THE LIGNEOUS SYSTEM. 



137 



of the green bark ? 414. Tissue of the brown bark ? Its varying colors ? 415. IIow ia it 

 in Cork Oak ? 416. What the medullary rays ? Their structure ? Tissue ? Show the 

 ' silver-grain 11 in fig. 509. What their service ? 418. Where the cambium layer ? What 

 is it? 419. Why called the generative layer? 420. Why ia the growth of Exogens un- 

 limited ? 



CHAPTER Y. 



THE LIGNEOUS SYSTEM CONTINUED. 



421. The endogenous structure. In the cross-section of 

 a monocotyledonous stem (Corn, Palm) there is no visible dis- 

 tinction of bark, wood, pith, or of annual layers of any kind. 

 It is composed of tissues quite similar to those of the exogenous 

 stem, but very differently arranged. 



513, Various kinds of vessels in a wot, * Ibre of Bamboo or Rattan a, Cells of parenchyma; b, annular 

 cells; r, spiral vessels; d, porous duct; e., wood-cells. 514, Section of an Acrogenous stem of Tree-Fein 

 (Cyathea), showing the vascular bundles imbedded near the circumference of the cellular mass. 



422. The body of the monocotyledonous stem consists of pa- 

 renchyma, within which tissue numerous threadlike bundles of 

 woody matter are imbedded. Each bundle consists of one or 

 more dotted ducts accompanied by spiral vessels, pleurenchyma, 

 and often cienchyma, variously arranged in different species. 



423. The formation of these bundles is dependent upon the leaves, from which they may 

 severally be traced downward, first tending toward the interior of the stem. Further 

 on they recurve outward again, and finally terminate near the surface, there interlacing 

 and combining with their fellows, and forming an excessively hard but inseparable rind 

 (false bark). From this entanglement of the fibres the cleavage of endogenous stems ia 

 difficult or impossible. In jointed stems (culms) this entanglement occurs only at tlie 

 nodes (Cane, Grasses). 



424. The growth of monocotyledonous stems thus takes place 

 by the addition of the new wood bundles to the interior, and 

 hence such plants are called Inside-growers or ENDOGENS. 



