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Volume 50, 1918
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Art. XXV.—Notes on the Autecology of certain Plants of the Peridotite Belt, Nelson: Part I—Structure of some of the Plants (No. 1).

[Read before the Otago Institute, 9th October, 1917: received by Editors, 29th December, 1917; issued separately, 24th June, 1918.]

Introduction.

At a short distance from the city of Nelson there is an area known as the “Mineral Belt.” This is a zone of boulder-strewn land-surface, often dun-coloured in appearance, underlain by peridotite and serpentine rocks, which extends from D'Urville Island, in Cook Strait, south-west for a distance of sixty miles. It is an almost continuous band, but it disappears for about a mile between the valleys of the Lee and Serpentine Rivers. At its narrowest part the Mineral Belt is 100 yards wide, and it reaches its maximum width of 3 miles 50 chains in the vicinity of the Dun Mountain. The area occupied by the Mineral Belt is about 29 ½ square miles.*

[Footnote] * J. M. Bell, E. de C. Clarke, and P. Marshall, The Dun Mountain Subdivision, N.Z. Geol. Surv. Bull. No. 12, 1911.

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The vegetation of the Mineral Belt presents a striking contrast with that of the neighbouring land-surface, which is clothed with luxuriant forests of southern-beech (Nothofagus spp.). On the Mineral Belt there are three principal plant-associations:—

1.

Shrubland.—This is usually found near the margin of the Belt, and is composed of many species that are found in the adjacent forests, but on the Belt they are much dwarfed—e.g., Griselinia littoralis is usually a tree 10–16 metres high, but in the shrub formation on the Mineral Belt it is reduced to a woody shrub ⅓—2 metres high; Nothofagus fusca, a forest-tree, is represented by small trees 2–3 metres high. In addition to these dwarfed representatives of the neighbouring forests there are in this association a number of shrubs which are not reduced. Such plants are Cassinia Vauvilliersii var., Coprosma propinqua, Dracophyllum longifolium var., and Leptospermum scoparium var. In this association there are a number of small herbs—e.g., Claytonia australasica, Colobanthus quitensis, and Epilobium pedunculare var.

2.

Open Scrubland.—In this association the most characteristic plants are Cassinia Vauvilliersii var., Dracophyllum rosmarinifolium, Exocarpus Bidwillii Hymenanthera dentata var. alpina, Veronica buxifolia var., V. Menziesii var., V. pinguifolia (?), Pimelea Sutern, and Muehlenbeckia axillaris. Among the herbs to be found in this association are Myosotis Monroi, Notothlaspi australe, Gentiana corymbifera, Anisotome aromatica, and A. filifolium.

3.

Tussock Grassland.—The dominant plant is Danthonia Raoulii var.; sub-dominant are Phormium Cookianum and Astelia montana var.

It is proposed to describe the anatomy of a number of the plants of the Mineral Belt in a series of short papers, and then the results obtained from these investigations will be considered.

In addition to the anatomy of the leaf and of the stem of the different species, a brief description of the growth-form of the plant is given. In those cases where the usual form of the species is found on the Mineral Belt this description is quoted from Cheeseman's Manual of the New Zealand Flora (1906). Where the species is modified in form, a description of the usual type is quoted, and then that of the plant as it is found on the Mineral Belt is given.

1. Nothofagus fusca Oerst.

Usual Growth-form.—“A noble forest-tree 60–100 ft. high; trunk 4–8 ft. diam.; bark dark-brown or black in old plants, deeply furrowed, smooth and greyish-white on young trees; branchlets and petioles pubescent. Leaves evergreen, petiolate, ¾–1 ½ in. long, broadly ovate or ovate-oblong, obtuse or rarely acute, cuneate at the base, rather thin but firm, pubescent above and glandular beneath when young, glabrous when old, deeply and sharply serrate, veins conspicuous; stipules linear-oblong, caducous.”

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Mineral Belt Growth-form.—A small tree 6–8 ft. high, with leaves ½–¾ in. long.

Anatomy.

Leaf.—The upper epidermis consists of small cells, more or less oval in transverse section. The cell-walls are thin, except the external walls, which are slightly thickened and also cuticularized. Some of the epidermal cells, in the vicinity of the vascular bundles, are produced into long unicellular hairs which have thin, slightly cutinized walls. There are no stomates on the upper surface.

The lower epidermal cells are small, oval, and thin-walled, the external walls being slightly thickened. A thin cuticle is present. Stomates are

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confined to the lower surface; the guard-cells are small and on the same level as the other epidermal cells, the stoma being protected by guard-cell ridges. On the lower surface there are hydathodes, which are sunk in slight depressions.

The chlorenchyma is differentiated into palisade and spongy tissue. The former consists of 3 rows of thin-walled cells, the outer layer with the cells very closely arranged so that there are no intercellular spaces; the 2 inner layers are arranged more loosely. The spongy tissue consists of small thin-walled irregular cells which have rather small air-spaces between them. Many of the chlorenchymatous cells contain tannin.

The midrib is slightly prominent. Surrounding the vascular bundle of the matous there is a sheath 1–3 cells thick, consisting of small sclerenchy-matous cells. Around this there is a sheath of larger cells, also with lignified walls. The xylem consists of vessels of moderately large diameter and of wood-fibres. Above the xylem there is a small amount of parenchyma. The phloem is in the form of a crescent; the parenchymatous elements contain tannin.

Stem.—The cork is a fairly wide band of tissue, consisting of small, very compact cells.

The cortical cells are large, and oval in transverse section. These cells are thick-walled, and many of them contain tannin. They are closely arranged, so that there are only small intercellular air-spaces.

The pericycle fibres form a wide band, in which the cells vary considerably in size in transverse section. Some are small, with their cell-walls so much lignified and thickened that the lumen is almost obliterated; connecting groups of these cells are much larger cells, also with thickened, lignified walls, but the cell-cavities are large.

The phloem forms a narrow band, and the parenchyma contains tannin. The spring wood consists of a large number of vessels of large diameter, together with wood-fibres. The autumn wood is formed of much smaller vessels, and of wood-fibres in which the lumen is almost obliterated.

The medullary rays are uniseriate, and the cells have thickened lignified walls, and contain tannin. The pith cells are large and round, have thickened lignified walls, and contain abundant starch.

2. Nothofagus cliffortioides Oerst.

Usual Growth-form.—“A small tree, usually from 20 ft. to 40 ft. high, rarely more, with a trunk 1–2 ft. diam., in alpine localities often dwarfed inter a much-branched bush 5–12 ft. high. Branches spreading, often distichous, especially in young trees; branchlets densely pubescent. Leaves shortly petiolate, distichous, ⅙–⅔ in. long, ovate-oblong or ovate or ovateorbicular, acute or subacute, rarely obtuse, always broadest at the unequally rounded or almost cordate base, quite entire, very coriaceous, glabrous and reticulated above, more or less clothed with greyish-white appressed hairs beneath, margins thickened, often recurved; stipules membranous, caducous.”

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Mineral Belt Growth-form.—A much-branched bush 4–8 ft. high, with leaves ⅙–¼ in. long.

Anatomy.

Leaf.— The upper epidermis consists of small cells which are more or less square in transverse section. These have their cell-walls thickened, and there is a thick cuticle. Some of the epidermal cells contain tannin. On the upper surface there are numerous glands which are formed from epidermal cells.

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The palisade and the spongy parenchyma, the lower epidermis, and the stomates are the same as in N. fusca, but on the lower surface some of the epidermal cells are produced into unicellular hairs, which have thin non-cutinized walls. There is a thick cuticle on the lower surface. Many of the mesophyll cells and the cells of both the lower and the upper epidermis contain tannin.

The vascular bundles are the same as in N. fusca, but the midrib is smaller.

Stem.—The structure is essentially the same as in N fusca, the only differences being—(1) There are more pericycle fibres; (2) the phloem forms a wider band; (3) the pith cells do not contain starch; (4) there are more numerous vessels of large diameter.

3. Exocarpus Bidwillii Hook. f.

Growth-form.— “A small much branched rigid procumbent shrub 6–24 in. high, branches ascending, short, stiff, terete, deeply furrowed. Leaves reduced to minute triangular scales, alternate, persistent,”

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Fig. 1.—Exocarputs Bidwifai, Portion of plant (½ natural size). aruit; b, loaves reduced to triangular scales.

A portion of the plant is shown in fig. 21, which also shows, the fruit, which is seated on a thin kenlargede thickened red and succulent peduncle. The perianth-segments are persistent under the fruit.

Anatomy.

Stem (figs. 24).—The structure of the stem is shown roughly in fig. 2. From this it will be seen that the furrow stiff hairs, that there is a thick cucculte & c. The more detailed slructure of the stem is shown in fig. 4.

The epidermis consists of small squarish cells with thin cell-walls and an extremely thick cuticle. In the furrows the epidermal cells are larger and there is only a thin cuticle. Many of the epidermal cells in the furrows are produced into stiff hairs, which have thick walls which are cuticularized. In the furrpws are the stomates, but these cannot be seen well in transverse section. as their long axes are placed transversely to the surface of the” stem.

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Fig. 3 gives the epidermis from a longitudinal section; from this it will be seen that the stomates are at the same level as the epidermal cells, and the opening is protected by guard-cell ridges.

The cortex is composed of closely packed more or less polygonal cells with thin walls. In the outer part of the cortex, and especially in the

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Fig. 2.—Excarpus Bidwhi. Diagrammatic transverse section of the stem (x 24). a, thick cuticle: b, furrow lined with hairs; c, pericycle fibres.
Fig. 3.—Exocarpus Bidwillii. Longitudinal section through epidermis (X 350). a, guard-cell ridge.
Fig. 4.—Exocarpus Bidwillii. Transverse section of stem (x 120). a, thick cuticle; b, tannin-containing cells; c, chlorenchyma; d, pericycle fibres; er phloem vessels of xyiem; g, hgnified pith.

ridges, the cortical cells containtannin; in the inner part of the cortex most of the cells contain numerous chloroplasts, but some contain tannin. At intervals there are large groups of pericycle fibres, composed of small cells with very thick lignified was and small lumen.

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The phloem forms a wide band in which the elements are very regularly arranged. Most of the parenchymatous cells of the phloem contain tannin. The xylem consists chiefly of wood-fibres of small diameter; these have very thick walls and small lumen. The number of vessels is small in comparison with the amount of wood, and they are not of wide diameter. The medullary rays are numerous and uniseriate; the cells have lignified walls, and contain tannin. The pith is solid, and consists of polygonal or roundish cells with pitted lignified walls.

4. Muehlenbeckia axillaris Walp.

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Growth-form” A small much-branched prostrate or diffuse shrubby plant, usually forming densely matted patches 3–12 in. diam., but sometimes open and straggling; stems and branches woody; branchlets puberulous. Leaves on rather long petioles, small, 1/10–⅓ in. long, broadly oblong or ovate-oblong or almost orbicular, obtuse or retuse, rounded at the base, flat, quite glabrous, dotted beneath.”

Anatomy.

Leaf.—The upper epidermis consists of large cells with thin walls, except the external ones, which are slightly thicker. There is a very thin cuticle. The epidermal cells form mucilage-sacs. The lower epidermis is similar to the upper. A few stomates are found on the upper surface, but they are much more numerous on the lower surface; the guard-cells are small and are level with the other epidermal cells. On both surfaces of the leaf there are hydathodes, which are sunk in small depressions; they are more numerous on the lower than on the upper surface.

The chlorenchyma is differentiated into palisade and spongy tissue. The palisade tissue is found on both surfaces of the leaf; there are 3–4 layers of cells on each side. The cells are small, their walls are thin, and they contain numerous small chloroplasts. The outer layers contain tannin. The cells are closely packed, so that there are only very small intercellular air-spaces. The spongy tissue consists of fairly large cells with thin walls and containing numerous small chloroplasts. The air-spaces in this tissue are larger, and some of the cells contain tannin.

The vascular bundles are frequent, but of small size. Above the xylem there is some stereome, and above this small-celled parenchyma. There is also small-celled parenchyma below the phloem. Each vascular bundle is surrounded by a sheath of large parenchymatous cells, which contain tannin.

Stem.—The cork forms a fairly wide zone of very small compact cells.

The cortex consists of oval cells which are closely packed together, so that there are only very minute intercellular air-spaces. Most of the cells contain tannin.

The pericycle fibres form a narrow, more or less continuous baud 1—2 cells wide. The cells are small, and have thick walls and small lumen. The phloem forms a wide band, in which the parenchymatous cells contain tannin. The xylem consists almost entirely of wood-fibres, but there are a few vessels of large diameter.

The medullary rays are multiseriate and are very wide. They consist of small cells with thickened lignified walls, and they contain abundant large starch-grains.

The pith consists of rounded or polygonal cells with thick lignified walls. They are closely packed together, and are full of large more or less polygonal starch-grains, and some contain tannin.

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5. Claytonia australasica Hook. f.

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Usual Growth-form.—“A perfectly glabrous tender and succulent usually matted plant, with slender creeping stems 1–6 in. long. Leaves very variable in size, ¼–1 ½ in. long, alternate or in distant pairs, narrow-linear or linear-spathulate, obtuse, dilated into broad membranous sheaths at the base.”

Mineral Belt Growth-form.–In the Mineral Belt plants the leaves are ¼—-⅓ in. long.

Anatomy.

Leaf (figs. 5 and 6).—The upper and the lower epidermis are similar; the epidermal cells are large and have thickened walls. A very thin cuticle is present. Stomates are confined to the upper surface of the leaf.

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Fig. 5.—Claytonia australasica. Diagrammatic view of leaf (X 40). a, palisade tissue; b, aqueous tissue.
Fig. 6.—Claytoma australasica. Transverse section of leaf, passing through midrib (X 160). a, guard-cell ridge; b, palisade tissue; c, xylem; d, phloem; e, aqueous tissue.

The guard-cells are at the same level as the other epidermal cells, and the opening is protected by guard-cell ridges. The cells of the epidermis contain a few small chloroplasts.

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Fig. 7.—Claytonia australasica. Transverse section of prostrate stem (X 210). a, guard-cell ridge cuticle; b, chlorenchyma; c, starch-grains; d, endodermis; e, pericycle; f, phloem; g, xylem.
Fig. 8.—Claytonia australasica. Transverse section of the outer part of setem; showing a stoma (x 210). a, guard-cell ridge.

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The chlorenchyma is differentiated. The palisade tissue consists of about 5 layers of somewhat irregular cells. These cells are very large and are compactly arranged, so that there are only small intercellular air-spaces. The cells contain a large number of small chloroplasts.

The spongy tissue is composed of very large thin-walled more or less rounded cells, with a thin peripheral layer of protoplasm in which are embedded the small chloroplasts. This tissue is for water-storage. There are frequent air-spaces in it, but they are small compared with the size of the cell.

The vascular bundles are small, and are surrounded by a sheath of parenchymatous cells which have thin walls and which contain a few chloroplasts. The xylem and the phloem are of the usual type. The amount of lignified tissue is small.

Stem (figs. 78).—The epidermis consists of small somewhat domeshaped cells which have very thick walls and which contain a few small chloroplasts. A ridged cuticle is present. The stomates are not numerous; the guard-cells are smaller than the other epidermal cells, and the opening is protected by guard-cell ridges (fig. 8).

Below the epidermis there is a single layer of large cells which contain numerous chloroplasts and which have thickened cell-walls. The rest of the cortex is a very wide zone consisting of large round cells with very thick walls and with air-spaces between them. These air-spaces are small compared with the size of the cells. These cells contain a large number of starch-grains, which are heaped at the base of the cell.

There is a well-marked endodermis, consisting of cells with thin suberized walls. The pericycle also is well marked; it is composed of 2 layers of smaller cells with thickened cell-walls.

The xylem is composed of wood vessels which form a more or less continuous cylinder, with a few small parenchymatous pith cells in the middle. The phloem forms a continuous band round the xylem and contains a fair amount of parenchyma.

6. Colobanthus quitensis Bartl.

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Growth-form.—“A small densely tufted much-branched plant 1–2 in. high, forming rather soft rounded patches. Leaves variable in size, lower sometimes over ½ in. long, upper often very small, ⅛–¼ in., narrow-linear or linear-subulate, acute or mucronate but not acicular at the tip, connate at the base, flat or concave above, convex beneath; texture soft.”

Anatomy.

Leaf (fig. 9).—Fig. 9 shows a transverse section of half the leaf. The upper epidermis consists of cells which in transverse section are fairly large and oval. The lateral and the internal walls are thin, but the outer walls are very much thickened. There is no cuticle. The lower epidermis is similar to the upper, except that the cells are slightly smaller and their external walls are not so thick. Stomates are found on both surfaces, but they are more numerous on the upper. The guard-cells are small, have thickened walls, and are raised above the other epidermal cells, but are below their thickened external walls.

The chlorenchyma is differentiated. The palisade tissue consists of about 3 layers of cells, containing numerous small chloroplasts. The cells are more or less oval in transverse section, and have thin walls; the intercellular air-spaces are small.

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The spongy tissue consists of fairly large roundish cells which have very thin walls and which contain only a few small chloroplasts. There are fairsized air-spaces between the cells of the spongy tissue, which forms a water-storage tissue. A few of the cells of the chlorenchyma contain crystal aggregates of calcium oxalate.

The vascular bundles are small, and have only a small amount of lignified tissue. Each bundle is surrounded by a sheath of parenchymatous cells, which contain a few small chloroplasts.

Stem (fig. 10).—The epidermis is composed of more or less rounded cells with thick cell-walls. A ridged cuticle is also present. Inside this layer there is a zone of tissue 5 or 6 cells deep; these, cells are empty, have thickened walls, and near the outside are regular in shape, but towards the inside are more irregular. The innermost cells of this tissue are suberized.

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Fig. 9.—Colobanthus quitensis. Transverse section of half of leaf (X 125). a, thick cuticle; b, small raised stcoma; c, aqueous tissue.
Fig. 10.—Colobanthus quitensis. Transverse section of stem (X 125). a, epidermis; b, dead cortex; c, suberized tissue; d, thick-walled cortex; e, phloem; f, xylem.

Next comes a band of small suberized cells. The 2 innermost layers of these cork cells are practically square, but the remainder of this band is composed of the usual flattened cork cells. All the cells of this tissue have thin walls.

The cortex consists of small cells with very thick mucilaginous cell-walls and small cavities.

The phloem forms a continuous wide band around the xylem; the cellwalls are thickened and mucilaginous. The xylem consists of vessels of fairly large diameter, together with a large amount of xylem parenchyma.

The pith is small and solid, and consists of rounded or polygonal cells which are closely packed together, so that there are only minute air-spaces between the cells.

7. Clematis Colensoi Hook. f. var. rutaefolia Hook. f. (?).

Growth-form.— A woody liane with slender flexuous branches. The stems and branches are glabrous, but silky at the tips. The leaves are biternate, with the secondary leaflets stalked. The leaves are slightly coriaceous.

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Anatomy.

Leaf.—The cells of both the upper and the lower epidermis are large, and oval in transverse section; the walls are slightly thickened, the external ones more so. A thin cuticle is found on both surfaces. Stomates are confined to the lower surface. The guard-cells are large and thick-walled, and are at the same level as the other epidermal cells.

The chlorenchyma is differentiated. The palisade tissue consists of about 3 layers of large irregular cells, which are somewhat loosely arranged, so that there are moderately large air-spaces between the cells. The spongy tissue consists of very irregular cells, which are loosely arranged. Both palisade and spongy tissues contain numerous fairly large chloroplasts.

The vascular bundles are small, and each is surrounded by a sheath of small thin-walled parenchymatous cells which contain a very few chloro-plasts. Associated with both ploem and xylem is small-celled parenchyma. The amount of lignified tissue is small.

Stem.—: The epidermis is composed of somewhat squarish cells with thickened cell-walls. A moderately thick cuticle is present. Some of the epidermal cells contain a few chloroplasts. Stomates are frequent, the guard-cells, which have thickened walls, being at the same level as the other epidermal cells.

The cortex consists of roundish or irregular cells. This tissue can be divided into two regions—an outer one containing numerous chloroplasts, and an inner colourless region. The cells of the chlorenchyma are rounded or irregular, and their walls are slightly thickened. The colourless cortex consists of somewhat squarish cells regularly arranged.

There are 6 vascular bundles, with a mass of pericycle fibres above the phloem.

The pith is solid, and consists of thin-walled parenchymatous cells. The medullary rays are wide, and are composed of large round or polygonal cells with thickened lignified walls.

8. Notothlaspi australe Hook. f.

Growth-form.—A small densely tufted alpine herb, 2–4 in. in height. It is “usually much branched from the base; branches leafy, spreading, 1–4 in. long. Leaves radical and cauline, numerous, ½-1½ in. long, petiolate, linear- or oblong-spathulate, entire or crenate, glabrous or with a few cellular hairs.”

Anatomy.

Leaf (figs. 1112).—The cells of both the upper and the lower epidermis are large, and oval in transverse section, and have their external walls thickened. There is a thin cuticle on both surfaces. Stomates are very numerous on both surfaces; on the upper surface of the leaf they are sightly sunken, but not on the lower. The guard-cells are small and have their walls thickened. On the younger leaves there are some hairs; on the older leaves there are a few on the lower part of the blade. The hairs are large, thin-walled, and slightly cutinized at their base. They contain protoplasm, and are probably water-absorbing hairs. They are shown in fig. 12.

The chlorenchyma is differentiated. The palisade tissue consists of 4 ayers of cells, the layer adjacent to the epidermis being composed of roundish cells and the other 3 layers of large elongated cells. Chloroplasts are numerous but small. There are small intercellular air-spaces between the cells. The cell-walls are thin.

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The spongy issue consists of large thin-walled roundish cells which contain only a few very small chloroplasts, and which form a water-storage tissue.

The vascular bundles are small, and each is surrounded by a sheath of small parenchymatous cells. There are very few vessels in the xylem, and these are of small diameter.

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Fig. 11.—Notothlaspi australe. Transverse section of leaf (x 100).
Fig. 12.—Notothlaspi australe. Upper epidermis of leaf (x 100).

Stem (figs. 1315).—The epidermis is composed of large cells, which have thin lateral and internal walls, but the external walls are slightly thickened. There is a thin cuticle. Some of the epidermal cells are considerably larger, and form special water-storage cells.

Beneath the epidermis there is a single layer of rounded cells, which contain numerous chloroplasts. The remainder of the cortex, which is a very wide zone, consists of very large round cells with thin cell-walls. All the cortical cells except the outermost layer form an aqueous tissue. The intercellular air-spaces are small.

The phloem and the xylem form continuous cylinders. The amount of lignified tissue in the xylem is small, and there is a moderately large amount of parenchyma in both the xylem and the phloem.

The pith is solid, and consists of large rounded thin-walled cells, which form an aqueous tissue.

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Fig. 13 gives a schematic view of a transverse section. There are 3–5 furrows in the stem, and beneath these furrows there are zones of smaller-celled tissue in which the cells are closely packed together and contain small chloroplasts, especially at the corners (see fig. 14).

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Fig. 13.—Notothlaspi australe. Diagrammatic view of a transverse section of the stem (x 20). a, phloem; b, xylem; c, small-celled tissue beneath furrows.
Fig. 14.—Notothlaspi australe. Transverse section of cortex below furrow (x 90).
Fig. 15.—Notothlaspi australe. Transverse section of stem (x 90). a, chlorenchyma; b, aqueous tissue; c, phloem; d, xylem.

9. Weinmannia racemosa Linn. f.

Usual Growth-form.—“A tree 50–80 ft. high or more, with a trunk 1–4 ft. diam.; glabrous when mature, except the raceme, which is pubescent. Leaves of young plants pinnately 3–5-foliolate, thin, and memebranous, often pubescent; of mature plants 1-foliolate, 1–4 in. long, oblong-lanceolate or oblong-ovate to orbicular-ovate, obtuse or subacute, coarsely and obtusely serrate, very coriaceous, quite glabrous.”

Mineral Belt Growth-form.—A shrub 4–8 ft. high.

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Anatomy.

Leaf.—Both the upper and the lower epidermis consist of small cells which in transverse section are oblong. The cell-walls are thin, but the lateral and external walls are cutinized. The cuticle is smooth and fairly thick. Stomates are confined to the lower surface. The guard-cells are at the same level as the other epidermal cells, and the stoma is protected by guard-cell ridges.

Below the upper epidermis there is a hypoderma composed of 2 rows of large cells with thick walls. These cells contain tannin. Above the lower epidermis there are 1–2 broken layers of hypodermal cells; these in transverse section are more or less rounded, have thick walls, and contain tannin.

The chlorenchyma is differentiated. The palisade tissue consists of 3 rows of cells with thin walls and numerous chloroplasts. The two outer layers are very compact, but the third layer has fairly large air-spaces between the cells. The spongy tissue consists of irregular thin-walled cells with abundant chloroplasts. These cells are loosely arranged, so that there are large intercellular air-spaces.

The midrib is prominent. Above and below it the epidermal cells are smaller and the hypoderma is thickened. On the lower surface there is collenchyma. Below the hypoderma there are a few small cells, which contain chloroplasts. The vascular bundle forms an irregular cylinder, in the centre of which there are round cells with lignified walls. The xylem consists of vessels of small diameter and of wood-fibres. The phloem is a narrow zone formed of small cells. Outside the phloem there is a narrow band of small sclerenchymatous cells with thickened walls.

Stem.—The cork is a wide band of tissue in which the cells are large and irregular. The phellogen is well marked.

The cortex consists of oval cells with, thick walls. These cells contain abundant starch-grains, and are compactly arranged, so that there are only small intercellular air-spaces.

The pericycle fibres form a continuous band around the phloem. These fibres vary considerably in diameter, and have their walls very much thickened, so that the lumen is small.

The phloem forms a fairly wide band with numerous uniseiate medullary rays which contain tannin passing through it. The xylem is well developed, and is composed of vessels of fairly large diameter and of wood-fibres with thick walls and small cell-cavities.

The medullary rays are numerous and are uniseriate, and have thickened lignified walls, except in the phloem, where the walls are not lignified. The pith is solid and consists of roundish cells with thickened lignified walls. The cells contain abundant starch-grains and are closely packed together, so that there are only very small intercellular air-spaces.