1 Dec., 1899.] QUEENSLAND AGRICULTURAL JOURNAL. 595 
When an ordinary timber tree is sawn across, it is seen to be made up of a 
series of more or less concentric rings—the annual rings, or rings of growth— 
which surround a central small core, the pith, and the whole is enclosed by a 
more or less thick layer of bark. Through the mass of the wood passes a 
number of minute radial lines, the medullary rays, which begin at the outer 
surface of the wood and pass inward, some few reaching to the pith. 
When the tree is living, the actual living growing part is confined to a thin 
layer—the cambium—lying between the wood on the inside and the bark on the 
outside, and forming both by the division of its cells. In this climate growth 
is at a standstill during the winter, but as soon as spring comes it recommences, 
the cambium cells divide up and become specialised, forming on the one side the 
wood and on the other the bark cells. At first, in spring, when the supply of 
nutriment is not very large, the cells are large and thin-walled, whilst, as the 
supply of nutriment becomes larger, the walls of the cells become thicker, and 
therefore the cavities smaller; thus the outer portion of each annual ring is 
different in colour to the inner portion, and as the dark portion of one ring 
comes against the lighter portion of the next, the rings are clearly marked. In 
tropical countries, where growth keeps on all the year round, the distinction 
between the annual rings may be quite obliterated. The cells of the cambium 
become very much modified in form, the exact form varying with the nature of 
the wood, but the wood is always made up of elongated cells packed closely 
together, thus producing the fibre or grain of the wood, the fibres being more 
or less broken up by the cellular patches of the medullary rays, and through 
the mass running parallel with its fibres there may be ducts or passages, or long 
vessels. 
Water in Wood.—W hen the tree is living it always contains a very large 
quantity of water (50 to 70 per cent.), this being much greater in the younger 
parts of the plant than in the older, and greatest of all in the leaves. The 
amount of water varies with the season, and ig always greatest when the 
growth is most active—in the summer. Even when there is most water present, 
however, the vessels are largely filled with air, so that the wood is lighter than 
water and will float, though the materials of which it is composed are actually 
heavier. 
Felling Limber.—When wood is to be used for timber it should be felled 
in winter or early spring, when vitality is least active, as then the amount of 
water present is much less than at other seasons. ‘The bark is then stripped 
off, and the wood is left exposed to the air, lifted off the ground and sheltered 
from the rain for months or years, tillit becomes air-dried, and in this condition 
it may contain from 10 to 15 per cent. of water. The more perfectly the wood 
is dried, the more durable is it likely to be when used in structures. 
Shrinkage of Wood.—As the wood loses water it shrinks, and the shrinkage 
is greater in the case of the newer wood than the older, and as this new wood 
is outside, the contraction necessarily produces radial cracks. If planks be cut 
out of the wood before the shrinkage is complete, or if the wood be subse- 
quently more thoroughly dried, they will warp, the warping always being 
determined by the greater contraction of the younger wood. Dry wood exposed 
to moist air will absorb moisture, and this causes an expansion exactly the 
reverse of the contraction produced on drying the absorption of water, and 
corresponding expansion being due to the presence of constituents in the cells, 
which absorb water and expand inso doing. The expansiou may be go great 
that the cracks in a dried dise may completely close up. 
Formation of Wood.—Vhe cells in the living and growing cambium layer 
have thin walls of cellulose, and contain the nitrogenous matter known’ as 
protoplasm. As these cells become converted into woody tissue, the walls 
become very much thickened and changed in character. The protoplasm to a 
large extent disappears, and the cells and vessels contain various elaborated 
products, such as starch, resins, gum, &c., the nature and quantity varying 
with the plant. 
