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Volume 58, 1928
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Some Crevice Plants from the Lava Field at Mt. Wellington.

By Joyce H. Wilson, M. Sc.

[Read before the Auckland Institute, 9th November, 1926; received by Editor, 31st December, 1926; issued seprately, 8th November, 1927.]

Plates 32, 33.
Introduction.

Aspect.—Although the lava seams run in various directions the majority are in a line roughly from east to west. Trees grow to a great size and appear altogether more luxuriant on the northern side, which is directly exposed to light and warmth, while those on the southern side are dwarfed and straggling. On the southern side crevice plants attain their greatest development, Astelias of various species growing in every available position, while Peperomia and ferns occupy the smaller crevices and the deep fissures.

Climate.—The summer is hot with a fairly low rainfall, while in the winter the rainfall is higher. There is no very great range of temperature. The prevailing westerly and north westerly winds blow from the sea. which is only a few miles away.

Soil.—This is formed by the weathering of the rock and the production of humus from decaying plants, especially astelias. It lodges in the crevices and in sheltered positions on flat rocks, where it is often held by the matted roots of the ferns (Cheilanthes Seiberi and Cyclophorus serpens. Owing to its fine texture it is well adapted for holding moisture.

Moisture.—The relative humidity of the air in the crevices is greater than that outside. Experiments were made under various conditions at the end of winter and in spring, using a wet and dry hulb hygrometer.

(a) Dull day with cold wind—

In open. In crevice.
Dry bulb 11° C. 10° C.
Wet bulb 9° c. 9° C.
Relative humidity .63 .87

(b) Calm day with bright sunshine—

In open. In crevice.
Dry bulb 17° C. 13° C.
Wet bulb 14° C. 12° C.
Relative humidity .69 .869
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1. General Remarks on the Plants of the Lava Field.

The plants found are very varied in character, a list being given below.

  • Ranunculaceae

  •   Clematis indivisa

  • Cruciferae

  •   Nasturtium

  • Violaceae

  •   Melicytus ramiflorus

  • Geraniaceae

  •   Geranium molle

  • Rhamneae

  •   Pomaderris phylicaefolia

  • Sapindaceae

  •   Alectryon excelsum

  • Leguminosae

  •   Ulex europaeus

  • Rosaceae

  •   Rubus australis

  • Myrtaceae

  •   Leptospermum scoparium

  •   Metrosideros hypericifolia

  • Onagrariae

  •   Epilobium Billardierianum

  • Umbelliferae

  •   Daucus brachiatus

  • Cornaceae

  •   Griselinia lucida

  • Utricaceae

  •   Parietaria debilis

  • Rubiaceae

  •   Coprosma robusta

  •   Galium umbrosum

  • Campanulaceae

  •   Wahlenbergia gracilis

  • Myrsineae

  •   Suttonia australis

  • Liganiaceae

  •   Geniostoma ligustrifolium

  • Convolvulaceae

  •   Calystegia tuguriorum

  • Solanaceae

  •   Solanum nigrum

  •   Physalis peruviana

  • Scrophularineae

  •   Veronica salicifolia

  • Myoporineae

  •   Myoporum laetum

  • Chenopodiaceae

  •   Chenopodium pusillum

  • Polygonaceae

  •   Muehlenbeckia complexa

  • Piperaceae

  •   Peperomia Urvilliana

  •   Macropiper excelsum

  • Liliaceae

  •   Cordyline australis

  •   Astelia Solandri

  •   Astelia Cunninghami

  •   Phormium tenax

  • Orchidaceae

  •   Thelymitra longifolia

  • Filices

  •   Asplenium flabellifolium

  •   Asplenium lucidum

  •   Asplenium adiantoides

  •   Polypodium Billardieri

  •   Cyclophorus serpens

  •   Pellaea rotundifolia

  •   Pellaea falcata

  •   Cheilanthes Sieberi

  •   Pteris aquilina

Astelia Solandri is by far the most abundant species, and possesses remarkable structural peculiarities which explain its success. Next in point of numbers comes Peperomia Urvilliana, which reaches its greatest luxuriance right in the crevices, sometimes two feet or more from the opening. The ferns Polypodium Billardieri, Pellaea rotundifolia, and Asplenium flabellifolium are also abundant, and grow best as absolute crevice plants, though they are found near and at the surface as well.

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Fig. 1 - Polypodium Billardieri. Fig. 2 - Leaf. T. S., showing gland at a.

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Fig. 3 - Astelia Solandri.

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2. Plant Structure.

A. Asplenium flabellifolium.

General description.—This species of Asplenium seems at first sight to be most unsuited for life among rocks. Small stunted plants are found in the shallowest crevices, but it grows best in more shel-tered positions and reaches its greatest size in narrow crevices where the light is feeble. Its ability to withstand the conditions of drought which must often prevail is explained after an examination of the roots and rhizome. In plants near the surface the rhizome is exremely short and stout, and from it the fine fibrous roots are given off in dense masses, forming a close mat in which particles of soil are held.

Leaf.—There is a thick cuticle beneath which are found the epider-mal cells, which are large and often irregular. There is no distinction of palisade and spongy mesophyll, and small intercellular spaces are seen all through. The placenta is a cushion of cells below which is a group of tracheids turned almost at right angles to the vein. The cells of the lower epidermis are smaller and more regular than those of the upper.

The reduction of stomata and the thickening of the cuticle are evidence of xerophytic modification.

B. Polypodium Billardieri.

General description.—Polypodium Billardieri (Fig. 1) occurs as an absolute crevice plant growing most luxuriantly in positions where it obtains as much shade and moisture as possible. It also appears growing on rock surfaces with the roots penetrating into any crevices too small for the whole plant. The creeping rhizome is stout and presents a mottled appearance through the growth of membranous brown scales closely pressed to the green surface. Young branches, leaf bases, and the growing point of the rhizome, are all thickly cov-ered with brown scales. From the somewhat flattened under-surface of the rhizome short fine fibrous roots are given off almost continu-ously except towards the growing point.

The leaves vary greatly in form, from the simple coriaceous leaf with short stipes found on exposed rock faces, to the large, deeply-lobed fronds with stipes sometimes twelve to eighteen inches long, found in the dampest, and most sheltered spots, the size and texture showing a direct dependence on the amount of moisture available. All leaves are alike, however, in one respect. The veins branch and ana-stomose, forming a network in the meshes of which at the ends of short branch veins are numerous white dots. These are minute scales apparently organic in nature, which cover and protect absorptive glands.

Structure.—Rhizome.—The most striking feature about the struc-ture of the rhizome is its adaptation for the storage of water and other reserve materials. A good deal of starch is found in the form

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of grains embedded in the protoplasm lining the cell-wall, especially in the cells of the outer cortex and the epidermis. Protein is also found as clusters of transparent granules, particularly in the sieve-tubes of the phloem.

There is a distinct cuticle beneath which are the epidermal cells, lengthened radially. From the epidermis project large multicellular scales, the thickened cell-walls of which are brown in colour.

Leaf (Fig. 2).—The epidermis consists of regular brick-shaped cells, longer tangentially than radially. The cuticle is thin, and although there are few stomata on the upper surface they are fairly numerous on the lower, and are of the usual type. There is no dis-tinction of palisade and spongy parenchyma, the mesophyll being made up of rounded cells with scattered chlorophyll grains. About midway between upper and lower epidermis are the vascular bundles, each surrounded by sclerenchyma. The most interesting feature of the leaf is the glands found on the upper surface. These agree very closely in the structural details with those seen in the leaves of one of the Saxifrages growing in positions where the supply of moisture is very small. Each gland is covered by white scales consisting of organic material, probably dead epidermal cells. It is composed of a group of cells forming a roughly hemispherical mass, the surface of which is a little below the surrounding surface of the leaf. Close to the lower part of the gland is the ending of a small vascular bundle. The gland is made up of cells of two kinds—short broad scalariform tracheids, and parenchyma cells with abundant cell contents, yellow-ish in colour. The epidermal cells are also densely packed with brown and yellow granules which suggests that they may consist of hygroscopic material.

C. Pellaea rotundifolia and Pellaea falcata.

General description.—The two species of Pellaea show very little difference in point of structure, but differ markedly in the situation in which they grow. Pellaea rotundifolia is an extremely common form widely distributed, while Pellaea falcata is rare, occurring on this lava field only in one spot, though it is very plentiful in that one place. The former grows near the surface in small cracks or among small stones, and the size of the plant varies greatly according to its position. Those plants with the smallest leaves are found growing right at the surface of the rock and the size increases with decreasing illumination until a maximum is reached at a depth of about one foot. Pellaea falcata seems to prefer a moister and more sheltered situa-tion, and reaches its maximum size growing among grasses where the mixed forest association has become established, thus forming a strik-ing contrast with the more hardy species which grows in isolation. Pellaea plants grow in all the intermediate positions between the bare stones and grass-covered earth, but cannot definitely be assigned to either species, forming an interesting series of transition forms between the two.

The only respects in which Pellaea falcata differs are the larger size of the leaves and the separate leaflets and the shape of the latter.

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The leaflets of the transition forms gradually increase in size, at the same time becoming longer and narrower and taking up an oblique position with regard to the rachis. In the extreme form described as Pellaea falcata the leaf is perhaps eighteen inches long and two inches broad, and the leaflets, which are almost sessile as in all the forms, are curved towards the tip.

Leaf.—The structure of the leaf presents several unexpected features, considering that the stem exhibits xerophytic characters. The first unusual feature is the presence of a large number of sto-mata in the lower epidermis, there being a hundred and forty-seven to the square millimetre. The stomata are of the usual type, having two guard cells which contain chlorophyll grains. They open into large intercellular spaces surrounded by the irregularly-shaped, almost stellate cells of the spongy mesophyll, a structure suggesting a water plant rather than one growing on rock. Where a vascular bundle is cut through, it is seen to consist of afew tracheids with parenchyma cells and phloem towards the lower surface. The pali-sade tissue is distinguished from the spongy mesophyll by having smaller intercellular speces and being composed of cells more regular in shape and more densely packed with chlorophyll. Most of the cells are broader than they are long, and are somewhat rounded. The cells of the epidermis are fairly regular and the cuticle is thin.

D. Astelia Solandri.

General description.—This Astelia forms a most characteristic feature of the vegetation of the lava fields, many parts of which it occupies almost exclusively. Astelia Solandri (Fig. 3) is a perennial, and indications point to its living for many years. The plant forms a tuft emerging from the erevice. The living leaves extend to about four or five inches back from the growing point and are arranged in a close spiral, the youngest ones being folded together to protect the growing point. The leaves are ensiform, about thirty inches long and an inch and a-half wide, and expand into bases about four inches wide and almost black in colour. Dense masses of silky white haris clothe the leaf bases where they join the stem. Their function would seem to be to help to check the evaporation of the water for which the leaf bases form a reservoir. Even in dry weather there is usually a con-siderable quantity of water to be found. Below the living leaves are the dry ones of the preceding years, the damp bases of which are in various stages of decomposition. An obvious adaptation to life as a crevice plant which is entirely lacking in epiphytic members of the species is the production of a woody stem in which secondary thick-ening is developed. The living part of the stem is perhaps eight inches long, but the decaying tissues retain their form for some dis-tance below. The plant does not project far above the surface of the rock, the decaying stem and leaves being apparently thrust deeper and deeper into the crevice. In the case of one specimen from a par-ticularly deep crevice the length from the growing tip of the stem to the end was very nearly two feet. For about half this distance the woody stem could plainly be seen while the rest consisted of a mass

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of humus bound together by the roots and leaf-fibres, and still pre-serving the original shape. This example shows the important part played by these Astelias in the formation of soil and the gradual fill-ing up of the crevices. As would be expected from the mode of growth of the stem, the roots are adventitious, a fresh root system being produced each year. In plants examined in the spring root tips were forcing their way through the leaf bases only an inch and a-half, or even less, from the growing point of the stem. The roots turn in all directions, some entering the cracks in the rock and form-ing a most efficient anchoring system, others hanging freely in the air, and a third set turning upwards and applying themselves closely to the leaf bases to absorb the water and dissolved salts held there.

Structure.—Root (Fig. 4.)—The cortex readily breaks away from the vascular cylinder at the thick-walled endodermis, so that the inner and outer parts of the root frequently appear separately in section.

The centre of the vascular cylinder consists of small rounded cells, surrounding which is a wide region of much thickened cells with lumina which become smaller towards the pericycle. At the outer edge of the ground tissue is a ring of numerous vascular bundles, con-sisting of xylem groups alternating with phloem. The xylem consists of large scalariform vessls with smaller celled protoxylem towards the outside. The protoxylem is made up of spiral tracheids. Between the xylem groups are the small thin-walled cells packed closely toge-ther, which form the phloem group. The whole vascular cylinder is bounded by the pericycle consisting of small square cells with extremely thick walls, which in longitudinal section are seen to be pitted. Outside the pericycle is the endodermis, made up of larger cells, also with thick walls.

The outer cortex may be divided into two regions. In the inte-rior are rounded living cells, many of which have dark brown cell contents, probably tannin compounds, while outside these is a zone of dead cells, most of which have thin walls though a few are very much thickened and show pits on their walls. The function of these dead cells, like that of the velamen of the epiphytic orchids, is to absorb and hold any moisture with which they come in contact. Experiment shows that they absorb the moisture with great rapidity. Towards the outside of the root is a layer of large rectangular cells and outside these are two or three rows of extremely irregular cells, many of which are prolonged into root hairs. These persist even in the older parts of the root and help to check the evaporation of any moisture absorbed. Twisted around many of the hairs are very fine fungal hyphae, which do not, however, penetrate the cells of the root.

Stem.—An older stem shows many differences from a younger, the chief being the appearance of secondary meristem (Fig. 5) which increases in amount with increasing distance from the growing point. Unlike the secondary meristem of Cordyline, which produces both ground tissue and more vascular bundles, that of Astelia Solandri pro-duces only ground tissue. This is readily explainable, since, as the func-tional part of the stem is only a few inches long, and a new root system is produced each year near the apex, extra conducting tissue

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Fig. 4. —Root. T.S. (× 80).
Fig. 5. —Cortex of Astelia Solandri stem, showing meristem at a. Drawn from a Photomicrograph.
Fig. 6. —Water-absorbing Organ of Leaf. T.S. (× 400).

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is not needed. The meristematic layer first appears just inside the epidermis as a row of large irregular cells which divide either tan-gentially or radially. They do not remain active for long, but before division ceases they cut off a new meristem on the outside. This new cambium produces cortical cells on its inner side so that it is soon separated from that first formed by several layers of cells, some of which have thickened walls in which are pits. When the second cam-bium is exhausted it gives rise to a third layer which divides just as the other two did, and may in its turn produce a fourth layer, and this a fifth. The layers produced later consist of smaller and more regular cells, and so approach more nearly to the typical cambium than do those formed first. The cambium does not form complete rings, but is frequently interrupted. Much of the parenchyma of the cortex, meanwhile, has become sclerized and the pericycle has greatly changed. Near the growing point it is made up of narrow, thin-walled cells, but by the time the stem has become woody and secondary thick-ening has begun, it consists of wide, very thick-walled cells, which in longitudinal section are seen to be pitted.

Leaf.—At the base the Astelia leaf is colourless, and the cells show very little differentiation. Much starch is stored in this region. Further away from the point of attachment to the stem the leaf broadens out into the dark-coloured portion mentioned above. The colour here is due to the extremely thick cuticle which is composed of scales of wax closely pressed together. When seen separately they are a golden yellow, but in the mass they appear black. Thin places or even pits are left in the cuticle, showing, in a section cut parallel to the surface, as pairs of rounded, light-coloured spaces. These correspond to the position of specialized epidermal cells, the numbers of which in the leaf bases where water is held are so much greater than in other parts of the leaf that the conclusion is reached that they are absorbing organs.

A transverse section of the blade of the leaf shows that under the thick cuticle there is an epidermis of two layers of small rounded, regular cells, those of the inner layer being slightly larger than the outer ones. At intervals are deeply sunken stomata of the usual type with two guard cells containing chlorophyll grains. Each stoma opens into an inter-cellular space. Between the stomata are the water-absorbing structures which consist of two rows of four or five cells, those nearest the centre and those on the outside being larger and rounded, while those between are smaller and flattened and have brownish cell contents (Fig. 6). Below the epidermis is the meso-phyll in which there is no distinction of palisade and spongy meso-phyll, though the cells towards the upper surface are smaller and contain more chlorophyll. The lower epidermis resembles the upper except that the cells are larger and there are more hydathodes. Bridges of sclerenchyma are found embedding the larger veins, and groups of sclerenchyma occur below the smaller ones, thus supplying the mechanical support necessary for a large leaf. Mucilage ducts occur towards the lower surface opposite the smaller veins and on each side of the sclerenchyma embedding the larger ones. The ducts are formed by the breaking up of cells and secrete large quantities of

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colourless mucilage which exudes rapidly from cut surfaces and sets like a jelly in the air.

E. Peperomia Urvilliana.

General description.—Peperomia Urvilliana grows as an epiphyte and also in all parts of the lava field from the surface to the deepest crevices. It grows more luxuriantly in the deeper crevices where it gets more shelter. Here the plants form large masses with long branching stems and leaves larger and of a paler green than those of plants nearer the surface. The bulk of the stem is made up of large-celled water-storage tissue, in which are scattered chlorophyll granules. A peculiar feature of the vascular system, common to the order Piperaceae, is that the bundles are arranged in an outer and an inner series. Each is bounded by pericycle and endodermis of small regular cells.

Leaf.—The upper epidermis has a thick cuticle and consists of small cells, which are barrel-shaped in transverse section. Under this are four or five layers of large water-storage cells devoid of chloro-phyll, among which are oil cells. There is a single layer of palisade parenchyma, long narrow cells closely packed with chlorophyll. Two layers of rounded cells also packed with chlorophyll are seen below the palisade cells and a broad band of irregularly-shaped parenchyma containing scattered chlorophyll grains, stretches to the lower epider-mis. This resembles the upper epidermis except for the presence of hydathodes, funnel-shaped cells beneath which are mucilage cells. Stomata are seen in both upper and lower epidermis, and are of the usual type. Leaves from the crevices differ from surface leaves in having the chlorophyll less strongly developed and in there being more oil-cells among the water-storage cells. According to Haber-landt these are probably of use in focussing light on the chlorophyll cells below.

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