ORGANS OF NTJTEITION. 7^ 



central one, in which the wood-cells and vessels have thickened 

 i walls, are impenioiis to fluids, hard in texture, of a dry nature, 



/' and commonly more or less coloured, which is called the Duramen 



I or Heart-wood ; and an outer portion, in which the wood-cells 

 and vessels have thin sides, are pervious to and full of sap, soft 

 in texture, and pale or colourless, to which the name oi Alburnum 



j or Sap-wood is given. 



' When the internal part of the wood has become of the nature 

 of heart-wood it ceases to perform any active functions in the 

 plant, its office being then chiefly to act as a support to it. All 

 the vital and essential functions of the stem are then carried on 

 by the sap-ivood. Hence we see the reason why a tree in which the 

 central part is completely destroyed, but with the outer part or 

 alburnum remaining, continues to live, put forth new branches, 

 and add to its substance. 



Age of Trees. — As each zone of wood in an exogenous stem is 

 produced annually, it should follow that by counting the number 

 of zones in a transverse section of any tree we ought to be able to 

 ascertain its age, and this is true with a few exceptions, when 

 such trees are natives of cold climates, because in these, as we 

 have seen, the annual zones are distinctly marked. In exogenous 

 trees, however, of warm climates it is generally difficult, and 

 frequently impossible, to ascertain their age in this manner, in 

 consequence of several disturbing causes ; thus, in the first place, 

 the zones are by no means so well defined ; secondly, more than 

 one zone may be formed in a year; thirdly, some trees, such as 

 Zamias, the Cycas, &c., only produce one zone as the growth of 

 several years ; fourthl}', some plants, as certain species of Cacti, 

 never form annual zones, but the wood, whatever its age, only 

 appears as a uniform mass ; while lastly, in some, such as 

 Guaiacum, the zones are not only indistinct, but very irregular 

 in their growth. 



It is commonly stated that the age of a tree may not only be 

 ascertained by counting the annual zones in a transverse section 

 of the wood, but that the mere inspection of a fragment of the 

 wood of a tree of which the diameter is known, will also affi)rd 

 data by which the age may be ascertained. The manner of pro- 

 ceeding in such a case is as follows: — Divide half the diameter 

 of the tree divested of its bark by the diameter of the fragment, 

 and then having ascertained the number of zones in that frag- 

 ment, multiply this number by the quotient previously obtained. 

 Thus : suppose the diameter of the fragment to be two inches, 

 and that of half the diameter of the wood twenty inches ; then if 

 there are eight zones in the fragment, by mxiltiplying this number 

 by ten, the quotient resulting from the division of half the dia- 

 meter of the tree by that of the fragment, we shall get eighty 

 years as the supposed age. Now, if the thickness of the zones 

 was the same on both sides of the tree, and the pith consequently 



