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 0°42 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- 
