Among the conditions necessary to successful cork-culture, climate and soil are foremost in im portance. In the Mediterranean basin, the tree favours altitudes varying from 1600 to 3200 ft. ; as regards latitude, it does not flourish beyond 45° N. ; while the minimum average annual tempe rature must not be less than 13° (55° F.). The most generally suitable aspect is southerly. Slopes are always preferable to flat lands, as affording a more free circulation of air and admission of light. Considerable care should be shown in the selection of the soil. It is said that the tree in R. wild state is found only on the older geological formations, as granite or clay-slate ; and the experience of cultivatore is that the best cork, and the most rapid growth, are produced on granitic, siliceous, and slaty (Silurian) soils, while the tree almost refuses to grow on calcareous soils. It requires abundant moisture eombined with efficient drainage.
Planting is usually performed with seed. Ae a rule, large sweet acorns develop into trees of regular growth and yielding the finest cork; while email and bitter acorns produce trees of a coarse and inferior nature. The most approved method of planting appears to be the " furrow " or "belt '' system, which consists in sowing the acorns at 20-40 in. apart in a furrow between two or more rows of grape-vines, placed at 5-7 ft. apart. The sowing and planting are conducted simultaneously, the vines affording the shelter which is so necessary to the cork tree during its early growth. Tile young cork trees are thinned out as required, so as to afford abundance of air and light to each.
French siviculturists recommend an average of 110-120 trees a hectare (about 2/ acres), and calculate the production of cork at about 8 kilo. (say 18 lb.) a tree. The trees should be barked according as they arrive at maturity for the operation, rather than at fixed intervals independently of their condition. It is highly important to keep the forests cleared of the naturally-shed virgin oork, on account of the chances it offers of creating a conflagration.
The distinguishing feature of the cork oak is that parenchyma forms the mass of the b irk. In the earliest stages of its growth, it is much less elastic than it ultimately becomes, owing to its con taining, in the first instance, a large proportion of woody matter. The outer casing of the bark is formed during the first year's growth, and does not subsequently increase ; but the parenchyma con tinues to grow, as long as the tree is alive. In consequence of this phenomenon, the pressure of the growing parenchyma beneath forces the outer shell to split and peel off in flakes. The substance
thus shed under natural conditions is known as " virgin cork " ; it is very coarse and of woody texture, its applications being, for these reasons, very limited. But the forcible removal of the cork bark, when performed in a judicious manner, is fortunately unattended with any evil consequence to the tree ; on the contrary, the operation seems to hasten and assist the growth of the bark, improving its quality, at the same time that the tree waxes more vigorous, and attains greater longevity, trees which are regularly barked living to 150 years and upwards.
The age at which the first stripping may be attempted varies, with the locality, from fifteen to thirty yeare, the former being the most general. The yield much resembles the naturally-shed virgin cork, and is commonly included uuder the same term. Subsequently the barking is repeated at regular intervals of eight or ten years, the quality improving on each occasion. The second crop is, also, still too coarse for any but inferior uses. The cork harvest, as it may be called, takes place in the months of July and August, when the second sap flows plentifully. It is conducted in the following manner. An incision through the cork bark is first carried round the tree near the ground ; then a similar cut, parallel to the first, is made just under the first branches ; these are united by others of equal depth drawn longitudinally, and dividing the bark into broad planks. The instrument employed in the barking operations is a sort of axe, Fig. 508, the handle of which is flattened into a wedge-like shape at the extremity ; in short, it is not unlike the axe used in this country for barking the common oak. After cutting, each plank is loosened from the tree by tapping it smartly, and, when thus isolated, its dislodgment is effected by inserting beneath it the wedge-shaped handle of the axe used in making the incisions. Occasionally the planks, after being cut out, are left to shed themselves, by the natural process resulting from the growth of the living bark beneath. The greatest care must be taken that the incisions do not penetrate to the inner bark, or the life of the tree would be destroyed. The thickness of the cork layer thus removed is seldom less than in. nor more than 3 in. According to the 23rd article of the laws regulating cork culture in France, the minimum thickness at which the bark may.be removed is 0.023 metre (say 0.9 in.) ; on the other hand, no good can be gained hy allowing it to exceed the ordinary thickness, as the extra amount would only cut to waste.