LESSON 20.] THE REMAINING TISSUES. 55 



sels, succeeded by woody fibre, as before ; but now the bark must be 

 formed : it has existed already twice before ; but, when no longer re- 

 quired as such, its structure has become transformed into the woody 

 fibrous tissue in many Exogens, however, its elements remain 

 permanently. 



320. To the woody fibres, cambium cells succeed, then, the inner 

 fibrous layer of the bark; next, the cellular envelope, , which forms 

 the middle layer of the bark; and, lastly, the outer corky layer, cov- 

 ered with epidermis. 



321. It is only necessary, now, to describe briefly the characteris- 

 tics of these tissues severally, by way of recapitulation, and first- 

 ly of 



322. The ivood. The layers of wood are formed outside the me- 

 dullary sheath, or the vascular zone which surrounds the pith. 



S23. They consist of woody fibres, mixed with dotted ducts, oc- 

 casionally mixed with annular, reticulated, and spiral vessels. In the 

 young state the tubes of the woody tissue are pervious, but by de- 

 grees they fill up by the deposit of lignine within them. 



324. In old, exogenous trees, the central wood is hard and dura- 

 ble, Constituting the Heart-ivood, while the external layer is soft, and 

 forms the Albernum or Sap-wood. 



325. The ligneous matter forming the heart-wood of some trees, 

 acquires color ; thus it is black in the Ebony, brown in the Black 

 Walnut, yellow in the Barberry and Judas-tree, purple in the Red 

 Cedar, and green in the Guaiac tree. 



326. The proportions between the heart-wood and alburnum dif- 

 fer greatly in different trees ; those, however, in which the hard wood 

 predominates are best suited for building, and better adapted to with- 

 stand the attacks of insects, or the wet or dry rot. 



327. The durability of wood depends on the nature of the lignine, 

 and this greatly varies in different trees. 



LESSON XX. 



THE EEMAINING TISSUES. 



328. The Medullary Bays. These consist of flattened, cellular 

 tissue, having the appearance of bricks in a wall ; in the young stem 



