Art. XXVI.—The Leaf-anatomy of some Trees and Shrubs growing on the Port Hills, Christchurch.
[Read before the Philosophical Institute of Canterbury, 6th August, 1913.]
The following paper contains an account of some of the points in the leafanatomy of some trees and shrubs growing in the patches of forest still remaining on the Port Hills, Christchurch. It forms a part of a more comprehensive report on the “Plant Ecology of the Forests on the Port Hills” which was prepared as an honours thesis for the University of New Zealand.
By way of introduction I give a brief description of the district and of its ecological conditions.
I wish to express here my indebtedness to Dr. L. Cockayne, F.R.S., for much assistance readily given in the preparation of the thesis.
Description of the District.
About seven miles south of Christchurch is Lyttelton Harbour, surrounded on its three sides by hills. “Port Hills” is the name given to that part of these hills which lies between Lyttelton and Christchurch, but the name may be extended to include a greater length of hills, stretching more to the south-west, as well as to the east. In this paper the name is used in this extended sense, and includes the part running round the head of the harbour, where the direction changes from east and west behind Lyttelton along the north shore of the harbour, to south-west and north-east till it becomes north and south at the head. To the north and north-east stretch the wide Canterbury Plains, in which Christchurch is situated, while on the south and south-west sides the hills slope down to the harbour, with practically no level ground between. The height above sea-level of the highest peak within the boundaries of the part studied is 593·3 metres, being that of Cass Peak, at Kennedy's Bush, which is taken as the south-west limit. Near the north-east boundary the highest peak is 495·6 metres. The peaks are, as a rule, not much higher than the surrounding hills, the average of which would be from 460 to 530 metres above sea-level. Here and there the hills are rocky.
The part of the district visited lies between Kennedy's Bush or Cass Peak to the south-west, and the Bridle-path to the east. The latter is a road over from Heathcote, a suburb of Christchurch nestling against the hills, to Lyttelton. The forest occurring within these limits may be taken as typical of all of that on the Port Hills
There is no meteorological station at the top of the Port Hills, so that no figures can be given as to the different climatic factors at work there, and it is near the top that most of the bush grows. However, the meteoro-
logical tables at two neighbouring stations have been consulted, and these will give a fair idea of conditions existing where the bush grows. These two stations are in Christchurch City, and at the Convalescent Home on the Port Hills, which is situated about 50 metres above sea-level.
Rainfall in Christchurch throughout the year is, on the whole, fairly evenly distributed, but on an average most rain seems to fall in July. The mean rainfall for July for the years 1902–10, inclusive, was 7·820 cm. The rainfall each month seems to vary considerably for each year, sometimes one month being the wettest one year and perhaps the driest the next. The average annual rainfall is about 85 cm., or a little over. The number of days with rain seems to be more equal for each month than the actual rainfall. The mean total number of days with rain from 1902 to 1910 for the winter months (June, July, August) was 38·8; for September, October, and November, 29·5; for summer (December, January, February), 26·1 for autumn (March, April, May), 31·9. Comparison of records taken in Christchurch with those taken at the Convalescent Home shows that there is a considerable amount of difference in the two stations, and that there is some variation in relative amount of rainfall, that at Christchurch being sometimes less, sometimes more, than at the Home. For instance, in July, 1910, the total rainfall for the month in Christchurch was 15·10 cm., while that on the hills was only 11·40 cm. Again, for August, 1907, Christchurch showed a total of 7·225 cm.; the hills, 12·525 cm. On the whole, rainfall is rather less at the Convalescent Home than in the city, but is probably greater higher up the hill than it is at the foot. When an easterly wind is blowing, on a clear day, clouds often hang over the spurs of the hills. In summer, and sometimes in winter, dry hot north-westerly winds blowing against the northern slopes of the hills must affect the amount of moisture in the soil.
Schimper* says that from an ecological point of view statistics as to temperature should give the mean daily minimum and maximum for each month of the year. These, however, are not available at the meteorological stations referred to; only the mean temperature for each month can be given, as a rule. At Lincoln, a station south of Christchurch and some few miles from the hills, the highest temperature recorded in 1903 was 83·8° F., in November; the lowest, 22·0° F., in July. The temperature for the different months does not differ so much in different years as does the rainfall. The coldest months are from May to September, with an average temperature of about 47°F.; the hottest, from November to February, with an average of about 59°F.
The temperature readings at the Convalescent Home are not taken in the same way as at the stations at Christchurch and Lincoln, and so are of no value for comparison. However, readings have been taken by residents near the Home, and these show that the temperature is slightly higher there than on the plains. There are fewer frosts near the foot of the hills than in Christchurch, but it is probably colder higher up than it is nearer the foot, for in winter snow often appears sooner near the top, and lies longer. For instance, on the 20th September, 1911, there was snow lying at the top, but none farther down.
The only statistics for relative humidity, and force and frequency of winds, are taken from Dr. Cockayne's paper on the Waimakariri River
[Footnote] * Schimper, A. F. W.: “Plant Geography.” Translated by W. R. Fisher. 1903, p. 175.
Basin,* and these were recorded some years ago. The force of the wind may be seen by records of the average daily force in miles for the year and of the maximum velocity in miles on any day in the year. For the years 1874 to 1879, inclusive, the average daily force for the year was 127 miles; the maximum velocity for a day, 717·5 miles. The most frequent wind, according to statistics, is the south-west, which is common in winter, and is a cold, strong wind, blowing in gusts, and generally bringing rain. The south side of the hills especially gets the full force of this wind. In summer it usually follows a north-west wind, bringing a heavy storm, and greatly lowering the temperature of the atmosphere. Though statistics show that the south-west is the most frequent wind, the east wind is generally considered to be the most prevalent, and addition of figures of frequency of the north-east, east, and south-east winds shows that the prevailing wind is from the east, though not always blowing from due east. The east wind, whether north-east, east, or south-east, is always steady, and often strong, but never blowing in gusts. Neither side of the hills is protected from it. A fairly frequent wind is the north-west, which is a great contrast to the south-west, which in summer it often precedes. It is a strong, gusty wind, laden with rain before it reaches the Southern Alps, but hot and dry after it has deposited the moisture on the mountains and sweeps across the plains to the east coast. The north-wester blows with force against the north side of the hills, and has a very drying effect. The average daily maximum relative humidity for the month before 1877 was in July, with 83, and minimum in October, with 72.
Some idea of the amount of sunshine may be obtained from the number of days with rain. New Zealand sees more of the sun than does Great Britain. For example, during 1902 there were 1,492·2 hours of sunshine in Surrey, one of the sunniest parts of England, and 1,749·59 hours in Christchurch. The light outside the bush, affecting the foliage of the trees, is more intense than that reaching the lower leaves of the trees, the shrubs, and the herbs but in most places the light in the interior of the forest on the hills is not very diffuse.
Another factor important in a study of plant-ecology is the altitude of the habitat. The average height of the hills, as stated above, would be between 460 and 530 metres above sea-level.
The soil varies considerably in physical and chemical character in different parts. Where there are no rocks near the surface the soil is a light loam for about half a metre or so, varying slightly as to depth, and below this there is clay. In some parts the substratum is composed mostly of small rocks with some soil between. This unevenness seems to serve a good purpose, or animals are not so fond of entering the “bush” and destroying the undergrowth; so that where the substratum is rocky erns seem to be more abundant than in other places where animals are not kept out. Most of the species seem able to grow on or between the rocks about as well as in the deep soil. The clay would increase the water-capacity of the soil, which, since it forms a sloping surface, would be inclined to allow the water to run away.
[Footnote] * Cockayne, L.: “Plant Geography of the Waimakariri River Basin.” Trans. N.Z. Inst., vol. 32, p. 95.
The plants of which the leaf-anatomy was studied were selected as representatives of the following plant-forms: (1) Lianes, (2) deciduous trees, (3) evergreen trees, (4) semi-parasites.
Parsonsia heterophylla A. Cunn.
This plant is a liane all through its life, not assuming different habits, but the leaves at different ages are of most diverse shapes. There are all transitions, from orbicular to linear, oblong, and partly lobed. The leaf of the mature plant is oblong or ovate, subacute, entire, varying in size from 4m. to 8m. long, 2m. to 3cm. broad, and may have a few hairs on the upper surface, which the young leaves seldom have. There is no essential difference in the anatomy of the young and adult leaf, but in the former often the row of cells next to the lower and upper epidermes contains anthocyan as well as chloropyhll, and this gives the leaf a copper-coloured appearance. (See fig. 1.)
Fig. 1.—Upper epidermis of Parsonsia heterophylla leaf, showing hair; × 418.
Fig. 2.—Lower epidermis of same; × 418.
Leaf-anatomy.—The leaf-anatomy of one of the lianes, Parsonsia heterophylla, was studied, and compared with that of Tetrapathaea australis, with a view to finding in them some of the anatomical characters peculiar to climbers.
The epidermis is covered with a thick striated cuticle, and is composed of rather large cells. Unicellular hairs often occur on the upper and sometimes on the lower surface, but not thickly enough to affect transpiration. These hairs take on a bright-red colour with eosin, while the other epidermal cells remain unstained. The epidermis is two-layered above the midrib-Stomata occur only on the lower surface, with which they are level.
The palisade layer is three or four cells deep, tightly packed, with chlorophyll grains distributed throughout the cell. Each cell contains a conspicuous oil-drop, soluble in ether. The palisade tissue composes about a quaiter or one-third of the thickness of the leaf. The cells of the spongy parenchyma are loosely arranged, and contain fewer chloroplasts than the
palisade layer, except near the lower epidermis. Rounded masses of crystals of calcium oxalate occur fairly abundantly in the spongy parenchyma.
The midrib is surrounded by round-celled parenchyma, amongst which, directly above and below the xylem-strands, and close to the latter generally, occur little groups of small cells. These may be part or all of the phloem, which Solereder* says occur in the stem on the inside of the xylem, and in groups in the pith, as well as outside the xylem. Staining with eosin failed to prove the presence of sieve-tubes. In the mesophyll the vascular bundles are surrounded by large parenchyma cells, which contain chlorophyll.
Fig. 3.—T.S. of adult leaf of Parsonsia heterophylla, passing through part of the midrib. The groups of small cells in the midrib are probably phloem. × 167.
Leaves of the young stage of this species often contain a red colouring-matter, besides the chlorophyll, in the upper row of the palisade cells and in the lowest of the spongy parenchyma cells. This colouring-matter is soluble in alcohol, dissolving out and leaving the green chloroplasts. It answered tests for anthocyanin, being changed to a blue by a strong base.
There is no other difference in the anatomy of the adult and juvenile leaf-form.
The anatomical characters of the leaf of Parsonsia heterophylla seem to be altogether mesophytic, and there seems to be little device to protect against or to increase the effect of radiation, except the fairly thick
[Footnote] * Solereder, H.: “Systematic Anatomy of the Dicotyledons.” Translated by L. A. Boodle and F. E. Fritsch, vol. 1, p. 531.
cuticle. This leaf might easily be taken as a typical mesophytic leaf. The leaf of Tetrapathaea australis is very similar to that of the former in essential ecological characters. It also has crystals of calcium oxalate in the mesophyll; but this point is more of morphological interest, common in the family to which it belongs. The cuticle is not quite so thick as in Parsonsia. (See figs. 1, 2, and 3.)
Fuchsia excorticate Linn. f.
Fuchsia excorticata Linn. f. is very abundant, and is found in all stations, out in the open and in the thick bush, in the latter case often forming a much larger tree than it ever does in exposed positions. It often roots amongst rocks, the roots growing along close to the rock and fastening in the crevices. In habit it is somewhat peculiar, the trunk being often almost quite horizontal for some distance, very irregular, and much branched, sometimes over 3m. round at the base. Leaves, which, are silvery underneath because chlorophyll is absent in the spongy parenchyma, are lost in May, and appear in August, which is also the beginning of the flowering season.
Leaf-anatomy.—The epidermis has little or no cuticle. It gives off a few unicellular trichomes, especially above the midrib. It consists of two layers of cells on the upper surface, except above the midrib, and both are without chlorophyll, and contain oil-drops. The cells of the outer layer are very wavy in outline as seen from surface view. Stomata are present only on the lower surface, where they are numerous, and are on a level with the other epidermal cells. They are not provided with special auxiliary cells.
The palisade layer is two or three cells deep, the third often consisting of rounded cells. It contains abundant chlorophyll, and measures about one-third the whole thickness of the leaf. The spongy parenchyma contains a large amount of air-space, and in the mature leaf is devoid of chlorophyll, giving the latter a silvery appearance on the under-surface.
Large cavities, with thin or no special bounding walls, occur at fairly regular intervals in the palisade layer. Bundles of raphides of calcium oxalate are abundant in the spongy parenchyma of the midrib.
The vascular bundle of the midrib is surrounded by large parenchyma cells, many of which contain mucilage, while raphides are found in a few. Anthocyan may occur in the epidermis above the midrib.
The raphides are probably a waste product, stored in the leaves, and got rid of when the leaf falls. Since the leaves are not on the tree when it is exposed to cold dry winds, at a time when root-activity is not so great, this may possibly be the reason why they have not developed a thick cuticle to protect against excessive transpiration. The leaf-anatomy of Fuchsia excorticata has several points in common with that of Plagianthus betulinus, the other deciduous tree on the Port Hills. Both have crystals of calcium oxalate in the mesophyll of the leaves, those of the latter being similar to the crystals in the leaf of Parsonsia. The cuticle of Plagianthus is also rather thin, and in its palisade layer are cavities or large empty cells, very similar to those of the Fuchsia leaf. Both leaves contain abundance of mucilage when the leaf is ready to fall, and also in Plagianthus when it is quite young. The occurrence of mucilage in Plagianthus is probably a character of systematic importance, however, and is not specially produced owing to the deciduous habit, as it is common in the family Malvaceae, according to Solereder.* Crystals also are common in most of the members of the two families Onagraceae and Malvaceae. No mention is made by Solereder of the cavities in the palisade layer of the leaves of these two species. (See figs. 4 and 5.)
Olearia Forsteri Hook. f.
This is a small tree, occurring only in the bush. It has a stringy bark. The leaves are leathery, and have a thick tomentum on the lower surface.
[Footnote] * Solereder, H.: “Systematic Anatomy of the Dicotyledons.” Translated by L. A. Boodle and F. E. Fritsch, vol. 1, p. 146.
When specimens of this tree are grown in the open, in gardens, the leaves are a little smaller, rather curled up, and of a brighter yellowish-green.
Leaf-anatomy.—The upper epidermis is covered with a fairly thick cuticle. If the upper epidermis be stripped off and stained with haema-toxylin, the cuticle remains colourless, and can be seen to have small openings here and there, surrounded often by markings which seem to be arranged in the shape resembling the guard-cells of stomata. Over the opening is a collection of a granular or scale-like substance, looking like broken pieces of the cuticle, which it probably is, since it turns yellowish - brown with chlor-zinc-iodide, and yellow in caustic potash, and is hardly soluble in chromic acid. It is not wax, being insoluble in ether. To the naked eye it appears on the upper surface of the leaf as a white granular substance. The upper epidermis is two cells thick, the inner cells being larger than the outer, and with the outer sometimes containing chlorophyll. The transverse walls of the outer epidermal cells have conspicuous pits. The lower epidermis is one cell thick, and its cells resemble those of the upper, and also have a thick cuticle. It is thickly covered with a layer of buff-coloured, two-celled stellate hairs growing from the epidermal cells. The stomata are raised above the other epidermal cells, and have large subsidiary cells.
Fig. 6.—T.S. through the leaf of Olearia Forsteri, with some of the stellate hairs on the under-surface showing; × 75.
The palisade layer is generally two cells thick, and about one-quarter of the thickness of the leaf. The spongy parenchyma forms a rather deep layer, and both it and the palisade layer contain abundant chlorophyll. The cells of the mesophyll contain conspicuous oil-drops, usually one in each cell. Bands of sclerenchyma occur at short intervals, extending from one epidermis to the other, and also above and below the vascular bundles. Pits can be easily seen in the walls of the sclerenchyma cells. The sclerenchyma assumes a very bright tint with iodine-green or saffranin. Below the vascular bundles of the midrib there are one or two canals.
A study of the anatomy of this leaf reveals several xerophytic characters. There is abundance of sclerenchyma, which makes the leaf very leathery. The thickness of the cuticle, the possession of a double epidermis (probably for water-storing purposes), and the great thickness of tomentum of empty hairs on the lower surface, where the stomata occur, are all common in plants adapted to dry situations. The position of the stomata above the surface of the epidermis would seem to show that the hairy covering is sufficiently effective in decreasing transpiration, and no further device, such as sinking of the stomata, is necessary.
Sections were made of leaves from garden specimens growing in an exposed, sunny position. The only marked difference in the anatomy was the far smaller amount of oil in the mesophyll of the garden-plant than in that of the forest-tree. No different arrangement of chloroplasts in the cells of the palisade layer could be detected, for in both cases they seemed to occur throughout the cell, and not only on the walls. (See figs. 6, 7, 8.)
Griselinia littoralis Raoul.
Griselinia littoralis Raoul, the broadleaf, is common in the bush and in the open. There are some very ancient trees of this species on the hills. One measured 4·75 m. in circumference. It has large buttress roots.
Fig. 9.—(a.) T.S. through leaf from a well-illuminated position on a tree of Griselinia littoralis. (b.) T.S. through leaf from a young tree growing in a shady position in the bush. Both × 60.
Leaf-anatomy.—This species grows into a comparatively large tree on the hills, and the leaves are, as a rule, fairly well illuminated. Leaves were taken and examined both from amongst those of a large tree and
from a young tree growing in a rather damp, shady situation. The following is a description of the anatomy of the first:—
Epidermis: The cuticle is conspicuous, and of a bright-green colour on both surfaces, and both lower and upper epidermes are double, the inner cells being larger than the outer. Stomata are numerous, small, only in the lower epidermis, difficult to see in transverse section. The guard-cells contain oil-drops, and have no special subsidiary cells. Hairs are altogether absent.
Mesophyll: The palisade layer is three or four cells deep, the cells being rather small, and containing abundant chloroplasts distributed throughout the contents of each cell, not arranged along the walls. The spongy parenchyma is rather deep, with the average amount of air-space, and the chloroplasts are not quite so abundant as in the palisade layer. No sclerenchyma is developed in the leaf. Clusters of crystals of calcium oxalate occur in the mesophyll. (See fig. 9, a, b.)
The leaves taken from a shady position were much thinner. The cells, of the palisade layer were rounded, but about the same size as in the former case, and more loosely packed. Air-spaces were a little more numerous, perhaps, but this was not so well marked as the difference in the palisade layer. The cuticle was slightly thinner, and colourless.
Tupeia antarctica Cham. & Schl.
Tupeia antarctica Cham. & Sol. is perhaps the most abundant of the three semi-parasites present. It is a shrub of about the same size and habit as Loranthus micranthus, but has smaller leaves, variable in size, of brighter colour, and also isobilateral.
Leaf-anatomy.—The leaf-anatomy of Tupeia antarctica and of Loranthus micranthus seem to be very similar, but this may be due to their belonging to the same family as much as to their having adopted the same mode of life.
The leaf of Tupeia antarctica is isobilateral, and shows no differentiation of mesophyll into palisade layer and spongy parenchyma. The cuticle is thick, and striated, especially near the stomata. The epidermis is com-
posed of a single layer of cells with conspicuous oil-dops, and numerous stomata, present on both surfaces, each with two subsidiary cells, and level with the other epidermal cells. The mesophyll is composed of green cells with few air-spaces. The parenchyma cells surrounding the vascular bunches contain a little chlorophyll. In being thick and isobilateral, and in having a small amount of air-space, the leaf shows characters of a heliophyll.
Explanation Of Lettering In Figures.
eut. str. striated cuticle.
f.v.b. fibro-vascular bundle.
m. medullary ray.
m.c. mucilage cell.
pal. palisade layer
par. palisade layer
s.c. subsidiary cell
sp. spongy parenchyma.