INTRODUCTION TO BOTANY. xK 



its liber in the lower part, will not strike root : remove the 

 liber, in a circular band, from the trunk of a tree, so as to 

 leave the woody body naked, the entire tree will ultimately 

 perish. 



The liber hardens, every year ; new layers are formed on 

 its inner surface by means of the cambium. 



§. 5. Of the Alburnum^ or False JVood. See Plate, 

 11, fig. 169, and Explanation. 



The external woody layers, in contact with the liber, consti- 

 tute the alburnum. The alburnum is real wood, still young. 



§. 6. Of the Wood t properly so called. 



The wood derives its origin from the innermost layers of the 

 alburnum, which successively acquire a greater degree of hard- 

 ness, and are ultimately converted into true wood. The latter, 

 therefore, consists of all the circular layers between the albur- 

 num and the medullary tube. At a certain period of the life of 

 vegetables, there are annually formed a layer of wood and a 

 layer of alburnum ; that is to say, the innermost layer of 

 alburnum is changed into wood, at the same time that a new 

 layer of alburnum is produced externally, so that every year a 

 new concentric zone is added. 



The alburnum is destitute of vessels ; the wood abounds 

 with them. These vessels are either false spirals, or porous 

 vessels, (see plate, 11, and explanation,) but with no true 

 spirals. It is by means of these tubes, that the sap is con- 

 veyed into the body of the stem. Through age, the cavity of 

 these vessels becomes diminished, and the course of the fluids in 

 tlie wood is for ever arrested. 



§. 7- Of the Medullary Tube. See Plate, 11. and 

 Explanation . 



This occupies the middle of the stem : its office is to con- 

 tain the pith. Its jyarieles, (or sides,) are the very long vessels* 

 parallel, and disposed in a longitudinal direction. Its vessels 

 are spirals, false spirals, and porous vessels. See Plate, 1], 

 and Explanation. 



