226 THE SECOND BOOK OF BOTANY. 



In inaple and beech the ducts are uniformly dis- 

 tributed, while in pines there are no ducts at all. 

 But the inner and outer portions of each layer 

 are still so different in compactness, that the line 

 which separates the new, large, vigorous cells of the 

 spring growth from the close, fine cells, formed the 

 last of the previous season, can usually be distinctly 

 seen. 



But, if the wood of pines has no ducts, it still 

 presents a peculiar structure. It is composed wholly 

 of dotted fibres, and the dots are produced by little 

 hollows in the sides of the fibres, like the cavity of a 

 watch-glass, these hollows being so placed 

 FIG. 397.* that, when fibres come together, one concavity 

 answers to another (Fig. 397*), making a lens- 

 shaped space, like two watch crystals, so placed 

 that the concavities face each other. These 

 little disk-like marks are the result of an 

 unequal deposit of the lining material of the 

 fibre, leaving thin places where the wall of 

 the fibres curves inward. This thinness, as 

 the fibre gradually fills up with deposits, pro- 

 duces in the centre of each cup a short canal, 

 opening into its interior. The cavity is usu- 

 ally filled with turpentine, which sometimes 

 finds its way through this canal into the fibre, 

 destroying it little by little, and often producing con- 

 siderable deposits of resin in the wood of green trees. 

 In all the pine family these marks are on the lateral 

 portions of the fibres, and never on the part toward 

 the centre or the outside of the tree. 



