34 THE CELL WALL 



3. Half-bordered pit-pairs represent "an intercellular pairing 

 of a simple and a bordered pit" (Glossary of Terms I'sed in 

 Describing Woods, p. 5) and occur when living parenchymatous 

 cells develop in contact with dead tracheary elements. According 

 to a frequent opinion expressed in texts, the pit-member on the 

 side of the living cell is simple while its mate on the side of the 

 tracheid or vessel is bordered. Frost (1929), however, in a study 

 of the nature of pitting between tracheary and parenchymatous 

 cells in angiosperm xylem. has found that this conception has no 

 general validity. He concludes that "fully bordered, half- 

 bordered and simple pits are characteristic features between 

 tracheary cells and vascular parenchyma" and that "the type of 

 pitting on the wall of the parenchyma cell is controlled largely 

 by the degree of specialization of the vessel or fiber which lies 

 next to it." Obviously the whole question of pitting in plant 

 cells demands further investigation, from both a comparative as 

 well as an ontogenetic point of view. 



Secure prepared slides of conifer and dicotyledonous sec- 

 ondary xylem and investigate under high magnification the na- 

 ture of the pitting between wood parenchyma or wood ray cells 

 and the connected tracheary elements. 



4. Vestigial pit-pairs are typical of thick-walled wood and 

 bast fibers. Tn these cells, the secondary wall is greatly thickened 

 and the pits are often so reduced in size and number as to appear 

 truly "vestigial" or functionless. The vestigial pit-pairs of 

 typical wood fibers are usually interpreted morphologically as 

 reduced and highly modified bordered pit-pairs. This conclusion 

 is based (1) on the belief that the wood fiber has developed phylo- 

 genetically from the tracheid, and (2) on the fact that a closely 

 graded series of intermediate conditions between typical "bord- 

 dered pit-i)airs" and "vestigial pit-pairs" can be seen in com- 

 paring the tracheids, fibcr-tracheids and fibers in the xylem of 

 the same plant. In typical vestigial pit-pairs of wood fibers, the 

 pit-cavities although circular are relatively small, the border is 

 greatly reduced in size or absent and a torus is frequently lacking. 

 The most distinctive feature of tliis type of pitting, however, 

 consists in the elongated slit-like apertures which instead of being 

 opposite fas is true of the circular apertures of the members of 



