26 PLANT LIFE. 



early observers as actual pits or perforations in the cell-wall, 

 hence the name, which has been retained, although the 

 apparent pits are now known to be due to the different 

 appearance of the circular unthickened portions of the 

 cell-wall. In ordinary pitted cells the opening through the 

 secondary thickening is everywhere uniform in shape and 

 size with the unthickened portion of the primary wall, 

 hence cylindrical ; but in a special form of pitted cells, 

 characteristic of the Conifer a, including fir-trees, yews, etc., 

 the pits are termed bordered pUs, and originate as follows. 

 The unthickened portions of the primary cell-wall are 

 circular in outline as before, but as the thickening increases 

 the cavity becomes smaller and smaller, consequently in a 

 section of the cell-wall the cavities are conical, widest at 

 the outside, so that a surface view of such a pit shows two 

 circles, one large and corresponding to the large outermost 

 ring, and enclosing a smaller ring corresponding to the 

 innermost portion of the pit'; in fact, exactly the same 

 appearance as presented by looking down a funnel. The 

 thickening matter, deposited by the protoplasm, consists at 

 first, like the primary cell-wall, of cellulose, but sooner or 

 later chemical changes take place, either partially or 

 entirely ; for example, the cellulose may be converted into a 

 substance called lignin, as in the wood of ordinary forest 

 trees, for the purpose of giving additional mechanical 

 strength to the structure — such modified cell-walls are yet 

 easily permeable by water; or the cell-wall may become 

 partially or entirely converted into cork, characterized by its 

 elasticity and water-proof nature — such changes occur in the 

 external cells of leaves, branches, etc., which are growing 

 in the air, the object being to prevent the loss by evapora- 

 tion of the water contained in the tissues of such structures : 



