BORDERED PITS. 



83 



recent plants. When the wood in a pine stem is cut radially, 

 the flattened sides of the wood-cells exhibit the dotted appear- 

 ance seen in Fig. 60. The number and mode of distribution of 

 the markings in the wood- 

 cells or tracheids of Co- a 

 niferai are so nearly con- 

 stant, that the}' may be 

 used with considerable 

 certaint}' in the discrimi- 

 nation of a few genera. 



269. In a transverse 

 section of the mature tra- 

 cheids the discoid mark- 

 ings are plainly seen to 

 be pits having an arched 

 border or incomplete 

 dome, and it is also 

 seen that the thin spot 

 or pit is common to 

 two contiguous cells. 

 Hence the two domes, 



being on opposite sides of a partition- wall, have a lens shape, 

 and the central perforations are nearly or exactly opposite each 

 other (Fig. 62). Even in the same speci- 

 men the bordered pits vary within com- 

 paratively narrow limits both as regards 

 the size of the disc and that of the central 

 aperture. 



The two domes making up a single dis- 

 coid marking are at first separated b} 7 a 

 delicate plate of unequal thickness ; but 

 later this middle lamella may be broken 

 down, and then a free passage extends 

 from one cell to the other. 



The character of the domes and the mid- 

 dle plate can be understood from the ac- 

 companying figures of sections of the stem of Pinus S3'lvestris 

 (Figs. 62 and 63). According to Sanio. the sections should be 

 boiled in acetic acid, in order to remove all cell- contents. 



FIG. 60. Areolated or disciform markings of the wood-cells (tracheids) of Pinus 

 Laricio : a, aspect of radial walls ; b, a transverse section : c, development of the 

 markings in Pinus sylvestris. (Sanio.) 



Fir.s. 61 and 62. Pinus sylvestris. Transverse sections of nearly perfect and perfect 

 discoid markings. (Strasburger.) 



