112 



TRANSPORT OF WATER AND SOIL SOLUTES 



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II 



Relation of Rings of Growth to Growth in Length. — 



When the terminal bud unfolds and adds another year's growth 

 in length to the stem, it, together with the new shoots from 

 lateral buds, produces the leaves of the 

 current year. All of the leaves, it will 

 be remembered, with the exception of 

 perennial leaves such as those of the 

 pine, are borne on these new shoots. 

 We must now proceed to enquire what 

 is the relation of the tracheal elements 

 of the new segments of stem to those of 

 the leaves and of the preceding years' 

 growths. 



While the internodes of the new shoot 

 are still elongating and the cambium has 

 not yet begun secondary increase in thick- 

 ness, only the small and few spiral and 

 annular tracheal tubes differentiated from 

 the procambium are present in the new 

 growth to take up and carry water from 

 last year's segment of stem into the unfold- 

 ing leaves of the new shoot (Fig. 57). And 

 it sometimes happens that the leaves are 

 more than half grown before these first 

 tracheal tubes are reenforced by others 

 from the cambium. In this fact we have 

 the evidence, since young leaves are known 

 to transpire water rapidly, that the spiral 

 and annular tracheal elements first laid 

 down are very efl&cient water conductors, 

 through short distances at least. Soon, 

 however, these earliest tracheal elements 

 are reenforced by others laid down by 

 the cambium, and on account of their 

 weakness they break apart and go out of 

 function (Fig. 57). 



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Fig. 57. — Diagram show- 

 ing the relation of the water- 

 carrying tissues of the leaves 

 to those of the stem, and 

 how the older rings of 

 growth give up their water 

 to the newer before this 

 water can enter the leaves. 

 The figures at the bottom 

 indicate in years the age of 

 the rings. 



