THE RING OF ANNUAL GROWTH. Ill 



and smaller tracheids of the later growth are a response to the 

 demand for greater strength. The ring of growth in its relation 

 to the transport of water and solutes demands our attention in 

 this chapter. 



The large tracheal tubes and tracheids of the early growth 

 are the most efficient in carrying the transpiration stream be- 

 cause of their relatively large diameters. That, however, the 

 tracheal tissues of the late growth are useful in carrying water 

 is shown by experiments with colored solutions, and by the 

 numerous pits in their tangential walls which are evidently 

 designed to deliver water to the early tracheal elements of the 

 succeeding year. Furthermore, there is frequent communica- 

 tion of the late tracheal elements with the early elements of the 

 following year by means of radial rows of tracheids; and at 

 the close of the season's growth groups of small tracheal tubes 

 are often produced which are to lie immediately against, or in 

 close juxtaposition to, the large tracheal tubes of the succeeding 

 year. 



The number of years the tracheal elements remain active 

 varies with different species. In trees like the oak, walnut, 

 etc., whose wood is differentiated into sapwood and heartwood 

 the heartwood has lost the power to conduct water through the 

 filling of the tracheal tubes by ingrowths of the wood paren- 

 chyma into their cavities, and by the infiltration of the walls 

 of the tissues in general with gums, resins, and coloring mat- 

 ters. On the other hand, in trees like the beech which produce 

 no heartwood the water highways remain functional through 

 many years. But always it is the wood of the current year 

 that is the most active in water conduction, while the part played 

 by the older rings of growth lessens with each succeeding year. 

 In perennial monocotyledonous stems, as in the case of the 

 palm, for example, where there is no annual increase in the 

 size of the bundles, the same tracheal elements must retain their 

 activity indefinitely. 



