162 Relation of Annual Rings to Age. 



IX. The Relation of Annual Kings of Exogens to- 



Age. 



By D. P. Pexhallow. 



That there is a certain relation existing between the well- 

 known phenomenon of the development of rings of growth in 

 exogenous trees and the age of the trees themselves, has been a 

 matter of common observation and speculation for a very long 

 time. Latterly, a few scattered observations have been made 

 to determine this relation more exactly, but there appears to 

 be nothing on record of a sufficiently systematic and exhaustive 

 character to permit the deduction of a general law. The diffi- 

 culties in the way of obtaining such data are considerable, one 

 of the greatest being that of getting together a sufficiently large 

 number of woods of different species, of which the exact ages are 

 known. During the past year, somewhat unusual opportunities 

 have been presented in this direction, and it has therefore been 

 deemed desirable to collect all the data possible, and see how far 

 they are in accord with the statements already made and the views 

 generally held by botanists. The question may be considered as 

 presenting itself under two aspects, viz : — 



1st. Upon what does the distinction of rings depend? 



2nd. Does every ring indicate one year of growth ? 



These primary considerations necessarily involve several others 

 which it will be desirable to consider ; but we will first pass in 

 review the general opinions held at the present time, as also some 

 of the more recent observations recorded. 



It has been commonly accepted by botanists that each ring 

 in the stem is generally determined by the annual cessation and 

 renewal of activity, and that it thus corresponds to one full season 

 of growth, and thus an estimate of the age of the tree may be ob- 

 tained by counting the full number of rings. Schleiden gave 

 expression to this view several years since. * More recently 

 Gray f says: " Each layer being the product of a single year's 

 growth, the age of an exogenous tree may, in general, be correct- 

 ly ascertained by counting the rings in a cross section of the 

 trunk." Bessey % is a little more explicit when he says that,. 



* Principles of Scientific Botany, 1849, p. 238. 

 f Text Book of Botany, p. 79. 

 X Text Book of Botany, p. 447. 



