BOTANY IN RELATION TO STUDY OF TIMBER. 39
At the end of each year's increment is a band of wood in which
very few vessels appear. Like oak, beech wood contains vessels,
fibres and parenchyma cells, but here all the elements are more
or less irregularly scattered, except for a band consisting almost
entirely of fibres at the end of each season's increment. Woods,
like beech, with vessels of about the same size throughout
the year, are termed diffuse porous, in contrast to woods like
oak, which have a ring of vessels at the beginning of each
season : such woods are termed ring porous.
The two timbers so far considered are produced by dicoty-
ledonous trees. Examination of the end grain of a coniferous
tree, like Scots Pine, shows concentric cylinders of wood made
up of alternating pale and darker coloured rings : vessels are
absent and the rays are very small, and only visible with a lens.
The differences are explained on the much simpler structure
of the pine wood. Here the rays are only a single cell wide
and the remainder of the wood consists of tracheids which are
both water conducting and supporting elements. In many
conifers parenchyma cells filled with resin are also present, or
parenchyma cells may be present round a central duct, the resin
canal: resin canals occur in Scots Pine, but are absent in many
coniferous genera. In Yew (Taxus spp.) neither resin ducts
nor resin cells are present. The light and dark rings consist of
spring and summer wood respectively: in the former the
tracheids have thinner walls.
Coniferous woods are called softwoods and dicotyledonous
woods hardwoods, but these terms arc purely a matter of
convenience. Many hardwoods are much softer than the
average softwood, while pitch pine and yew, although both very
hard timbers, are technically softwoods.
SOME TIMBERS AND THEIR SUBSTITUTES.
Users of timber are conservative. It is often an extremely
difficult matter to get a new timber on the market ; the result
is that a new wood frequently appears in commerce under an
old name. This may not be a matter of great moment, if the
wood has the same texture as the better known one, and is
equally suitable for the purpose for which it will be used, but
with a new wood one cannot be sure that this is so : the risk of
using it is considerable.