The Structure and Development of Astelia nervosa var sylvestris.

By Elma M. McCarthy, M.Sc.
(Communicated by Dr. J. E. Holloway.)

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.

Fig. 6.—Transverse section of base of leaf × 40.

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.

Fig. 7.—Transverse section of young leaf × 50.

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

Fig. 8.—Diagramatic section of peduncle of flowering shoot × 12.

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.

Fig. 9.—Individual flower × 6.
Fig. 10.—Small portion of flowering spike (nat. size).

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

Fig. 11.—Transverse section of young anther × 500.

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

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.

Fig. 19.—Longitudinal section of an ovule × 380.

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

Fig. 21.—Longitudinal section of seed × 295.

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

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.

Fig. 24.—Transverse section through the leaf-bases of a young plant × 60.

Summary.

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.