(For the methods of boring see Boring.) The working of coal may be conducted either by means of levels or galleries driven from the outcrop in a valley, or by shafts or pits sunk from the surface.
Each strand of spiral or annular first-formed tracheids is called a protoxylem strand, as distinct from the metaxylem or rest of the xylem, which consists of thick-walled tracheids, the pits of which are often scalariform.
These fibre-tracheids are easily confused on superficial view with the true wood-fibres belonging to the parenchymatous system; but their pits are always bordered, though in the extreme type they are reduced to mere slits in the wall.
It is thickened more in some places than in others, and thus are formed the spiral, annular and other markings, as well as the pits which occur on various cells and vessels.
Pit-threads whIch traverse the closing membrane of the pits in the FIG.