252 INDUSTRIAL PLANTS 
the fibrils we see circular holes many times larger than the 
fibril-cavities. These are long, tubular reservoirs called 
resin-ducts from the material they contain which oozes out 
at a wound. On longitudinal surfaces they appear as more 
or less conspicuous yellowish or brownish streaks. 
In many woods there are no resin-ducts present, but there 
are numerous, commonly empty, canals sometimes con- 
siderably larger than resin-ducts and sometimes much smaller 
in diameter. They form a continuous system of tubes 
throughout the wood. Their appearance, viewed endwise, 
is shown for various woods in Figs. 235-241. They are known 
as pores or vessels, and in the sap-wood serve as pipes or 
reservoirs for conveying upward the crude sap absorbed 
by the roots or for storing it, together with more or less air, 
temporarily till needed. They thus share with the wood- 
fibrils the office of conduction which is performed alone by 
the fibrils of such woods as pine. 
Figure 232 shows in a somewhat diagrammatic way the rela- 
tive position of the various structural elements found in 
pine wood, with reference to one another and to the pith- 
cylinder within and the bark without. Between the bark and 
the wood is found a thin layer of soft, living material called 
the cambium (c) which is of vital importance because from it, 
after the first year, all the wood and bark is formed. At the 
beginning of each season’s growth the cambium works vigor- 
ously and forms numerous full-size wood-fibrils, but as more 
and more new wood is added to the old, an increasing pressure 
results unless the bark yields readily to the strain. In many 
cases the bark holds firmly and this pressure is partly ac- 
countable for the fact that summer wood is commonly more 
compact than spring wood, which as we have seen results 
from the progressive flattening of the fibrils in the radial 
direction. Through the winter the outer bark becomes suffi- 
ciently cracked by the action of the weather to relieve the 
pressure upon the parts within; consequently at the return 
of spring the cambium can resume its work of wood-building 
under the most favorable conditions. As a result of these 
alternating changes of conditions, which in our climate are 
connected with the annual changes of temperature, we have 
Fhe, 
