20 



Elementary Botany 



arranged in transverse layers like the rungs of a ladder. Such 

 cells are well seen in the stems of Ferns. 



A very important form of pitted cell is met with, especially 

 in the wood of the Fir trees and their allies. In these cases 

 the pit is funnel-shaped, being wide on the outside and gradu- 

 ally tapering till it ^_ 

 forms a tube or 

 pore canal leading 

 into the centre of 

 the cell. 



Fig. 32. — Piece of a 

 reticulately pitted 

 tracheide from the 

 W-avziTilia gran- 

 difolia). 



Fig. 33. — Scalariform 

 cell from the under- 

 ground stem of the 

 Brake {Pteris aqni- 

 iina). 



Fig. 34. — Longitudinal section of wood 

 of "Pitlits sylvestris. Bordered pits, 

 t t' f't increasing in age : a~e, wood 

 cells, e, eldest, a, j oungest : cb, wood 

 cells of cambium ; st, large pits, 

 where medullary rays touch wood 

 cells. (After Sachs.) 



On examining these cells by means of a microscope, they 

 present the appearance of a central pit surrounded by a border, 

 and they are called cells with bordered pits. This appearance 

 is caused by a varying amount of light being transmitted through 

 the central pore which forms the pit, and the funnel-shaped 

 upper portion which forms the border. 



Very often the cell wall between the two ' borders ' becomes 

 absorbed, in which case a complete passage is made between 

 the two cells. 



