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 
