UKDEBGKOUND AND AEBIAL STEMS 



73 



Some of the principal forms of underground stems have for 

 convenience been given special names. The elongated forms 

 like that of the May apple (Fig. 59), mints, couch grass, and 

 many other plants, and some stouter kinds like that of trillium 

 and Solomon's-seal (Fig. 60), are known as rootstocks or rhizomes. 

 The very short shoots with disk-like stems and a covering of 

 scales, familiar in some lilies, the hyacinth (Fig. 61), and the 

 onion, are called bulbs. 

 Much like bulbs, ex- 

 cept that the stem is 

 more developed and 

 that the scales are 

 almost lacking, are tu- 

 bers, like those of the 

 Jerusalem artichoke 

 (Fig. 67), the potato, 

 and the crocus. 1 The 

 potato is a particularly 

 good tuber for study, 

 as it has well-defined 

 nodes and internodes ; 

 the buds ("eyes") are arranged in a distinctly spiral manner, 

 and are borne in the axils of little scales which represent leaves, 

 and not infrequently the tuber is considerably branched. 



65. Aerial outgrowths of underground stems. Some under- 

 ground stems produce a leafy aerial stem, while others send 

 up leaves but have no stem above ground. A good example 

 of the former class is the lily or the Jerusalem artichoke ; of 

 the latter, the ferns of temperate regions, many grasses, wild 

 ginger (Fig. 43), and some of the commonest violets (Fig. 

 124). In any case the aerial parts of herbs, in cold or tem- 

 perate climates, usually die to the ground at the beginning 

 of winter. In regions with a long, rainless summer they fre- 

 quently die soon after the end of the spring rains. The buried 



1 Such very short underground stems as that of the jack-in-the-pulpit 

 and the crocus are often called corms. 



FIG. 60. Rootstock of Solomon's-seal 



rh, rhizome or rootstock ; b, b', buds ; r, roots ; 



s, stem. The scar where an old stem was attached 



is seen just above b' 



