SUBTERRANEAN STEMS. 145 



like in character. The buds, or "eyes," 

 upon the potato, as well as its mode of 

 growth, prove it to be a subterranean stem. 

 So also are the underground runners of 

 quack or quitch grass, Canada thistles, and 

 other pests. If we look into the more mi- 

 nute life histories of these underground 

 stems, we find additional and even more de- 

 cisive proofs that they are in no sense roots ; 

 we find that they do not imbibe nourishment 

 for the support of the plant, but are simply 

 means for propagating it. It is at once ap- 

 parent that the underground stems of the 

 quack grass and the thistle naturally serve 

 as very active agents in plant propagation, 

 and a moment's reflection will reveal the 

 same fact in regard to the potato, which is 

 native to countries where frost does not de- 

 stroy the tubers. Plants which possess un- 

 derground runners, and which also bear 

 seeds, are doubly prepared, other things be- 

 ing equal, to overcome obstacles of environ- 

 ment. Another class of underground stems 

 are those which become thick and more or 

 less woody, and which are best illustrated 

 in strong perennial herbs. The heavy rhi- 

 zomes — for so are subterranean stems called 

 — of rhubarb, May-apple, and blue-flag are 



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