24 POTATO. [CH. II 



Although the tuber of the potato is formed under- 

 ground, it is essentially a stem and not a root. It is only 

 one instance of a common state of things, namely, that 

 the underground parts of plants are not necessarily roots. 

 Many plants have creeping underground stems, like the 

 sedge shown in fig. 6. A similar morphological arrange- 

 ment will meet us in the fern. In the case of the 

 potato the thing is not so evident ; perhaps the most 

 striking proof that can be offered to one who has no 

 knowledge of morphology is that under certain conditions 

 tubers are formed on the aerial stem of the plant as in the 

 specimen sketched in fig. 7. 



T 



Fio. 7. 



FORMATION OF TUBERS ON THE AERIAL STEM OP A POTATO-PLANT. 

 T, T, tubers : L, the stalk of the leaf in whose axil TT appear. 



Moreover fig. 8 suggests that the elongated organs 

 which end in tubers are branches, since they spring from 

 the axis above the cotyledons and therefore a fortiori 

 above the line dividing root from shoot. 



