THE INNER ORSANIZATION OF TREES. 43 



activity, however, only continues for a short time. Their 

 walls are soon thickened by earthy matter, which goes on 

 accumulating, until their cavities are finally closed, and 

 the sap ceases to circulate through them. The color then, 

 changes, and they no longer take any further part in the 

 vital operations of the tree. Their function is now purely 

 a mechanical one ; for the very same matter which termi- 

 nates life endows them with force and persistence. The 

 fibre-cells thus lignified, form, as it were, the skeleton or 

 framework of the tree, and withstand outer infiuences as 

 well as inner decomposition much longer than the other 

 tissues. They are the very last to yield to dissolution. 



The vasiform or duct-cells are spread through the mass of 

 fibrous tissue. They originate out of a row of cells, the 

 cross walls of which are absorbed, so that, when fully de- 

 veloped, they form one continuous tube. These ducts may 

 be readily distinguished from the wood-cells among which 

 they are interspersed, as their interior diameter or bore is 

 much more considerable, and they remain permanently 

 open. The open mouths of the ducts are very conspicuous 

 on the transverse section of common pine-wood, where 

 they resemble pores. There are several varieties of this 

 species of cell termed by botanists, dotted, annular, spiral, 

 and scalariform ducts. The whole of these vessels at first 

 contain sap, which is afterward displaced by air. Tfcey 

 may be regarded, in fact, as the air-vessels of plants, by 

 means of which the sap in their interior is brought into 

 communication with the atmosphere. This is the reason 

 why these ducts or air-tubes are placed amongst the fibre- 

 cells or sap-tubes, as inspection plainly shows. 



The ducts and fibre-cells are, however, arranged in the 

 stem according to a definite law, as inspection plainly 

 shows ; for the former preponderate in number toward the 

 interior portion of the ring, whilst the fibre-cells are most 

 abundant towards its exterior. The bounding line of each 

 year's growth is easily distinguished by this internal ar- 

 rangement of the ducts, and its cause is to be sought for 

 in the vital economy of the tree itself. The inner portiori 



