Buatr.—On the Building Materials of Otago. 135 
except in supporting them. The leaves are the lungs of the plant, but, 
instead of making the original fluid thinner, and purifying it by the extrac- 
tion of carbonic acid and the addition of oxygen as in animals, they make 
the sap thicker, and add carbonic acid, which is the food of plants. The 
precise nature of the chemical process carried on in the leaves, and the 
exact constituents of its product, are imperfectly understood. After the 
sap has acquired the necessary ingredients, it returns through the outer 
layer of the wood and the inner layer of the bark, leaving in its course a 
deposit of ligneous matter on each, and permeating to a greater or less 
extent all the rings of sap-wood. The deposits made on the bark and wood 
harden into rings of timber and bark, the former to increase the size of the 
tree, and the latter to replace the scales that are continually falling off the 
outer surface. The conversion of sap into heart-wood is attributed to the 
combined action of the juices and the compressive force exercised by the 
shrinkage of the outer rings and bark; but against this idea we have the 
fact of the change being generally sudden: one ring may be perfect heart, 
and the next sapwood of a very inferior quality. Whatever be the cause of 
this ripening of the timber, the process is not simultaneous with its growth, 
for the rings of sapwood always decrease in number as the tree approaches 
maturity, and there are frequently fewer rings on one side than the other. 
Climate, situation, and soil, exercise a great influence on the character of 
timber. Among different trees the best timber is obtained from tropical 
countries, but in the same species the product of cold climates is found to 
be the strongest and most durable. Most authorities, ancient and modern, 
pronounce in favor of slow growth in timber trees as essential to perfection ; 
but I observe that Mr. Laslett, Inspector of Timber to the Admiralty, 
entertains an opposite opinion formed from observations on oak and fir 
trees. I can easily understand the possibility of rapid growth being con- 
ducive to strength and durability, as it proves that the plant is well fed 
and in vigorous health. Although the wood may be soft and porous in the 
young tree, it does not follow that the old one will inherit these qualities ; 
the energy that puts forth strong shoots is in all probability sufficient to 
provide them with a proportionate supply of woody fibre and the other 
essentials of strength. 
Timber grown in open ground is stronger and more durable than that 
from the dark forest, but, on the other hand, it is more subject to twists, 
shakes, and irregularity of composition, and the trees are often stunted and 
crooked. The effect of the weather is well shown on the southern side of 
the Otago Peninsula, where the trees are blown into shapes as grotesque as 
could be seen in a Dutch garden. 
The influence of situation and soil on the growth of trees is very re- 
