232	painting materials
rounded by a wall of cellulose. In the woody structure of these higher plants,
where cell growth has ceased, the cells become cemented together with a substance
called lignin. This lignin, which is somewhat similar to cellulose and is generally
spoken of as one substance, is in all probability made up of several closely related
substances. It is the substance that is removed chemically in preparing cellulose
for paper. The woody tissue of plants is made up of cells which exhibit great
diversity of form, size, and markings. The paper maker is interested in the true
wood fibres or libriform cells, and in the tracheids. The wood fibres characteris-
tic of deciduous trees or hard-woods are elongated cells with strongly thickened
walls; they are variable in length in different woods, ranging from 0.14 mm. to
2.0 mm. and in all cases they are the longest elements present. The woody tissue
of coniferous trees consists almost entirely of tracheids which are elongated,
tapering cells, more or less lignified and characterized by bordered pits or discoid
markings. These pits are so constant in number and mode of distribution that they
may be used as distinguishing characteristics for some woods. The tracheids of
coniferous woods are considerably longer than the wood fibres of hard-woods,
frequently reaching a length of 4 or 5 mm. This is one of the reasons that spruce
wood is so valuable for making sulphite pulp.
Bast fibres are those fibre bundles found in the inner bark of various plants.
They are held together by incrusting materials and by partial identity of their
cell walls. Some kind of chemical or mechanical treatment is necessary to separate
them. The commonest method is that of retting. The bast tissues of dicotyledonous
annuals furnish such staple materials as flax, hemp, ramie, and jute. The walls of
bast fibres are generally of considerable thickness and the central canal varies
greatly in different species. The fibres are characterized by irregular thickenings of
the walls at intervals to give joints or nodes.
In the seed hairs, like cotton, the fibres are individual cells or units. Although
they occur as complex aggregates they are not cemented together as are wood
or bast fibres. This is the reason that cotton is such a valuable source of fibre and
cellulose.
Fk (Picea and Abies). The name of this wood now correctly includes the
spruces (Picea) as well as the true firs (Abies) and is also frequently applied as a
general term for all of the true conifers (Abietineae). In northern Europe the Nor-
way spruce (P. excelsa) is the most important timber species but in southern
Europe the silver fir (A. pectinata) is an important rival. This tree appears to be
the true Abies of the Latin writers. It is abundant in most of the mountain ranges
of southern Europe. The wood is inferior to P. excels a but is soft and easily worked.
The wood structure of the spruces and firs is similar to the pines except that the
resin ducts are smaller, more scattered, and less distinct. Woods from various firs
have no doubt been used rather extensively in the panel paintings of Europe. It
is little mentioned in the records but there perhaps it has been often confused
with pine.