STRUCTURE.] ALBURNUM DURAMEN. 199 



first, with which it agrees exactly ill structure, except that 

 there is no medullary sheath ; the third year a third zone is 

 formed on the outside of the second, in all respects like it ; 

 and so on, one zone being deposited every year in cold 

 countries as long as the plant continues to live. As each 

 new zone is formed over that of the previous year, the latter 

 undergoes no alteration of structure when once formed. 

 Wood is not subject to a distension by a force beneath it, as 

 the bark is ; but, whatever the first arrangement or direction 

 of its tissue may be, such they remain to the end of its life. 

 The formation of the wood is, therefore, the reverse of that of 

 the bark ; the latter increasing by addition to the inside of 

 its strata, the former by successive deposits upon its outside. 

 It is for this reason that stems of this kind are called 

 Exogenous (from two Greek words, signifying to grow out- 

 wardly). According to Dutrochet, each zone of wood is 

 in these plants separated from its neighbour by a layer of 

 cellular tissue, forming part of the system of the pith and 

 bark : but although this is true in certain plants, such as 

 arborescent nettles and others, it is by no means a general 

 law. 



After wood has arrived at the age of a few years, or some- 

 times even sooner, it acquires a colour different from that 

 which it possessed when first deposited, becoming what is 

 called heartwood or duramen. For instance, in the beech it 

 becomes light brown, in the oak deep brown, in Brazil wood 

 and Guaiacum green, and in ebony black. In all these it was 

 originally colourless, and owes its different tints to matter 

 deposited gradually in all parts of the tissue ; as may be easily 

 proved by throwing a piece of heart-wood into nitric acid, or 

 some other solvent, when the colouring matter is discharged, 

 and the tissue recovers its original colourless character. 

 That part of the wood in which no colouring matter is yet 

 deposited, and consequently that which, being last formed, 

 is interposed between the bark and duramen, is called 

 alburnum. The distinction between these is physiologically 

 important, as will hereafter be explained. 



