90 OTHER TYPES OF STEMS 



the wood cells of the stem remain alive for only a few years. 

 This fact is usually apparent in any log or stump where it will 

 be seen that the outer annular rings are of a lighter color, the 

 sap wood, and the older parts are of a darker color, the heart 

 wood. The sap wood is active in the transport of water and 

 contains many living cells. For this reason it is not so valuable 

 for lumber. The heart wood is composed of dead cells and 

 while the most serviceable for building purposes, it is of little 

 use to the tree as we often see a vigorous tree whose heart wood 

 has been largely destroyed by decay. 



Fig. 6o. Diagram showing a common method of sawing in producing 

 quartered oak. The log is first squared to remove the sap wood and then a 

 number of boards, depending upon the size and grain of the tree, are removed 

 at a and b. These are the best boards obtained from the log, being broad and 

 approximately radial and therefore showing the medullary rays to good ad- 

 vantage. The remaining portions of the log are cut radially, as shown at c. 

 One of the ends of quartered lumber is oblique and must be squared, as shown 

 in the diagram. 



41. Other Types of Steins. — The cone bearing trees, such as 

 the pines and spruces, have essentially the same arrangement 

 of tissues and mode of growth as noted above in the dicotyledons. 

 The xylem, however, consists entirely of tracheids (Fig. 59) 

 with the exception of a few small spiral vessels that are formed 

 as the first cells of the vascular bundles. 



The monocotyledons, plants distinguished usually by their 

 parallel veined leaves and single seed leaf, like the palms, lilies, 

 grasses, etc., are characterized by stems that do not increase 

 materially in diameter. Growth is largely confined to the top 

 of the stem and consequently it can only elongate, forming a 

 very regular, columnar trunk. The reason of this is apparent 

 when cross sections of such stems are examined (Fig. 46, D). 



