THE SWEET POTATO. 79 



The bundles, or wood elements, are not visible on the 

 freshly cut surface. That gives us K.— 3. Thus our 

 formula runs : Al— B2— C2— D2— E2— Fl— Gl— 

 H2-3— II— Jl— K3. AVhen we refer to the alpha- 

 betical arrangement of the formulas we find that it 

 is the formula of Ticotea. To make entirely sure, 

 we compare our plant with the photograph of Ticotea 

 and with its description. If all these points agree, 

 we may be certain that the variety is Ticotea. 



Advantages of This Key Over the Ordinary 

 Botanical Key. ♦ 



At the beginning of the chapter the writer prom- 

 ised to give his reasons for choosing this key. This 

 key is preferred on account of the following advan- 

 tages : 



1. It is a key and a classification combined. All 

 varieties are determined by the same eharacter^^ and 

 can be readily compared, while in the ordinary key 

 each variety is thrown out at an opportune moment 

 by a character which may be common to many others 

 already separated by some other character. One 

 comes to associate that particular character with 

 that variety, while in reality it depends entirely on 

 the arbitrary arrangement of the key whether the 

 character is used at all. Similarly there is little 

 attempt made in the arrangement of the ordinary 

 key to keep together the varieties most resembling 

 each other, while in this key they stay together nat- 

 urally. For example, Bronze Spanish, as the name 

 indicates, has a peculiarly colored tuber. In the 

 ordinary key this character would very likely be used 

 at an early stage to separate it. Black Spanish has 

 a tuber which is similar to several others, and it 

 would be hard to separate it by that character. Yet 



