4^ Elementary Botany 



phloem. Thus year by year fresh rings of wood and liber are 

 formed (fig. 68), the oldest wood being towards the centre of 

 the tree, whilst the oldest bark is towards the exterior. Growth 

 such as this is sometimes spoken of as exogenous, and dico- 

 tyledonous plants are sometimes termed exogens. It is, how- 

 ever, only the wood which is exogenous in growth ; the liber 

 is endogenous. 



As fresh rings of wood are as a rule formed every year, the 

 age of a tree can generally be approximately ascertained by 

 counting the number of the rings. The annual rings of various 

 trees differ very much in the extent of their thickness, much 

 depending not only on the nature of the plant itself, but also 

 on its age and the atmospheric conditions of the climate. 

 Also in the same plant the rings are not of equal thickness all 

 round, so that the pith instead of being geometrically in the 

 centre of the tree is generally more or less excentric. 



When these wood rings are fully developed they consist of 

 three elements, viz. : — 



a. Wood fibres or wood prosenchyma. 



b. Vessels, either spiral, annular, pitted, or otherwise. 



c. A variable quantity of wood parenchyma. 



These three elements are variously arranged, and any one 

 of them may be absent. Generally there is plenty of secondary 

 deposit, so that the cells have become hard. The inner wood is, 

 as we have seen, the oldest, and hence the hardest, and is often 

 coloured by the secondary deposit having colouring matter. 

 This is especially well seen in such wood as Ebony, Mahogany, 

 Rosewood, &c. In other cases, as in the Poplar and Willow, 

 the old wood is nearly as colourless as that of the exterior. 

 This inner wood is known as the duramen or heart-wood. 



The xylem on the exterior, which is younger, is permeated 

 with sap, and is known as the alburnum or sap-wood. 



The heart -wood is principally useful in supporting the plant, 

 so that it may be absent as well as the pith without interfering 

 with the vital activities of the tree. 



In the sap-wood the vessels are air carriers, whilst the fibres 

 are sap distributors carrying a current of sap from the root up 

 towards the leaves (see Chapter XII.), 



