CHAP. IV. FUNCTION OF NUTRITION. 243 



all the different organs by which its various functions 

 are carried on, and which are consequently as vigorously 

 performed in the oldest tree as in the youngest. But 

 although the organs which every animal possesses are 

 continually sustaining a certain degree of repair, yet 

 they are gradually wearing out, or ultimately become 

 choked up in old age ; and thus a definite period is 

 naturally allotted to the existence of the individual 

 from this cause alone. But the period of life to which 

 plants attain is no way dependent on these conditions ; 

 but is regulated by a combination of external causes 

 and internal influences of a very different kind. Those 

 trees are most likely to endure the longest, which grow 

 the slowest, and which attain the least height in pro- 

 portion to the diameter of their trunks ; and the anti- 

 quity of some trees of this description appears to be 

 prodigiously great. 



(240.) Estimation of the Age of Trees. It is only the 

 ages of Dicotyledons which can be ascertained with any 

 degree of certainty. In Monocotyledons the diameter of 

 the tree is not enlarged by annual additions of fresh 

 cylinders of wood, as is the case with the former, whose 

 ages may be accurately ascertained by inspecting a 

 transverse section of their trunks. By placing a strip 

 of paper upon this section from the centre to the cir- 

 cumference, and marking it along the edge where it 

 intersects the concentric circles on the section, a con- 

 venient register may be obtained, not only of the ages 

 of different trees, but of their comparative rates of in- 

 crease at different periods of their growth. As the pith 

 is seldom exactly in the centre of the tree, the best mode 

 of obtaining the average annual growth is by measuring 

 the circumference of the trunk, and then calculating for 

 the mean thickness of each layer by dividing the semi- 

 diameter by the whole number of layers. These mea- 

 surements should be made at a little distance above the 

 soil, generally about four feet, where the trunk is free 

 from protuberances and of an average thickness. 

 K 2 



