UNDER-GROUND SHOOTS 115 



If we examine an ordinary potato, we shall find on 

 the surface a number of depressions (a few or many 

 according to its size) and by each depression and on the 

 same side of it, a curved line. If the potato, or 

 any part of it that has one of these depressions, be 

 put into the earth, a branch will grow up out of the 

 hollow, and pushing its way up to the surface of the 

 ground, develop into an ordinary leaf-bearing stem 

 (shoot). This must have arisen from a bud, which 

 shows us that the depression is the axil of a leaf and 

 the curved line, next it, a leaf-scar. And this is 

 why the line is always on the same side of the 

 depression. 



The potato is formed under ground, but the leaf- 

 scars and buds (in the depressions) show that it is of a 

 shoot-nature, not a root. It is in fact a specially 

 thickened portion of an under-ground shoot or runner, 

 and if the whole potato plant be carefully taken up 

 the shoot-nature of the runners on which the potatoes 

 are formed will be shown still more clearly by the small 

 scale-leaves on them. 



A thickened part, whether of a shoot or of a root, is 

 termed a tuber. The Sweet-potato is on the other 

 hand a root, not a stem-tuber, being formed by the 

 enlargement of a true root, and having in consequence 

 no leaf-scar or bud on it. 



The under-ground part of a CANNA, ACORUS CALAMUS 

 the Sweet flag, or of a Ginger-plant, consists of a thick 

 whitish or brown horizontal axis from which roots 

 arise and spread out into the soil. On this under- 

 ground horizontal axis are thin brown scales, which 

 are larger towards the end where the axis turns up 



