,THE VEGETATIVE OEGANS OF PLANTS. 269 



the firmer vascular bundles unaltered in form. A portion 

 of the base of such a stalk, cut lengthwise, is represented 

 in figure 47, where are seen the duct-fibers arranged par- 

 allel to each other in the internodes, and curiously inter- 

 woven and branched at the nodes, either those, a and &, 

 from which roots issue, or that, c, which was clasped by 

 the base of a leaf. 



The endogenous stem, as represented in the maize-stalk, 

 has no well-defined bark that admits of being stripped off 

 externally, and no separate central pith of soft cell-tissue 

 free from vascular bundles. It, like the aerial portions of 

 all flowering plants, is covered with a skin, or epidermis, 

 composed usually of one or several layers of flattened 

 cells, whose walls are thick, and far less penetrable to 

 fluid than the delicate texture of the interior cell-tissue. 

 The stem is denser and harder at the circumference than 

 towards the center. This is due to the fact that the fibers 

 are more numerous and older towards the outside of the 

 stem. The newer fibers, as they continually form, grow 

 in the inside of the stem, and hence the designation endog- 

 enous, which in plain English means inside-grower. 



In consequence of this inner growth, the stems of most 

 woody endogens, as the palms, after a time become so in- 

 durated externally, that all lateral expansion ceases, and 

 the stem increases only in height. It grows, nevertheless, 

 internally, new fibers developing in the softer portions, 

 until, in some cases, the tree dies because its interior is so 

 closely packed with fibers that the formation of new ones, 

 and the accompanying vital processes, become impossible. 



In herbaceous endogens the soft stem admits the indefi- 

 nite growth of new vascular tissue. 



The stems of the grasses are hollow, except at the 

 nodes. Those of the rushes have a central pith free from 

 vascular tissue. 



The Minute Structure of the Endogenous Stem is ex- 

 hibited in the accompanying cuts, which represent highly 



