We now proceed to give some account of the substance which fills up the vacant spaces that are left between the longitudinal fibres. It is called by Grew the parenchyma or pulp, by Malpighi, the vesicular tissue or web; both of them consider it as formed of small bladders or reticles, that are in contact with each other, lying in an horizontal position, or at right angles to the longitudinal fibres: they do not suppose them to be all of the same size, or even of the same figure: Grew compares it to the froth of beer or eggs. The flesh of fruits consists for the most part of this substance, very much filled with juice, though with considerable difference in its organization. Be this as it may, the nature of this substance, its form and structure, are at present but very little known. It is floccose, and varies in colour in different species.
OF THE VASA PROPRIA INTERIORA.
Besides the lymphatic vessels and the cellular substance, we find the juice vessels, or vasa propria, in the bark. In those trees which are famous for medicinal virtues, they are usually very large; they carry the milky juices of the sumach, and in them is lodged the finest and highest-flavoured turpentine in all the kinds of pine. Dr. Hill thinks that a tree of that genus exhibits them best, and the more, as the turpentine which fills them may be perfectly dissolved in spirit of wine. The pinus orientalis is the species in which these vessels are most distinctly seen.
OF THE BLEA.
This is that part of the tree which is formed into wood, and therefore lies between it and the bark, and may be separated from them by maceration.
A longitudinal piece of the blea, when examined by the microscope, exhibits a number of vessels running parallel to each other, the interstitial spaces being filled with a floccose, white, formless substance, of which Dr. Hill suspects even the vessels themselves to be formed. Innumerable small openings or mouths may be discovered in these vessels, suited to imbibe the moisture which is so essential to the life and health of plants. These mouths cannot be well discerned, except when they are opened by the season of the year, either before the first leaves of spring, or in the midsummer shooting time; though a small quantity of moisture will keep them open at that time, yet no quantity would be sufficient at an improper season.[136]
[136] Hill’s Construction of Timber, p. 47.
The blea is a zone more or less perfect, which lies under the bark, and covers or surrounds the wood, and is principally distinguished from it by being less dense. In some species the difference between the blea and the wood is very remarkable, in others it is less so.
The ancient botanists, struck with the difference they observed between the wood and the blea, compared this substance to the fat in animals. Malpighi, Grew, and Du Hamel considered it as the wood not yet arrived to a state of perfection. It is organized in a manner similar to the wood, and possessing the same vessels disposed nearly in the same manner. The juice vessels of this part may be separated from it by maceration; Dr. Hill says, that in this state they appear perfect cylinders, with thick white coats, the surface perfectly uniform.