24 



BOTANY 



PART I 



Fig. 23. — Rhizoiue of Polygonatum miiUiflorum 



next year's aerial shoot ; i, scar of this year's, and c, d, 

 e, scars of three preceding years' aerial shoots ; w, roots. 

 Q nat. size.) 



The Metamorphosis of Subterranean Shoots. — Shoots that live 

 underground undergo characteristic modifications, and are then termed 



ROOT-STOCKS or RHIZOMES. 



By means of such subter- 

 ranean shoots many herb- 

 aceous perennial plants are 

 enabled to persist through 

 the winter. A rhizome 

 develops only reduced 

 leaves in the form of larger 

 or smaller, sometimes 

 scarely visible, scales. By 

 the presence of such scale 

 u, Bud of leaves, with their axillary 

 buds, and by the absence 

 of a root-cap, as well as by 

 its internal structure, a 

 rhizome may be distinguished from a root. Rhizomes usually pro- 

 duce numerous roots ; but in a few cases these are wanting and the 

 rhizome itself performs the functions of a root. Rhizomes often 



attain a considerable thickness and store 



up nutritive material for the formation of 



aerial shoots. In Fig. 23 is shown the root- 

 stock of Solomon's Seal (Polygonatum multi- 

 florum) which has been already referred to as 



an example of a sympodium. At c, d, and e 



are seen the scars of the aerial shoots of the 



three preceding years ; and at b may be seen 



the base of the stem growing at the time the 



rhizome was taken from the ground, while 



at a is shown the bud of the next year's 



aerial growth. The rhizome of Corallorhiza 



innata, a saprophytic Orchid, which grows in 



soil rich in humus, affords a good example of 



a rhizome performing the functions of a root 



(Fig. 24). Bulbs, also, belong to the class of 



subterranean metamorphosed shoots. They 



represent a shortened shoot with a flattened, 



discoid stem (Fig. 25 zl-), the fleshy thickened 



scale leaves (zs) of which are filled with re- 

 serve food material. The aerial shoot of a 



bulb develops from its axis, while new l)ulbs 



are formed from buds (k) in the axils of the scale leaves. Another 



form of underground shoot, allied to bulbs and connected with them 



by transitional forms, is distinguished as a tuber. The axis of a 



typical tuber, in contrast to that of a bulb, is fleshy and swollen. 



Fio. 24.— Rhizome of Corallorhizn 

 innata. a, Floral shoot ; b, 

 rudiments of new rhizome 

 branches. (After Shaciit, nat. 

 .size.) 



