CHAPTER IV 



BORDERED PITS 



In a preceding chapter it has been shown how the bordered 

 pit originates in the spiral structure of the protoxylem through 

 a more general and continuous development of the secondary 

 layer of the cell wall. We are now concerned with an inquiry 

 into the detailed structure of these markings, their variations 

 under different conditions of growth and situation, and their 

 relations to taxonomy as well as to phylogeny. 



It has been seen that in the genesis of the bordered pit the 

 bands of adjacent spirals or the bars of a scalariform structure 

 generally enlarge toward one another so that the intervening 

 area contracts about a common center, but the edges never com- 

 pletely meet, so that a pit is left at the central point. In such 

 contraction, however, the secondary wall is not joined to the pri- 

 mary, but is free and springs from it as an arch which has a 

 circular outline and a central orifice (fig. 8). As such pits are 

 always paired on opposite sides of the primary wall, the entire 

 structure, when seen in tangential section, is lenticular in form 

 (fig. 1 1), with a membrane traversing the central plane and two 

 openings opposite to one another at the extremities of the minor 

 axis. From this it is obvious that the pit as usually seen is a 

 double structure which does not at first present a direct opening 

 from one tracheid cavity to the other, for so long as the tracheids 

 are growing or the protoplasm is present communication between 

 adjacent tracheids is cut off by the primary wall, which consti- 

 tutes a closing or pit membrane. By a subsequent change in this 

 latter it may eventually become displaced from its central posi- 

 tion and then lie against one of the arches. Under such circum- 

 stances it is often not readily discernible, and the pit appears as 

 if the primary wall had been obliterated within its limits. In the 



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