THE STONE CELLS 



8l 



quality of the wood depends much upon its visible characters, 

 such as the frequency of its wood fibers and the thickness of 

 their walls, other invisible characters, such as the hardness^ 

 elasticity, and breaking strength of the walls are factors so im- 

 portant that whether a wood should be classed as hard or soft. 



m 



Fig. 37. — Cross section of yellow poplar wood. E, early; L, late growth; m, medullary ray. 



Photomicrograph. X20. 



weak or strong, could not be determined by microscopic exami- 

 nation alone. 



Sometimes the wood fibers are entirely lacking, as in Aris- 

 tolochia sipho, where their place is taken by tracheids, and as 

 in pine and other Gymnosperms w^here tracheids perform the 

 double function of wood fibers and tracheal tubes. 



The Stone Cells.— The stone cells are formed by the thick- 

 ening and lignification of the walls of originally thin-walled 

 parenchyma cells (Fig. 15). They may be found in the primary 

 and secondary cortex, pericycle, medullary rays and pith, in 



the integuments of many seeds, and in the shells and stones 

 6 



