246 INDUSTRIAL PLANTS 
the immensely 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 sap-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 inferiority to heart-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 on 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 seen, either replaced by 
fibers or shares with them more or less the service of mechani- 
cal support. Viewed broadly, it may be said that wood cor- 
