POTATO 459 



abundance. The extensive development of the latter depends 

 upon the presence of moist air; in dry air they do not 

 appear. 



The tubers themselves never bear roots, and are, therefore, 

 unlike the Jerusalem artichoke tubers in this respect. 



Stem and Tuber. — The stems are herbaceous; two forms 

 are present upon the potato plant, namely, the upright stem 

 above ground and the horizontal rhizome below. 



Although their geotropic behaviour is not the same, they are 

 essentially the same in structure ; the rhizome can be changed 

 into an ordinary shoot with green foliage-leaves by bringing it 

 above ground. 



The rhizome is usually not more than from i to 3 inches long 

 in early varieties, and the tubers consequently appear heaped up 

 round the stem when dug. Those giving heavier crops have 

 longer and more branched rhizomes, while varieties with very 

 long rhizomes usually give an unsatisfactory yield, although 

 individual tubers may reach a large size. 



Leafy stems resembling that from which Fig. 142 was drawn, 

 and showing tuber development in the axils, are readily produced 

 by allowing old tubers to germinate in spring in a darkened cellar 

 kept somewhat damp. Moreover, if the potatoes are picked 

 off below ground as fast as they form, the plant develops tubers 

 in the axils of the leaves above ground. 



The first internodes of the rhizome below ground are of con- 

 siderable length ; those produced later at its tip remain shorter, 

 but increase in thickness rapidly, and form a tuber. 



Tuber. — That the potatoes are thickened pieces of stems is 

 seen from a study of their origin ; the rhizomes, of which they 

 are merely the ends, arise in a normal manner in the axils of 

 leaves below the soil and although they occur under ground, 

 they have no connection with the root-system of the plant. 



A well-grown tuber usually shows at its base or ' heel ' a piece 

 of the withered rhizome^ and on its surface many ' eyes ' which 



