162 CLASSIFICATION OF ROOTS AND STEMS 



Order of Development. — Primary and Secondary Stems. — As to their 

 order of development in time, stems are Primary, Secondary, and so 

 on, terms which are self-explanatory. 



Place and Nature of Origin. — Aerial and Svbterranean Stems. — As to 

 their position and nature of origin, stems are Aerial or Subterranean, 

 which terms are also self-explanatory. A secondary stem assuming an 

 erect position from the base of the primary, like those of the Indian 

 corn, is a Sucker. Such an one arising from a rhizome at a considerable 

 distance from the original erect stem, as in the blackberry, is called a 

 Stolon. A short secondary stem developing from the base of the primary 

 is called an Offset. An elongated, slender one, hing prostrate and 

 rooting at some of its joints, is called a Runner (Fig. 44.5). 



444. 44S. 4^6. 



Fig. 444. Tubers of Curcuma. 445. Runner of strawberry plant. 446. Fibrous roots attached to 



scaly rhizome of Gesneria. 



The Rhizome. — An underground stem, fleshy-thickened at least during 

 the first year, so as to serve as a storage receptacle, and giving origin 

 to an aerial summit or branch, is a Rhizome (Figs. 447 to 4.52). 



Distinctions between the Rhizome and the Root. — A rhizome is very 

 frequently mistaken for a root, but the differences, both internal and 

 external, are well-marked. The internodes of the rhizome are com- 

 monly quite as uniform in length as those of the aerial stem. The nodes 

 are usually conspicuous. Leaves exist upon them, commonly in the 

 form of scales. Occasionally these scales are numerous and well formed 

 (Fig. 440), but usually they are rather obscure, as in the potato, where 

 they are mere semicircular or crescent-shaped ridges about the eyes. 

 In the axils of the scales, buds are usually to be distinguished. These are 

 the so-called "eyes" of the potato, and their development into branches 

 is a famihar phenomenon. Internally, the structure of the rhizome is 

 in general that of the stem, though fleshy tissue predominates. 



The growth and duration of a rhizome may be indefinite, like those of 

 stems, as in the case of the rhizome of Podophyllum (Fig. 449), or they 

 maj' be restricted to one or to a definite number of years, after which the 



