BOTANY. 



100 



is now transformed into a viscid glutinous matter called cam- 

 bium, is deposited between the liber and the wood, becomes 

 organized into cells, ajid forms a new layer upon each. Soon 

 afterwards, the new layers are pervaded by woody tubes and 

 fibres, which commence at the leaves and grow downwards. 

 " The number of layers in the bark and wood will always be 

 equal." (Wood.) The outer bark of young twigs seems to 

 perform the same function as the leaves : in the cactus, sta- 

 pheliu, and other plants which produce no leaves, the bark must 

 perform the same office as the leaves do in plants which pro- 

 duce them. (Johnston.) 



^ Fig. is. 



4 



C C 



[Fig. 18. 3, horizontal section of an endogenous stem, exhibiting the 

 bundles of woody fibre, spiral vessels and ducts, irregularly disposed in 

 the cellular tissue : 5, a, a, cellular tissue ; b, spiral vessels on inner side 

 of dotted ducts, c, c; d, woody fibre on the exterior side: 4, stem of 

 three year's growth; a, pith; e, bark; b, c, d, successive annual layers: 

 6 a, pith; b, spiral vessels of the medullary sheath; c, dotted ducts; d, 

 woody fibre; e, bark.] 



The endogenous stem exhibits no distinction of bark, wood 

 and pith, and no concentric annual layers or grains. It is 

 composed of cellular tissue, woody fibres, spiral vessels and 



10 



