STRUCTURAL BOTANY. 39 



and the trachenchyma are imbedded vertically / tliey run 

 longitudinally through the tranches. The parenchyma 

 •we may call the horizontal system of the Btem, and the 

 pleurenchyma and trachenchyma together the vertical 

 system. 



80. In Ph^enogams the woody system is arranged in 

 tioo widely different modes, the one mode characterizing 

 the Exogens, and the other the Endogens. 



In ExoGENS the woody system is arranged, if their 

 stems last one year, in one zone ; if they last from year 

 to year, in several (annual) concentric layers between a 

 central pith and a separable bark, which forms a peculiar 

 layer of tissue forming the circumference of the stem. 

 (PI. II., 29.) 



In Endogens, on the other hand, the woody tissue is 

 never disposed in a zone, nor in concentric layers, but 

 runs, in the form of separate and scattered bundles, 

 throiigh the ordinary cellular tissue, and not between a 

 well-marked central pith and a separable bark. (PI. 

 11,28.) 



The stem of the Exogens, if it lasts longer than one 

 year, increases in diameter by the annual formation of 

 new pleurenchyma and trachenchyma around the prior 

 circles — that is, grows on the outside {Exogens : outside- 

 growers) ; but that of the Endogens increases by the de- 

 position of new woody fibres toward the centre — that is, 

 by a gradual distention of the whole system of woody 

 bundles {Endogens : inside-growers). 



81. A cross-section of the stem, or of a branch of any 

 exogenous plant presents zones of diflEerent structure, 

 known as Pith, Medullary Sheath, Wood, amd JSa/rk. 



The Pith, occupying the centre of the stem, consists of 

 parenchyma. 



