142 VEGETABLE LIFE AND WORK. [SECTION 16. 



434. The Wood of an exogenous trunk, having the old growths covered 

 by the new, remains nearly unchanged in age, except from decay. Wherever 

 there is an annual suspension and renewal of growth, as in temperate cli- 

 mates, the annual growths are more or less distinctly marked, in the form 

 of concentric rings on the cross section, so that the age of the tree may be 

 known by counting them. Over twelve hundred layers have been counted 

 on the stumps of Sequoias in California, and it is probable that some trees 

 now living antedate the Christian era. 



435. The reason why the annual growths are distinguishable is, that the 

 wood formed at the beginning of the season is more or less different in the 

 size or character of the cells from that of the close. In Oak, Chestnut, etc., 

 the first wood of the season abounds in dotted ducts, the calibre of which 

 is many times greater than that of the proper wood-cells. 



436. Sap-wood, or Alburnum. This is the newer wood, living or 

 recently alive, and taking part in the conveyance of sap. Sooner or later, 

 each layer, as it becomes more and more deeply covered by the newer ones 

 and farther from the region of growth, is converted into 



437. Heart-wood, or Duramen. This is drier, harder, more solid, 

 ana" much more durable as timber, than sap-wood. It is generally of a 

 different color, and it exhibits in different species the hue peculiar to each, 

 such as reddish in Red-Cedar, brown in Black-Walnut, black in Ebony, etc. 

 The cnange of sap-wood into heart-wood results from the thickening of the 

 walls of the wood-cells by the deposition of hard matter, lining the tubes 

 and diminishing their calibre; and by the deposition of a vegetable coloring- 

 matter peculiar to each species. The heart-wood, being no longer a living 

 part, may decay, and often does so, without the least injury to the tree, 

 except by diminishing the strength of the trunk, and so rendering it more 

 liable to be overthrown. 



438. The Living Parts of a Tree, of the exogenous kind, are only 

 these : first, the rootlets at one extremity ; second, the buds and leaves of 

 the season at the other ; and third, a zone consisting of the newest wood 

 and the newest bark, connecting the rootlets with the buds or leaves, how- 

 ever widely separated these may be, — in the tallest trees from two to four 

 hundred feet apart. And these parts of the tree are all renewed every year. 

 No wonder, therefore, that trees may live so long, since they annually re- 

 produce everything that is essential to their life and growth, and since only 

 a very small part of their bulk is alive at once. The tree survives, but 

 nothing now living has been so long. In it, as elsewhere, life is a transi- 

 tory thing, ever abandoning the old, and renewed in the young. 



§ 4. ANATOMY OF LEAVES. 



439. The wood in leaves is the framework of ribs, veins, and veinlets 

 (125), serving not only to strengthen them, but also to bring in the sap, 

 and to distribute it throughout every part. The cellular portion is the 



