1834.] on the Longevity of Plants. 197 



madreporic islands, might they not give some idea of the date of their origin ? 

 But ceasing our conjectures on subjects of such magnitude, if we reflect on the 

 means of attaining the solution of the question, we shall see that they are all 

 founded on an exact appreciation of the laws which govern the growth of trees ; 

 and this knowledge may throw light on many parts of vegetable physiology 

 and of the forester's art. I believe therefore that such researches may become 

 useful ; but even should they prove but curious, I should still not think them 

 unworthy of being offered to the public ; for curiosity is an insatiable appetite 

 that the mind of man takes pleasure in satisfying, in proportion to the quantity of 

 food which has already been provided for it. 



It is well known that plants destined to attain the character of trees may all be 

 classed under two heads. The first, which are the most numerous, have the trunk 

 composed of a body of wood coated with bark ; they grow by the annual addition of 

 a new layer of wood, which is produced outside the old wood, but within the bark ; 

 these layers of the young wood being the most exterior, the name exogenous has been 

 given to such plants when speaking of their growth, and that of dicotyledonous when 

 alluding to their germination. Under the second head are placed, on the contrary, 

 all those plants whose trunks, being sensibly cylindrical and generally unadorned 

 by branches, show only a body of wood without any bark properly so called ; of 

 which the exterior fibres are the oldest and most hard, and the interior fibres the 

 softest and youngest. They have obtained from this last circumstance the name of en- 

 dogenous, by which they are distinguished when alluding to their growth, and which 

 is synonymous to that of monocotyledonous, used when speaking of their germination. 

 We will rapidly examine the means of ascertaining the age of individuals belonging 

 to these two classes, and will afterwards add a few remarks on vegetables more 

 humble in their appearance, but whose duration offers matter for special consider- 

 ation. 



Almost all trees that are natives of the temperate zones, and consequently of the 

 most civilized parts of the world, are exogenous : their nature and history have 

 therefore been examined much more closely than any others, and may afford us 

 the most interesting data. 



It is now ascertained beyond a doubt, that exogenous trees increase annually by 

 a new layer of wood, and consequently the number of concentric zones visible on 

 the transversal or horizontal section of a trunk may give an idea of the number 

 of years that have elapsed since the part of the tree under examination began to 

 vegetate. It follows that a slice cut at the bottom of the branch will give the age of 

 the branch ; another made at the bottom of the trunk, or at the neck, will give the 

 age of the tree. If, as has been asserted, irregularities may occasionally occur, and 

 this is a very doubtful point, it may at least be affirmed, that the probability of 

 deviation from the law are so slight, that we may boldly argue on the hypothesis 

 that a given number of layers indicates the same number of years' growth ; conse- 

 quently, whenever a clean section of the trunk can be attained, this very simple cri- 

 terion is sufficient to discover the age of a tree. But the inspection of these 

 concentric zones ought to be made with greater care than has hitherto been bestow- 

 ed on it. The zones, by their number, give the age ; but by the proportion of their 

 thickness they give the mean rate of increase. It is not sufficient therefore to count 

 them, they must be measured. The following is the very simple means I made use 

 of to attain this end. When I met with a clean cut of an old tree, sufficiently heal- 

 thy to observe its layers, I placed on the branch a slip of paper, reaching from the 



