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Volume 19, 1886
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Art. XXXVI.Observations on the Glands in the Leaf and Stem of Myoporum lætum, Forster.

[Read before the Wellington Philosophical Society, 10th September, 1886.]

Plate XX.

Distribution of genus.—The genus Myoporum is widely distributed throughout Australia and the Pacific Islands. The species M. lœtum is common in the North and South Islands of New Zealand on the sea coasts as far south as Otago. It is also found in the Kermadec Islands (M'Gillivray), and Chatham Islands (W. Travers).*

Description of the leaves.—One character of the whole genus is the occurrence of pellucid glands in the leaves. The mature leaf is 2–4 inches long, lanceolate or obovate-lanceolate, acute or acuminate, serrulate above the middle, narrowed into petioles, bright-green and lucid.

Glands in leaves.—The following I extract from the “Journal of the Royal Microscopical Society,” October, 1884, page 769:—

“The various causes of transparent dots or lines in leaves are:—Secreting cells, round intercellular secreting spaces of either lysigenous or schizogenous origin, secreting passages, epidermal or parenchymatous cells with mucilaginous cell-walls, cells containing mucilage, raphides cells, cells with single crystals or clusters of crystals, cystoliths, spicular cells, branched sclerenchymatous bundles, groups of sclerenchymatous cells, depressed pits with or without hairs, crevices in the tissue, stomata. The secreting cells, spaces, or passages may contain resin, gum-resin, balsam, or an essential oil. Secreting cells are an extremely common cause of transparent dots, and are usually characteristic of whole families, or at least genera. Round intercellular secreting spaces may be lysigenous, as in Rutaceœ, or schizogenous as in Hypericineœ, the two kinds showing no difference in the mature condition. Both kinds are of great importance from a systematic point of view, furnishing distinguishing characters for entire families. Thus, lysigenous secreting spaces occur in the Rutaceœ, Myoporineœ and Leguminosœ; schizogenous are constant in the Hypericineœ, Myrcineœ, Sacmydeœ and Myrtaceœ.”

Also the following, from De Bary's “Comparative Anatomy of the Phanerogams and Ferns,” p. 202:—

“Resin, ethereal oil, emulsions of gum-resin of different quality, according to the special case, and often little known as regards chemical relations, occur:—

[Footnote] * Vide Hooker's “N.Z. Flora,” pp. 225 and 739.

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Myoporum LæTum.

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“(b.) In short cavities in the group Rutaceœ, in species of Hypericum, many species of Oxalis, Myrtaceœ, Myoporeœ.”

Also, p. 209:—

“(e.) Among the Myoporeœ, the species of Myoporum have numerous round oil-cavities of unequal size in the leaves and the outer cortex of the branches. The cavities are superficial and separated only by 1–2 layers of cells from the epidermis, which is arched convexly outwards; e.g., M. parvifolium. In M. tuberculatum, on the contrary, according to Unger, they occur in the middle of the chlorophyll parenchyma of the leaf. They are surrounded by 1–3 layers of flattened cells. As far as investigated, their origin appears to be lysigenetic.”

Observations.—I made observations on the leaf and stem of M. lœtum, through the autumn and winter, examining sections through fresh specimens and specimens preserved in alcohol, with the following results:—

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In a piece of mature leaf, 4/25ths of a square inch, there were 164 glands. This gives 1,025 to the square inch, so that about 1,000 to the square inch appears to be a fair average of their number. They are, however, much fewer towards the base of the leaf, near the insertion of the petiole on the stem, but are not altogether absent. In the lamina they occur both in the parenchymatous tissue and also in the midrib (see fig. 4), and are entirely surrounded by two or three layers of compressed colourless cells (see figs. 1 and 2).

The upper epidermis is often arched outwards over the glands (see fig. 1), but there are always at least two rows of cells between the epidermis and the compressed cells surrounding the gland, one of the rows being palisade cells. The cells of the palisade parenchyma above the gland are also colourless.

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

The glands vary in size from 1/32–1/16 inch in diameter in the lamina; in the midrib and stem they vary even more, some being found considerably smaller. The shape of the gland is spherical, and in the dried specimen which I examined the contents had all fallen away from the centre, forming a granular colourless coating round the inside of the gland, enclosing a vacuole. In the specimens preserved in alcohol, the contents had coagulated into little brownish-yellow masses, apparently of gum. (See fig. 2.)

In very young leaves there are no glands (see fig. 5), but they begin to appear when 10 or 12 leaves are formed nearer the apex (fig. 6). The rapid growth of the young leaves appears to retard the formation of the glands, but after their first appearance they are soon found in considerable numbers (fig. 6).

In the stem the glands appear both in the pith and cortical parenchyma (fig. 7). Their occurrence in the pith is not mentioned by Hooker or De Bary. They are scattered irregularly

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and vary in size throughout the secondary stems. In the younger wood of the autumn specimens I found them occurring very rarely for the space of two or three nodes, but after that they appeared both in pith and cortex, though few and irregular in size. In the older wood much larger glands are found, but these are also irregularly scattered, and usually among others considerably less in size (fig. 7). They appear to develope very late in the wood, and appear more frequently in the pith. The contents of the glands of the specimens which had been preserved in alcohol had, as in the case of the leaves, coagulated into little brownish-yellow masses, apparently of gum.

My observations on the development of these glands were imperfect, but they seem to point to a lysigenous origin, for the following reasons:—When first the glands appear in the young leaf, or stem, they appear as two or three colourless cells. These cells increase in size, and appear to divide repeatedly, until they form a mass of colourless cells of the size of the mature gland (see fig. 8). [This is easily distinguishable from a vein, as the latter consists of an external bundle-sheath enclosing a bundle of thick-walled cells, slightly pear-shaped with the pointed end upwards, the whole being surrounded by chlorophyll cells (see fig. 3)]. The central cells of the colourless mass seem now to become absorbed, leaving two or three rows of flattened cells (colourless) on the outside, surrounding a vacuole with gummy contents. In the glands of the stem I repeatedly noticed ragged cells and portions of cell-walls projecting into the vacuole of the gland, as if the interior cells had been partially absorbed but the absorption had not been completed. Whether this absorption were partial or almost complete, the surrounding colourless layers in the case of a full-sized gland always assumed a spherical outline.

Explanation of Plate XX.

Fig. 1. Transverse section through portion of mature leaf of Myoporum lœtum × 160: e, upper epidermis; p.p, palisade parenchyma; p, spongy parenchyma; g, gland.

Fig. 2. Transverse section through portion of mature leaf preserved in alcohol × 120. Letters as before. Gland shows gummy contents.

Fig. 3. Transverse section through portion of leaf showing vein (v) × 50. The red substance above the vein is some apparently gummy substance, which is very common in the leaves.

Fig. 4. Longitudinal section through midrib (slightly inclined), showing gland (g) × 166.

Fig. 5. Longitudinal section through young bud × 83, showing growing-point, g.p.

Fig. 6. Longitudinal section through young bud × 83, showing young glands, g.

Fig. 7. Transverse section through stem × 25; p, pith; f.v.b, fibro vascular bundles; c, cortex; g, glands.

Fig. 8. Transverse section through leaf × 83, showing young gland, g.