THE INNER ORGANIZATION OF TREES. 43 



vertically. It is through the fibre cells of the wood that 

 the main current of the sap flows in the spring. Their vital 

 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 vi- 

 tal operations of the tree. Their function is now purely a 

 mechanical one; for the very same matter which termin- 

 ates life endows them with force and persistence. The 

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

 frame work of the tree, and withstand outer influences as 

 well as inner decomposition much longer than the other tis- 

 sues. 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 devel- 

 oped they form one continuous tube, fhese 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 trans- 

 verse 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. They 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 j 

 for the former preponderate in number toward the interior 

 portion of the ring, whilst the fibre cells are most abundant 

 toward its exterior. The bounding line of each year's 

 growth is easily distinguished by this internal arrangement 



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