92 



PRArTTCAL COURSE IN BOTANY 



ing of the rhizome gives rise to the tuber, of which the 

 potato and the Jerusalem artichoke are famiUar examples. 

 Can you give any evidence to show that the potato is a 



modified stem? Find the 

 })oint of attachment of the 

 tuber to its stem and stantl 

 it on this end, which is its 

 natural base. Notice that 

 the eye sits in the axil of 

 the little scale that forms 

 the eyelid. What does the 

 scale represent? "WTiat is 



Fig. U)( 



)cr showing Iciiti- 



cels, A, A, or pores for air ou the surface ; 

 <S, leaf scale, or scar. 



the eye? (100.) Do the 

 scales occur in any regular 



order — that is, opposite, or alternating with, each other, like 



the leaves on a stem ? Look on the surface for a number of 



small, lens-shaped dots (A, A, Fig. 106) scattered irregularly 



over it. These are aerating pores called lenticels, and are 



found in most dicotyl 



stems. Does their 



presence help to throw 



light on the real nature 



of the tuber? If any 



sprouts occur on your 



specimen, where do 



they originate? Where 



do buds and sprouts 



originate on plants 



above ground ? Make 



a sketch of the outside 



of a potato, showing 



the lenticels, eyes, and 



scales, or the scars left 



by the scales in case they have fallen away, as has probably 



happened, if your specimen is an old one. 



Cut a small slice from the stem end of two potatoes, stand 



Figs. 107, 108.— 

 Transverse and longi- 

 tudinal sections of the 

 potato: A, skin; B, 

 cortical layer ; C, outer 

 pith layer ; D, inner pith 

 layer. 



