THE TREE ITS SHOOT-SYSTEM. 109 



But, in addition to these differences in diameter with- 

 in one and the same annual ring, a gradual increment 

 in the average size of certain of the elements (both in 

 length and diameter) occurs as the tree becomes older 

 in other words, the average width and length of the ele- 

 ments increases year by year up to a certain age ; after 

 reaching a definite size they enlarge no more. These 

 changes differ according to the part of the tree con- 

 cerned. In the stem of the oak the chief changes in 

 this connection are : 



The fibers increase in length as follows, according to 

 Sanio's measurements : While they average 0-43 mm. in 

 length in the first annual ring, they increase to 0-60 

 mm. in the second, 0*74 mm. in the fourth, and go up 

 to 1-22 mm. after a great age (one hundred and thirty 

 years ?) The tracheids in the same annual rings were 

 found to average 0-39, 0-43, O53, and 0'72 mm. respect- 

 ively ; and the individual members or segments of the 

 argar vessals averaged 0-25 mm. in the second annual 

 ring, 0-26 mm. in the fourth, and 0*36 mm. in the three 

 outer rings. The mean radial diameter of these vessels 

 also increased : in the third year it was 0-08 mm., and it 

 rose year by year until in the sixth year the definitive 

 width of 0-31 to 0-33 mm. was attained. After this the 

 width of these vessels is practically constant. These in- 

 crements in size appear to take place after the element 

 has passed out of the strictly cambial condition. 



The passage of the older wood in the center of the 

 stem into the condition known as " heart- wood " (dura- 



