider- 

 ably smaller than those of the medulla, and hence the density 

 of the cortex is greater. The cortex is darker in color than 

 the medulla, which is probably due to its greater density. A 

 thick and dense cortex indicates a potato of good quality. 

 A thinner, more translucent cortex is said to indicate lower 

 quality. The periderm, or skin, and the outer layers of cor- 

 tex are removed when potatoes are peeled. 



Medulla. — The medulla consists of all of the tuber inside 

 of the vascular ring. It is divided into two parts, the exter- 

 nal and the internal medulla. When a thin slice of potato is 

 held up to the light, these two areas are easily distinguished ; 

 the external medulla appears darker and denser; the lighter 

 color of the internal medulla is due to its greater percentage of 

 water, and considerably less starch and other solid matter. 



