292 DECOMPOSITE ORGANS. CHAP. i. 



joints that form transverse diaphragms, interrupting 

 the continuity of the tube even though it is f\ir- 

 nished with no pith/ as in the Grasses. 



Plants of this class are all furnished with a dis- 

 tinct and conspicuous root, the structure of which 

 is often similar to that of the stem, as may be seen 

 in the horizontal, section of the Common Brake ; 

 in which some of the bundles are flattened and 

 bent so as to form a section of a circle, the rest 

 being placed irregularly around them. But though 

 the stem of such plants is often tubular, yet the 

 roots are not often tubular (though the root of 

 Water Hemlock furnishes an exception), but wholly 

 filled up either with pulp or pith contained within 

 the cylinder of fibres, which the bark or epidermis 

 externally invests. 



SUBSECTION III. 



The Mass consisting of Bark, Wood, and Pith. 

 A second variety of this mode is that in which 

 a strong and often thick bark encloses a circular 

 layer of longitudinal fibres, or several such circular 

 and concentric layers, interwoven with thin trans- 

 verse and divergent layers of pulp, so as to form a 

 firm and compact cylinder, in the centre of which 

 is lodged a pulp or pith. This mode is best exem- 

 plified in trees and shrubs (PL VIII. Fig. 13.), 

 though it is also applicable to many plants whose 

 texture is chiefly or almost wholly herbaceous, 



