i.] GENERAL CHARACTERS AND STRUCTURE. 7 



will be closer-grained, and heavier, than an equal 

 volume of the wood where the thin-walled tracheides 

 with large lumina predominate. 



FIG. 3. Portions of four annual rings from a thin transverse section of the wood of a 

 Conifer, such as the spruce, fir. M, a medullary ray ; b andc show the entire breadth 

 of two annual rings ; a, autumn wood of an annual ring internal to b (and there- 

 fore older than b) ; d. spring wood of an annual ring external to c (and therefore 

 younger than c). Bordered pits are seen in section on some of the tracheides. 

 Magnified about 100 times. 



Returning now to the section (Fig. 3), it is to be 

 observed that the differences in the zones just referred 

 to enable us to distinguish the so-called " annual 



