46 COMPOUND ORGANS OP PLANTS. 



CHAPTER IV. 



ON THE ORGANIZATION OF THE STEM. 



WHEN we examine anatomically the stems of phanerogamous 

 plants, we find them to be remarkably similar in their internal 

 structure. The stems of forest trees, ligneous and persistent 

 for centuries, differ only from the stems of the herbaceous and 

 humble plants which grow beneath their shade, in the degree 

 of their development; they are constructed on precisely the 

 same plan, and in all the varieties of their growth, for the most 

 part, are reducible to either one or the other of the two follow- 

 ing forms of vegetable organization. 



The exogenous (, outward, and yewdsw, to produce,) or 

 outside-growing stem, so called, because this kind of stem 

 increases in diameter by successive annual additions of bun- 

 dles of vascular and fibrous tissue to its outside. Such a stem 

 exhibits on the cross-section a number of concentric tongs of 

 wood, which mark the successive annual growths of the tree, 

 surrounding a central column of pith, the whole enclosed 

 by a hollow cylinder of bark. The forest trees of the northern 

 United States, and the major part of our herbaceous plants, 

 are all constructed on this plan ; and the cross-sections of an 

 oak branch, or of any other tree, will show the rings, and the 

 nature of an exogenous stem. 



The endogenous (evSov, within,) or inside-growing stem, so 

 called, because it increases in diameter by successive additions 

 of fibro-vascular and cellular matter to its inside. The growth 

 of these plants is carried on by means of the thick cluster of 



