THE TREE ITS SHOOT-SYSTEM 109 



But, in addition to these differences in diameter 

 within one and the same annual ring, a gradual incre- 

 ment 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 elements 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 

 concerned. In the stem of the oak the chief changes 

 in this connection are : 



The fibres increase in length as follows, according to 

 Sanio's measurements : While they average O42 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 (130 years ?). The tracheids 

 in the same annual rings were found to average 0*39, 

 0'43, 0-53, and 0*72 mm. respectively; and the individual 

 members or segments of the larger vessels 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 increments 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 centre of the 

 stem into the condition known as < heart-wood ' (dura- 



