But, in addition to these differences in diameter within 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 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 fibers 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 (one hundred and thirty 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 argar 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 center of the stem into the condition known as "heart-wood" (dura-
