246 INDUSTRIAL PLANTS 



the immenseljf heavy crown especially when loaded with 

 snow and ice, and severely strained by wind. So also must 

 the branches be joined with great firmness to the trunk, and 

 be stiff enough to hold the foliage well in place. Even the 

 leaves require a woody framework or skeleton to keep their 

 soft, green parts spread open to the sunshine. The woody 

 parts of leaves are continuous with the new wood of the stem 

 which in turn connects with the new wood of the root. What 

 is absorbed by the root is conducted as crude sap mostly 

 through the new wood of root and stem to the food-making 

 parts of the foliage. When as in many trees the new wood is 

 formed next to the bark in successive layers it is distinguished 

 as say-wood so long as it retains its power of conducting sap. 

 After a certain number of years, varying greatly in different 

 kinds of trees, the wood is no longer useful in this way, but 

 becomes more useful mechanically because of increased 

 dryness, compactness, and strength. It is then known as 

 heart-wood and is commonly distinguished from the sap-wood 

 by a marked change in color. The color is due to the presence 

 of substances formed as by-products of the plant's activities 

 but of no further use to it, and therefore best accumulated 

 in wood which has ceased to be a channel for sap. The sap- 

 wood is also used by the tree to some extent for the storage 

 of food substances, which have but little color, as for example 

 the .sweet sap of the sugar-maple. Such food makes the 

 sap-wood a particularly good feeding ground for wood-boring 

 insects and other parasites which injure or destroy the wood. 

 Its greater liability to the attacks of these destructive agents, 

 together with its infei-iority to lieart-wood in strength lead 

 commonly to the rejection of sap-wood for constructive 

 purposes; while for ornamental uses as well, heart-wood is 

 furthermore preferred <.>n account of its more attractive 

 coloring. A still further advantage of heart -wood for econom- 

 ic use is the much larger masses of it which may be obtained 

 from large trees. Thus we see that wood, especially heart- 

 wood, is the great massive and resistant material of plants. 

 In slender parts it is, as we have sch'h, eitlier n^jilaced b\- 

 fibers or shares with them more or less th(^ service of mechani- 

 cal support. Viewed broadly, it may be said that wood cor- 



