The Structure and Development of Astelia nervosa var sylvestris.
[Read before the Otago Institute 8th May, 1928; received by Editor, 12th May, 1928; issued separately, 30th August, 1928.]
The following account of certain aspects of the structure and development of Astelia nervosa var. sylvestris is taken from a thesis written at the Botany Laboratory, University of Otago, under the direction of Mrs. M. W. Aitken, M.Sc. It owes its present form to the help and encouragement extended by Dr. J. E. Holloway.
Astelia nervosa var. sylvestris is a stout perennial herb growing abundantly under mesophytic conditions on low hillsides in both islands. The leaves are arranged in dense tufts, a number of which combine to form the large head which comprises the plant. The growth-form of the Asteliads is very similar but Astelia nervosa can be distinguished from other species by its orange berry, the perianth of which is enlarged and coloured. The leaf is long and narrow, varying in length from 3 — 5 ft. and in breadth from 2 — 3 inches. It is many nerved but three nerves are very conspicuous, and are coloured red with anthocyanin, which is dissolved in the cell-sap of the superficial cells. The leaf as a whole is flexible, but the base is white and succulent and clothed by a tomentum of long silky hairs.
Astelia nervosa var. sylvestris is very similar to Astelia Cockaynei (Astelia nervosa var. montana). The latter is found mostly in subalpine situations as a small sturdy plant covered with hairs. The structure of the leaf of Astelia Cockaynei as found in the Peridotite Belt, Nelson, is described in “Notes on the Autecology of Certain Plants of the Peridotite Belt, Nelson” (Trans. N.Z. Inst., vol. 52, pp. 305-308) by Miss M. Winifred Betts, M.Sc. It is practically the same as that of Astelia nervosa var. sylvestris.
It also has some resemblance to Astelia Solandri, as described by Miss J. H. Wilson, M.Sc. in “Some Plants from the Lava-Field at Mt. Wellington” (Trans. N.Z. Inst., vol. 58, pp. 259-263). This resemblance lies chiefly in the anatomy and function of the leaf bases (Wilson, p. 262).
The only other paper dealing with the anatomy of the Asteliads is one by Miss E. M. Herriott, M.A. (Trans. N.Z. Inst., vol. 38, pp. 377-422), “On the Leaf Anatomy of Astelia linearis var. sublata.” This leaf has no close resemblance anatomically to that of Astelia nervosa var. sylvestris.
Microscopic structure of mature leaf. (Figs. 1-6).
Over the midrib the upper epidermis is composed of small cells, beneath which is an aqueous tissue consisting of 4-6 layers of colour-
less palisade-shaped cells very regularly arranged. The chlorenchyma is below this, and is represented by about 10 layers of spongy parenchymatous cells. The lower epidermis consists of 2 rows of small colourless cells. The fibro-vascular bundle forming the midrib,
stretches from the aqueous tissue to the lower epidermis, where it forms a conspicuous ridge on the under-surface of the leaf. The upper flange of mechanical tissue is a crescent-shaped mass of sclerenchymatous cells. It is smaller than the lower flange which would be circular but for a small area occupied by the phloem.
Fig. 3.—Transverse section of prominent side vein × 60.
Fig. 4.—Transverse section of leaf at margin × 200.
The most prominent nerves of the leaf are the two lateral ones. This is due to a very conspicuous anchor-shaped girder of mechanical tissue stretching from one epidermis to the other. The upper flange is the larger and the vascular bundle occupies only the lower half of the web. The aqueous tissue is slightly wider near these veins, and the cells are almost spherical and loosely arranged. Two layers
extend to the web of the girder underneath the upper flange. Towards the midrib the chlorenchyma increases greatly in thickness the upper limits being almost parallel to the surface of the leaf
which here projects as a ridge. Towards the margin of the leaf the epidermal cells become larger and the aqueous cells smaller. The aqueous tissue is here only one layered, the chlorenchyma occupying most of the leaf. Between each bundle is a large colourless cell several times larger than an ordinary cell of the chlorenchyma. The fibro-vascular bundles are similar to all the other minor bundles of the leaf, that is, they have mechanical tissue in the form of two crescent-shaped areas, one above, one below the vascular bundle. A slight variation occurs in the end-bundle where the upper flange is practically undeveloped.
At the base the leaf is soft and white and covered with a tomentum of silky hairs. These occur on both surfaces of the leaf, but are more numerous on the inner or upper surface. The hairs arise below the level of the epidermis and consist, below the epidermis, of a short stalk 2 or 3 cells deep and 2 wide. Beyond the level of the epidermis the hair branches out in several directions, but as a whole continues at right angles to the surface, branches being given off to the right and left. The branches, which are unicellular, remain close to the parent axis. On some hairs a number of branches are
given off together forming a whorl. The mesophyll is composed of round loosely-arranged cells which near the upper epidermis form a sponge-like tissue with large intercellular spaces. The three main veins are not conspicuous, but above each large vein is a triangular mass of collenchyma. Below the veins are crescent-shaped gum-passages. The gum is apparently secreted by special mucilage vesicles which project into the gum-passages. These passages, unlike those of A. Solandri (Wilson p. 262), are found only in the leaf-base.
Microscopic structure of leaf of seedlinu. (Fig. 7.)
The leaf of a 4-inch-high seedling is slightly thicker in the proximity of the main vascular bundles. On the upper surface there are 2 layers of colourless polygonal cells, the lower representing the aqueous tissue. The chlorenchyma which consists of from 3-6 layers of cells is in the form of spongy parenchyma. The lower epidermis consists of a single layer of cells except near the midrib where there are two layers. At this stage the leaf has 7 veins. These are the three main veins with a small group of sclerenchymatous cells above and below the vascular strand, two fairly large veins with no
mechanical tissue, and two small veins between the main laterals and the midrib. At the base of the leaf there are no gum-passages. These appear together with the hairs when the plant is about 2 ft. high.
Microscopic structure of peduncle of inflorescence (Fig. 8).
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
The triangular peduncle has a ground-tissue of round cells rich in starch. The outer layers contain a few chloroplasts and are surrounded by a dermis of tabular-shaped cells. About 1/50 - 1/75
of an inch from the dermis and parallel to it lies a row of vascular strands. Along this row large vascular bundles occur at fairly regular intervals, the space between each being occupied by 2 or 3 smaller ones. Inside this row vascular bundles are scattered through the ground-tissue. The mechanical tissue, especially in plants grown in the shade, is not well developed.
Flowering (Figs. 9 and 10).
Cheeseman (p. 317) states that Astelia nervosa is a dioecious plant. All the specimens examined by the writer, however, seemed to be hermaphrodite. This was confirmed by microscopic examination. All the anthers contained pollen-grains and in most cases the ovaries contained ovules. In some cases however the ovules were not well developed and were possibly functionless. An examination of the plants in the fruiting season shows a luxuriant development of fruit on some plants, while others have none at all. This would certainly seem to indicate the dioecious habit. Astelia nervosa var. sylvestris, however, does not flower and fruit every year. Investigation over a number of years is necessary before the position can be stated definitely.
About February and March individuals about to flower appear slightly swollen at the base. The inflorescence is formed at the apex of the shoot, a new shoot arising laterally to carry on the growth of the tuft. The flowering-scape appears above ground at the begining of September, and when only 4 or 5 inches long the pollen-grains and ovules are already formed. The buds open shortly after they emerge from the bracts and surrounding leaves, so that the tip of the spike is often in full bloom before the rest of the spike
has appeared. The flowers are not conspicuous, the colours being very dull, but they have a sweet scent. In contrast to the flower the fruit is very conspicuous, each spikelet being crowded with brilliant orange berries. The berries soon drop off when the succulent perianth, which at first is closely pressed to the berry, turns back. The number of seeds per berry varies from 3-10, 8 being the most frequent number. The seeds are smooth, black, and angular, with a bright polished appearance. The testa is very hard, thick, brittle, and practically impermeable to water
Structure and development of anther (Figs. 11 and 12).
In a very young flower, when the whole inflorescence is about an inch long, the stamen is quite undifferentiated, being represented by a mass of homogeneous tissue, kidney-shaped in transverse
section. A longitudinal section at the same stage shows a cone-like structure with no distinction between anther and filament. A little later some of the cells of the hypodermal layer divide to form the primary wall-layer and the primary sporogenous layer. This celldivision takes place, as in most Angiosperms, in 4 regions of the anther. Transversely the anther is still the same shape, but in longitudinal section shows a slight constriction at the base. This is the beginning of the differentiation into anther and filament. The primary sporogenous layer divides until several layers of sporogenous cells are formed. Some of the wall-layers form a tapetum which is partially disorganised in the spore-mother-cell stage. In longitudinal section at this point the anther and the filament are quite distinct.
The filament, however, is very short while the anther is almost the mature size. The opening of the bud therefore entails a great elongation on the part of the filament. The stamens pass the winter in the spore-mother-cell stage.
At the beginning of spring the spore-mother cells divide to give the tetrad of pollen-grains which have sculptured coats. Directly below the epidermis of the anther is the endothecium which is composed of a layer of palisade-cells. Between the adjacent sporogenous tissues of an anther lobe are 2 or 3 layers of thin-walled cells. In dehiscence this thin-walled tissue breaks down with the result that there is now a single compartment in the anther-lobe. In the region
Fig. 12.—Transverse section of anther × 100, showing the coalescence of the 2 adjacent pollen-chambers.
of the thin-walled cells the epidermis and endothecium are bent inwards, the apex of the bend being occupied by the thin-walled tissue. Consequently when this breaks down, the wall is not continuous, and with a slight increase of pressure may be bent backwards setting the pollen-grains free.
Structure and development of ovule (Figs. 13-19).
The ovule originates in the ovary about the same time as the division to form the spore-mother-cells occurs in the anther. The nucellus appears first as a protuberance on the placenta. A transverse section at this stage shows the three carpellary leaves with the ovules as small outgrowths at their margins. As the ovule develops the nucellar mass protrudes further and gradually bends sideways and downwards. The first integument soon appears as a ring round the base of the nucellus. Later, but before the first integument envelops
Fig. 13.—Longitudinal section through a very young ovary × 380 showing the ovule as a slight protrusion of nucellar material.
Fig. 14.—Longitudinal section through ovary × 380, showing young ovule bending sideways and downwards.
the nucellus, the second makes its appearance on the outer side of the ovule. At the same time the embryo-sac can be distinguished in the third layer, below the apex of the nucellus, as a round cell slightly larger than the others. About the time that the first integument has grown beyond the nucellus, a cavity is formed below the
Fig. 16.—Longitudinal section through ovary × 260, showing ovule with the first integument appearing as a ring round the base of the nucellus.
Fig. 17.—Longitudinal section through ovary × 250, showing an ovule with the first and second integuments and the embryo-sac.
Fig. 18.—Longitudinal section of a young ovule × 300.
enlarging embryo-sac, probably by the breaking down of some of the cells of the nucellus.
The mature ovule is anatropous with an inner and outer integument. The former is 2 cells wide, while the latter varies from 3 cells at the micropylar end, to five at the chalazal. On the inner side of the ovule, the funiculus and the outer integument cannot be distinguished. The embryo-sac occupies only a smal portion of the ovule, lying immediately below the micropyle. The cavity below the embryo-sac has greatly increased in size by the breaking down of the nucellar tissue. At the base of the cavity is a peculiar starshaped structure which consists of 3 or 4 small cells with thick walls, round which radiate elongated cells. Below this the nucellus is prolonged into the tissue of the chalaza as a tongue-like outgrowth
of small radially-extended cells. The walls of the loculi contain in their cells bundles of very long raphides, a type of crystal found in several other parts of the plant.
Structure of fruit and seed (Figs. 20, 21).
A berry, when still small and green, is seen in longitudinal section to have a dermis of rectangular cells inside which polygonal cells interrupted by an occasional vascular strand stretch to the cavity of the berry. The young seed has the cavity and star-shaped structure typical of the mature ovule. The testa, however, is now distinguishable and appears to be striated.
In the ripe seed the testa is hard and brittle. The embryo is situated at one end, with its long axis parallel to the long axis of the seed. It is of the usual mono-cotyledonous type, with a terminal
cotyledon and a lateral growing point. Round the embryo radiate in regular rows the endosperm cells.
Germination (Figs. 22-24).
The seed may not germinate for some time, but eventually the first root and the first foliage-leaf make their appearance. The
cotyledon remains inside the testa and acts as an absorbing organ of the young plant. The endosperm is gradually disorganised, a milky-looking fluid being formed.
The first leaf is enclosed in a sheath, through which it breaks its way, and shows above ground as an acicular blade. Later the second leaf appears enclosed by the base of the first, then the third enclosed by the base of the second, and so on. As the leaves
develop new roots make their appearance at the point of origin of the primary root.
As the seedling grows its hypocotyl becomes more complex. Near the root its shape is circular, in cross section, in contrast to the triangular form found further up the hypocotyl. The limits of the leaves can be distinguished by a layer or layers of cells less crowded
Fig. 22.—A. Seedling with seed still attached × 2.
(1.With seed split open. 2 and 3. Front and back views respectively.)
B. Slightly older seedling × 2.
C. Still older seedling (nat. size).
with starch-grains than the bulk of the hypocotyl. The leaves themselves are ring-shaped in transverse section and the structure of the two outermost can be clearly seen. They have an upper and lower epidermis of large cells containing no starch-grains. The vascular bundles are present but there is no mechanical tissue in connection with them. The rest of the leaf is occupied by spongy parenchyma crowded with starch-grains.
Further up the hypocotyl the leaves, in transverse section, are shaped like a hollow triangle, the third side being composed of 2 layers of colourless epidermal cells. The outer leaves as in the lower hypocotyl, are crowded with starch-grains, but inside these are seen
the upper portions of younger leaves with the starch-granules replaced by chloroplasts, and the third side absent. The leaves are spirally arranged, the fourth leaf being vertically above the first.
Astelia nervosa var. sylvestris has certain well-defined xerophytic structures. There is a conspicuous aqueous tissue, and the chlorenchyma is composed entirely of spongy parenchyma containing very few air-spaces. The leaves are almost vertical, hairy at the base, and are combined to form tufts, the base of which serves to collect and store water. It is possibly due to this adaptation that the root system is not extensive. The leaf-bases and hypocotyl form a storageplace for food as is indicated by the copious starch in these regions. The functions of the hypocotyl are therefore the same as those of A. Solandri (Wilson, pp. 259, 262). These xerophytic characteristics of the mesophytic A. nervosa var. sylvestris are probably due to the fact that the genus which, as a whole, contains so many epiphytes, has a tendency towards xerophily.
The ovule resembles the typical monocotyledonous type, being anatropus and having two integuments. The embryo-sac is smaller than in most Liliaceae, and the nucellus is correspondingly larger, and surrounds the large central cavity at the base of which is the star-shaped structure which can still be seen in young seeds. The tongue-shaped projection of the base of the nucellus into the chalaza can only be seen in a median section, and is probably a nutritive device.
Betts, M. W., “Notes on the Autecology of Certain Plants of the Peridotite Belt, Nelson.” Trans. N.Z. Inst., vol. 52, pp. 276-314.
Cheeseman, T. F., Manual of the N.Z. Flora, pp. 312-319.
Herriott, E. M., “On the Leaf Structure of Some Plants from the Southern Islands of New Zealand.” Trans. N.Z. Inst., vol. 38, pp. 377-422.
Wilson, J. H., “Some Crevice Plants from the Lava-Field at Mt. Wellington.” Trans. N.Z. Inst., vol. 58, pp. 255-263.