UNDERGROUND STEMS 133 



that the oldest of them is two or three years old, as the 

 case may be. 



Examine a rhizoma of the iris, or any other specimen 

 obtainable. How many joints do you find? Where is the 

 oldest ? How old is it ? Are they all entirely under- 

 ground ? Where do the true roots spring from ? The 

 flower stems ? Notice the rings or ridges that run across 

 the upper side of each joint. These are the leaf scars, 

 and each scar marks one of the very short internodes of 

 the past season's growth. At the nodes, in the axils of 

 the leaf scars, buds frequently occur, producing other 

 joints, which may be considered branches, and it is these 

 branches that give to the rootstocks of the iris and black- 

 berry lily their thick, matted appearance. How many 

 leaves did last year's joint of your specimen bear, and 

 how many internodes had it ? 



190. Tubers. When a rhizoma is very greatly enlarged, 

 as in the artichoke and potato, it is called a tuber. Its 

 real nature in such cases is often very much disguised, 

 but a little study will make it clear. The so-called root of 

 wild smilax shows very plainly 

 the gradations from leaves to 

 scales and from stem to tuber. 



In the typical tuber, of which 

 the potato is the most familiar 

 example, the internodes are so 

 thickened and shortened as to 

 have lost all resemblance to a 

 stem, but their nature is revealed 

 by the eyes. These are really 

 nothing else than buds growing 275, 276. - Tubers (after 

 in the axils of leaves, which are GRAY): 275, forming potatoes; 



276, young potato enlarged. 



represented in the potato by the 



little scale that forms the lid to the eye. (In an old potato 

 the scales will probably have disappeared ; try to get 

 fresh ones for examination, and if possible, with some of 

 the attaching stems still remaining. The artichoke and 



