ROOTS AND THEIR FUNCTIONS 77 



xylem which are concerned with translocation 

 of water. The' walls are thickened, but nearly 

 always show thin spots or " pits." These are 

 of special use inasmuch as the water from 

 one tracheid can more easily and rapidly pass 

 through a thin than a thick membrane. Now 

 there are considerable variations of pressure 

 conditions in these conducting channels, and 

 an unprotected thin membrane would stand 

 a good chance of becoming ruptured. The 

 risk is obviated by a partial roofing over the 

 thin spots by the thickened parts of the walls, 

 which gives the pits a curious appearance 

 under the microscope, and has caused them 

 to be known generally as " bordered pits." 

 Pits of this kind are, as we might now antici- 

 pate, of almost universal occurrence in water- 

 conducting tissue. They are more easily seen 

 in some woods than others, and perhaps in 

 none better than in a bit of deal or pine wood 

 (Fig. 12). ^ 



A striking character of these conducting 

 tracheids and vessels lies in the absence of 

 living protoplasm from them. All functional 

 tracheids and vessels are therefore merely 

 the dead skeletons of once living cells. The 

 protoplasm disappears from them as soon as 

 the thickening and lignification of the walls 

 is complete. It is good that this should be 

 so, for the presence of viscous protoplasm 

 within the channels would greatly impede the 

 flow of water through them. 



In addition to the conducting tracheids and 



