38 PBAVTIOAL BOTANY. 



MODIFIED STEMS. 



Tlie stem is, wliolly or in part, sometimes imder ground 

 (subterranean), and then it is distinguished from the root 

 by having some sort of leaves, sheaths, scales, sears, and 

 also by growing upward. 



Rhizomes, bulbs, tubers, and corms are the types of 

 undergroimd stems. 



Iiliizom.es or rootstocks are horizontal subterranean 

 stems. Aoorus calamus has a jointed, Scfrophvlaria nodosa 

 a knotty, and Polygonatum, a scarred rhizome. (PI. L, 9.) 



Bulbs are abbreviated underground stems, furnished 

 with an oval mass of thick, fleshy scales, closely packed 

 together above, and with adventitious roots at the base, as 

 the bulbs of the lily. (PI. I., 12.) 



Tubers are annual, thickened portions of a subterranean 

 stem or branch, with minute scales, and often buds, sunk 

 in small recesses, and called eyes. (PL I., 6.) 



Corms are rounded or oval, fleshy, but compact under- 

 . ground stems, provided with more or less obvious buds. 

 (PI. 1,11.) 



INTEEIOE STEUCTUEE OF THE STEM. 



79. The internal structure of the stem exhibits the 

 various forms of elementaiy tissue in certain combina- 

 tions. 



Among the Cryptogams, only the Acrog'ens have true 

 stems — that is, stems containing woody fibres and ducts. 

 The lower grades of the flowerless plants have either no 

 stems, or stems consisting of parenchyma only. 



The stems of the Phmnogams have woody fibres and 

 ducts. 



The parenchyma of the stem grows equally fast verti- 

 cally and Jiorizontally. Into this both the pUurenchymo, 



