Internal Modifications. 267 



Internal Modifications. 



The internal structure of the stems of flowering plants is sub- 

 ject to two principal and to several subordinate modifications. 

 The former arc well illustrated by such plants as the oak and the 

 cane, specimens of which can be easily obtained for comparison. 

 A transverse slice of the former exhibits a central cellular sub- 

 stance or pith, an external cellular and fibrous ring or bark, an 

 intermediate woody mass and certain fine rays radiating from 

 the pith to the bark, through the wood, and called the medullary 

 rays; this is called exogenous structure, and the plants 

 in which it occurs, outside growers. In the cane, on the 

 contrary, neither bark, pith, wood, nor medullary rays are dis- 

 tinguishable ; the transverse section exhibits a large number 

 of hard spots caused by the section of bundles of woody 

 tissue, and a mass of cellular substance in which they lie embed- 

 ded. This kind of structure is named endogenous, and the plants 

 in which it occurs are termed inside growers. In both cases 

 there is a cellular and vascular system distinct from each other; 

 by a diversity in the respective arrangements of which the differ- 

 ences above described are caused. 



The cellular system in an exogenous stem chiefly occupies the 

 centre and the circumference, which are connected by thin verti- 

 cal plates of the same nature as themselves. The central part 

 (fig. a) is the pith, that of the circumference (b) is the bark, 

 and the connecting vertical plates (c) are medullary rays. 



The pith is a cylindrical or angular column of cellular tissue, 

 arising at the neck of the stem, and terminating at the leaf buds, 

 with all of which, whether they are lateral or terminal, it is in direct 

 communication. Its tissue, when cut through, almost always 

 exhibits a six-sided character, and is frequently larger than in 

 any other part. When newly formed, it is green, and filled with 

 fluid ; but its color gradually disappears as it dries up, and it 

 finally becomes colorless. 



The bark is the external coating of the stem, lying immedi- 

 ately over the wood, to which it forms a sort of sheath, and from - 

 which it is always separable distinctly. When but one year old 

 it consists of an exterior coating of cellular substance, called the 



