Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/369

 Rh 1) and is best known in the Araucaria excelsa, or Norfolk Island Pine.

These discoveries are highly important, as they afford examples among the earliest remains of vegetable life, of identity in minute details of internal organization, between the most ancient trees of the primeval forests of our globe, and some of the largest living Coniferæ. The structure of Araucarias alone has been as yet identified

The transverse section of any coniferous wood in addition to the radiating and concentric lines represented Pl. 56a, Fig. 7, exhibits under the microscope a system of reticulations by which Coniferæ are distinguishable from other plants. The form of these reticulations magnified 400 times is given in Pl. 56a, Figs. 2, 4, 6. These apertures are transverse sections of the same vessels, which are seen in at longitudinal section at Pl. 56a; Fig. 8, cut from the centre towards the bark, and parallel to the medullary rays. These vessels exhibit a characteristic and beautiful structure, whereby a distinction is marked between the true Pines and Araucarias. In such a section the small and uniform longitudinal vessels, (Pl. 56a, Fig, 8) which constitute the woody fibre, present at intervals a remarkable appearance of small; nearly circular figures disposed in vertical rows (See Pl. 56a, Figs. 1, 3, 5.) These objects under the name of glands or discs, are differently arranged in different species; they are generally circular, but sometimes elliptical, and when near each other, become angular. Each of these discs has near its centre a smaller circular areola. Pl. 56s, Fig. 1, represents their appearance in the Pinus strobus of North America.

In some Coniferæ, the discs are in single rows; in others, in double as well as single rows, e. g. in Pinus strobus, Pl. 56s, Fig. 1.

Throughout the entire genus of living Pines, when double rows of discs occur in one vessel, the discs of both rows are placed side by side, and never alternate, and the number of the rows of discs is never more than two.

In the Araucarias the groups of discs are arranged in single, double, triple and sometimes quadruple rows, see Pl. 56, Fig, 3. 5. They are much smaller than those in the true Pines, scarcely half their size, and in the double rows they always alternate with each other, and are sometimes circular, but mostly polygonal. Mr. Nicol has counted a row of not less than fifty discs in a length of the twentieth part of an inch, the diameter of each disc not exceeding the thousandth part of an inch; but even the smallest of these are of enormous size, when compared with the fibres of the partitions bounding the vessels in which they occur.