2ry2 INDUSTRIAL PLANTS 



the fibrils we see circular holes many times larger than the 

 fibril-cavities. These are long, tubular reservoirs called 

 rcsin-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, commonlj' 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. Thej' are known 

 as pores or vessels, and in the sap-wood serve as ])ipes or 

 reservoirs for convej'ing upward the crude sap absorbed 

 by the roots or for storing it, togetlier 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 sucli woods as jjine. 



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 j^iith- 

 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 ^'ital 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 spiing wood, which as we have seen results 

 from the progressive flattening of th(> fibrils in the radial 

 direction. Through the winter the outer bark I)ecomes suffi- 

 ciently cracked by the action of the Aveather to relieve the 

 j)ressure upon the parts within; consequently at the return 

 of spring th(; 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 



