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FUNDAMENTALS OF CYTOLOGY 



Fig. 54. — Thickened cell walls in secondary xylem of plants, a, wood of spruce {Picea). 

 b, transverse section of wood of Trochodeiidron; the primary walls are deeply stained, the 

 secondary walls and the intercellular substance are not. c, transverse section of wood of 

 Trochodendron photographed with polarized light through crossed nicols; outer and inner por- 

 tions of secondary wall bright, middle lamella and thick middle portion of secondary wall 

 dark, (a, courtesy of U. S. Forest Products Laboratory; b, c, after T. Kerr and I. W. Bailey.) 



