572 ' ' BOTANY OF CROP PLANTS 



plant (potato) have been found to _ correspond to these 

 characters. 



Varieties. — There are at the present time over 500 named 

 varieties of potatoes in the United States. Many of these 

 variety names are found to belong to potatoes which are 

 identical in all respects. Usually, new varieties are the 

 seedlings of estabUshed varieties. 



The latest attempt at a classification of American potatoes 

 is that of Stuart. In his "proposed system of classification" 



vascular 

 ring 



-eye 



external 

 medulla 



Fic. 236. — Diagrammatic section of potato tuber. 



he gives the following "groups": Cobbler, Triumph, Early 

 Michigan, Rose, Early Ohio, Hebron, Burbank, Green 

 Moimtain, Rural, Pearl, and Peachblow. Tuber, sprout 

 and flower characters are made the bases of distinction ^ of 

 the groups. The student is referred to Bull. 176, U. S. 

 Dept. Agr. (Professional Paper) for the descriptions of these 

 groups. Fitch has also proposed a classification using about 

 the same characters. 



Tuber Morphology. — The potato tuber is made up of a 

 number of zones or layers which are commonly grouped as 

 follows (Figs. 236 and 237) : 



1. Periderm or skin. 



2. Cortex. 



3. Vascular ring. 



