574 BOTANY OF CROP PLANTS 



Periderm or Skin.^ — The stolon, which develops into the 

 tuber, possesses the true stem structure. It has a thin 

 epidermis, an outer parenchyma tissue or cortex, fibro- 

 vascular bundles, and an internal parenchyma or medulla. 

 As the tuber develops, the cortex becomes relatively reduced, 

 the vascular bundles separate and the medulla becomes 

 larger. The outer layers of cells of the cortex also undergo 

 changes. The cells of these layers become corky and 

 flattened, and so arranged that the vertical walls form 

 straight lines and do not overlap. Their walls become 

 suberized. The original true epidermis gradually dies and 

 disappears entirely. These outer corky layers of cells 

 constitute the periderm or skin of the potato. The outer- 

 most layers of periderm spHt off, giving some varieties a 

 characteristic rough appearance. The cells of the different 

 layers of periderm vary in size and shape in different varieties. 

 The number of layers is usually 7 or 8, but it varies from 5 

 to as many as 13 and even 17. At the eyes, the periderm 

 (skin) becomes thicker. Lenticels are scattered over the tuber. 



Some claim is made that thick-skinned varieties are of 

 better quality than thin-skinned ones, but such is not always 

 the case. A netted or rough skin develops on tubers of some 

 varieties as they mature in storage, which suggests, that a 

 rough or netted skin in these cases denotes maturity. Pos- 

 sibly this is sometimes the source of the common idea that a 

 rough-skinned potato is of superior quality. The size and 

 type of netting is found to vary with the variety and the con- 

 ditions under which grown. Smoother skins are usually 

 found on potatoes grown on sandy soils than on those grown 

 on heavy soils. It has been found that the thicker and 

 rougher-skinned varieties stand up better in shipping, and are 

 preferable for this purpose even though they may have no 

 greater merit in other ways. 



