464 



A TEXTBOOK OF THEORETICAL BOTANY 



organs, where elongation has ceased, are called pitted, because the wall is 

 fully lignified except for a number of pits. These may be either simple pits, 



B C 



451. — Types of tracheid wall thickening. A, Spiral. 

 B, Reticulate. C, Pitted. (After Zimmerynann.) 



as in parenchyma cells, or may be bordered pits (Fig. 452). In this latter 

 case the pit membrane, i.e., the primary wall, is roofed over on each side by 



B 



C 



D E F 



Fig. 452. — Bordered pits. A, B and C, Stages in the growth of 

 border and torus in three-dimensional view. D, Bordered 

 pits in transverse section. E, Bordered pit with torus 

 displaced laterally, filling aperture of pit. F, Pit membrane 

 in surface view with central torus and radial striations. 

 {After Stevens.) 



I! 



