38 PRACTICAL BOTANY. 



MODIFIED STEMS. 



The stem is, wlioUv or in part, sometimes imder ground 

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

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

 also by growing upward. 



Bhizomes^ hidhs, tubers , and corms are the types of 

 underground stems. 



Rhizomes or rootstocks are horizontal subterranean 

 stems. Acorus calamus has a jointed, Scrophularia nodosa 

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



Bulbs are abbreviated underground stems, furnished 

 with an oval mass of tliick, 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. (PI. I., 6.) 



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

 ground stems, provided with more or les3 obvious buds. 



(PI. 1, 11.) 



INTERIOR STRUCTURE OF THE STEM. 



TO. The internal structure of the stem exhibits the 

 various forms of elementary tissue in certain combina- 

 tions. 



Among the Cryptogams, only the Acrogens 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 PhcBnogains have woody fibres and 

 ducts. 



T\\Q jparenchyma of the stem ^row?5 equally fast ve/'^i)- 

 cally and horizontally. Into this both the pleurenchyma 



