BORDERED PITS. 



83 



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

 the flattened sides of tiie 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- 

 niferae are so nearly con- 

 stant, that they may be 

 used with considerable 

 certaint}- in tlie 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 contiouous cells. 



60 



Hence the two domes, 



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

 and the central perforations are nearlj- or exactly opposite each 

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

 men the bordered pits vary witliin com- 

 paratively narrow limits both as regards 

 tlie size of the disc and that of the central 

 aperture. 



The two domes making up a single dis- 

 coid marking are at first separated by a 

 delicate plate of unequal thickness ; but 

 later this middle lamella maj^ be broken 

 down, and then a free passage extends 

 from one cell to tlie 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 sylvestris 

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

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



Fig. 60. Ai-eolated or disciform markings of tlie wood-cells (tracliei'Js) of Pinus 

 Xjariclo : a, aspect of radial walls; 6, a transverse section; c, development of tlie 

 marldngs in Pinus sylvestris. (Sanio.) 



Figs. 61 and 62. Pinus sylvestris. Transverse sections of nearly perfect and perfect 

 discoid markings. (Strasburger.) 



