2o: 



In Figure 139, a longitudinal, radial section, the bordered pits are seen to 

 be on the radial walls of the tracheids, their diameter filling up the whole of the 

 lumina. Only one row was found to occur in each tracheid. A portion of a 

 medullary ra\' is also shown, running across the figure from left to right. 



\Mien working over the longitudinal sections an interesting feature in 

 connection with the dark substance present in some of the prosenchymatous 

 cells, was noted, namely that in such cells the walls diftered in no way from those 

 of the contiguous or empt}- ones, having bordered pits just as equally distributed 

 on their radial walls as those where no substance occurred. 



The substance itself was found not to be restricted to any particular portion 

 of the cells, but at certain inter- 

 vals, was broken into parts, each 

 bounded by a septum composed of 

 this material at right angles to the 

 walls of the tracheids. Now, if these 

 cells are followed along in the 

 opposite directions to the cell sub- 

 stance they will be found to have 

 acute, angular terminations at the 

 other end, showing their prosen- 

 chymatous nature. Our observa- 

 tions on these particular cells lead 

 to the conclusion that there is 

 probably some functional agree- 

 ment between the lumina content 

 of the prosenchyma with that of 

 the parenchymatoiis cells of the 

 medullary rays ; the simple slits or 

 pits of the latter being perhaps 

 the avenue of exchange or supply 

 of cell contents between these two 



r 



Figure 139. — Radial section of timber. Tlie uniform character of all 

 the ceils of the rays is clearly defined. The pits of the 

 bordered cells are just focussed. C. caUarata, x no. 



organs. 



The mural pits are of two kinds — bordered and simple, the former occurring 

 as a rule on the radial walls, although they do occasionally occur in the tangential 

 walls of the prosenchymatous cells, whilst the latter are found on the radial walls 

 of the parench}nTiatous cells of the rays. 



The aperture of the simple cells is a narrow, o^'ate, oblique slit between 

 the walls of the lumina, and these means of communication vary in number from 

 one to four, but mostly two to four. The bordered pits are well shown in section 

 on the radial walls in Figure 13S. 



