THE LIGNEOUS SYSTEM. 



133 



whereby the vegetable kingdom is distinguished into as many 

 great classes, viz. : 



THE OUTSIDE-GROWEKS (ExOGENS), 



TlIE INSIDE-GROWERS (ENDOGENS), 



THE POINT-GROWERS (ACROGENS), 



TlIE MASS-GROWERS (TlIALLOGENs). 



405. The exogenous structnre. A cross-section of the 

 Btem or branch of any dicotyledonous plant (Mustard, Maple) 

 exhibits zones of different structures, which are distinguished ae 

 pith, medullary sheath, wood, and bark. 



507 5(i 



507, Exogens Oak, Fir, etc. 508. Endosiens Palm, Agave, etc. 



406. The Pith occupies the central part of the stem. It con- 

 sists of parenchyma, is chiefly abundant in herbaceous plants 

 and all young stems. When new, it is filled with fluids for the 

 nourishment of the buds until they can make food for them- 

 selves. As the plant advances in age, the pith loses its vitality, 

 is filled with air only, is often torn into cavities, or disappears. 



