STEM. 



15 



known by the number of concentric circles of the wood. But 

 this rule is of uncertain application, owing to numerous dis- 

 turbing causes, especially in countries in which the period 

 of rest is less distinctly marked than in the winter of northern 

 latitudes. 



116. The secretions of plants are deposited most abundantly 

 in the oldest concentric layers ; while those layers which are 

 most recently formed contain but a slight deposit. 



117. When the tissue of the concentric layers is filled with 

 secretions, it ceases to perform any vital functions. 



118. The dead and fully formed central layers are called 

 the heart-wood. 



119. The living and incompletely formed external layers 

 are called the alburnum. 



120. Upon the outside of the wood lies the BAKK, which, 

 like the wood, consists of concentric layers. 



121. It consists of four distinct parts : 1. the Epidermis 66 "; 

 2. the EpipJilceum 65 ^ 3. the Mesophlceum 65 66c ; and 4, 

 the Endophlmum or Liber 65 ^ d . 



122. Each of these parts increases by successive additions 

 to its own inside, except the epidermis, which is never re- 

 newed. 



123. The Epiphloeum and Mesophlosum are both formed of 

 cellular tissue only ; but their cells are placed in different di- 

 rections with respect to each other. The former is often 

 large and soft, and may separate spontaneously from the young 



