106 



THE STEM. 



besides, the interminable subterranean branches bear tubers, or 

 reservoirs of nutritive matter, in their course, which have still 

 greater powers of vitality, as they contain a copious store of food 

 for the development of the buds they bear. The name of 



188. Rhizoma or RootstOCk is apphed in a general way to all these 

 perennial, horizontally elongated, more or less subterranean, root- 

 like forms of 

 the stem ; and 

 more particu- 

 larly to those 

 that are consid- 

 erably thick- 

 ened by the ac- 

 188 cumulation of 



starch or other forms of nutritive matter in their tissue, such as the 

 so-called roots of Ginger, of the Iris or Flower-de-luce (Fig. 291), of 

 the Calamus or Sweet Flag, and of the Blood- 

 root. They grow after the manner of ordinary 

 stems, advancing from year to year by the 

 annual development of a bud at the apex, and 

 emitting roots from the under side of the whole 

 surface ; thus established, the older portions die 

 and decay, as coiTesponding additions are made 

 to the opposite growing extremity. Each year's 

 growth is often marked, as in some species of 

 Iris (Fig. 291), by a narrowing at the place 

 where the growth of the season is suspended, 

 followed by an enlargement where it recom- 

 mences ; or else, as in the curious Diphylleia of 

 the Alleghany Mountains (Fig. 1C7), and the Polygonatum or 

 Solomon's Seal (Fig. 168), it is more indelibly stamped by an im- 

 pressed circular scar (which has been likened to the impression of a 

 seal), left annually, in autumn, by the death and separation from the 

 perennial rootstoek of the herbaceous stalk of the season which bore 

 the foliage and blossoms. In Diphylleia the growth is so slow, and 

 the ascending stems so thick, that the scars of successive years are 



FIG. 168. Rootstock of Polygonatum or Solomon's Seal, with the terminal bud, the base of 

 the stalk of the season, and three scars from "which the latter has separated in as many former 

 years, 



FIG. 169. The short and upright rootstock of Trillium erectum, or Birthroot, -with its ter- 

 minal bud. 



