
THE STEM lol 
substance itself has approximately the same weight in all species, 
being about 1.6 times as heavy as water. Therefore all woods 
would sink if they did not contain a considerable amount of ‘air 
in the dead cells and intercellular spaces. When this air is 
removed, as after long submergence in water, the wood sinks. 
From the above it follows that the weight of a given wood will 
depend on its density, that 
is, on the relative proportion 
of wood material and air. 
The density and weight nat- 
urally vary with the number 
of wood fibers and the thick- 
ness of their walls. Wood 
that contains a great deal of 
parenchyma and few fibers 
is light (Fig. 146), while 
wood composed largely of 
thick-walled fibers is heavy. 











Fuel value of wood. ‘The 
fuel value of wood varies | | | : WAU AA 
with its weight, as equal ! 11 | : r VJ 
weights of wood produce Fig. 148. Radial section of wood of coral 
equal quantities of heat. tree (Erythrina fusca) 
In general it may be said ‘The wood fibers and wood parenchyma have 
of woods otherwise equal the same characteristics as in the tangential 
that those light in weight Sitio Me), The gry of al con 
kindle easily and flash (x 95) 
quickly, the fire spreading 
rapidly, while those that are heavy behave in the opposite way. 
The longitudinal arrangement of the fibers of wood explains 
why it is much easier to split it longitudinally than to cut 
across it. Wood splits most easily along the radii, on account 
of the radial arrangement of the pith rays. 
Wood pulp. Paper made from wood consists essentially of 
the fibrous elements, which are separated and then pressed to- 
gether into sheets. In making paper from wood the fibers are 

