THE LIGNEOUS SYSTEM. 



133 



whereby the vegetable kingdom is distinguished into as many 

 great classes, viz. : 



TlIE OUTSIDE-GROWERS (ExOGENS), 

 TllE INSIDE-GROWERS (ENDOGENS), 

 TlIE POINT-GROWERS (AcROGENS), 



THE MASS-GROWERS (THALLOGENS). 



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

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

 exhibits zones of different structures, which are distinguished as 

 pith, medullary sheath, wood, and bark. 



507 5U8 



507, Exogens Oik, Fir, etc. 08, Endogens 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. 



