Art. LVII.—Notes on Nothopanax arboreum, with some Reference to the Development of the Gametophyte.
[Read before the Wellington Philosophical Society, 28th October, 1914.]
Closely allied to the order Umbelliferae is the order Araliaceae, differing from it chiefly in being arboraceous in habit and tropical in distribution, though it is found in Japan, Canada, north-west America, and New Zealand. Among other differences between the two orders is the presence of usually three or more carpels in the ovary. In Schefflera digitata, a New Zealand species, there are five carpels. This plant also bears its flowers in strongly scented racemes. In another New Zealand member of the order, however, Nothopanax arboreum, the appearance of three carpels, though not uncommon, is not usual: it usually produces only two. Moreover, the fruit does not, as in Umbelliferae, split up into separate mericarps, but remains somewhat fleshy when mature. The three orders, Umbelliferae, Araliaceae, and Cornaceae, agree in having the ovules solitary in each carpel. Bracts, also, are absent in Nothopanax arboreum, though, indeed, they are not always present in all members of Umbelliferae. There are no vittae in the seed.
Life-history of Nothopanax arboreum.
Nothopanax arboreum usually begins life as an epiphyte on tree-ferns, though one was observed growing on the felled stump of a large timber-tree. Young seedlings are, however, quite able to grow on the ground, and frequently do so in nature when sufficient light is available on the ground. The plant was not observed to bear flowers while still an epiphyte. In the epiphytic trees roots are given off often at a great height. These seek the ground, clinging closely to their support. Later they completely envelope the host, as in rata, and when the host dies away the plant remains as a Nothopanax tree.
The mature fruit-bearing form has leaves 5–7-foliate, with serrate edges. The seedlings show, however, that, as in many other New Zealand plants, this condition is not the original one. The cotyledons are usually small and rounded (occasionally longer and narrower), with entire margins (fig. 1, A). These remain till eight leaves are formed. The first foliage leaves are unifoliate, and, though produced singly, can easily be regarded as pairs. They are larger than the cotyledons, and have serrate margins (fig. 1, A–E). Four other pairs of leaves are produced closely resembling these, but when the sixth pair is produced it is found to be trifoliate, the
two lateral leaflets being much smaller than the terminal leaflet. The seventh and eighth pairs are also trifoliate (fig. 1, F–G). When the ninth pair appears it is seen to be 5-foliate (fig. 1, H). The terminal leaflet is the largest, and the two lowest the smallest, as in the trifoliate leaves. In the mature forms the leaves are 5–7-foliate, and this condition is attained at about this stage; some when they have achieved the 5-foliate condition do not alter further, others form 6- or 7-foliate leaves straight away (fig. 1, I). In the specimens examined the variations were for the most part constant, though in one or two the sixth pair of leaflets was normally trifoliate, the seventh returning to the unifoliate condition.
The inflorescence of Nothopanax arboreum is composed of compound umbels, three times divided. These spring from the tips of the branches, and are shorter than the leaves. Quite often two peduncles spring laterally from below the origin of the others. The number of branches in each umbel is very irregular—the primary peduncles numbering from twelve to fifteen, the secondary branches from fifteen to thirty, and the actual flower-pedicels from five to fifteen. Though very irregular in number, these stalks are very uniform in length. In some of the male trees examined the peduncles and pedicels were much flattened.
The plant is dioecious, male trees bearing brown rounded buds with five microscopic sepals, five petals, five stamens, and a gynoecium with two carpels and two style-arms. The petals, which are valvate in the bud, are brown on the outer surface and greenish-brown on the inner, small, and pointed. The stamens have large anthers packed upright in the buds, and there are corresponding depressions in the disc to accommodate their bases. They alternate with the petals. The style-arms are two, upright in the bud, small and thick, which diverge when the flower opens. The buds are protected in the young stage by masses of colourless mucilage, which later falls away. This is said to arise from hairs which are developed with the flower-buds. In about three months (April–June) from their first appearance the buds open. The petals expand like a star, the filaments of the stamens lengthen considerably till about as long as the petals, and the stigmas diverge somewhat. They possess a red colour, which was also seen while in the bud.
The flower soon becomes distinctly scented, and small quantities of nectar are seen on the disc. They are then visited by blow-flies, and very occasionally by bees. The latter, however, on account of the length of the proboscis, cannot make use of the nectar. A short time after the opening of the flower the ovaries become compressed, and often much deformed. When all the pollen is shed, an absciss layer is formed at the base of the receptacle, and the flower is cast off. Later the pedicels and peduncles are also cut off. The ground below male trees becomes strewn with flowers, especially after rain.
On one male tree examined there was found an umbel of seven well-formed fleshy fruits, with two seeds each. Also, later in the season (September), when the crop of male flowers had all fallen, an inflorescence of hermaphrodite buds was found on the same tree.
Compression of the receptacle is accompanied by a swelling in the ovary-wall, but the ovule itself withers before the flower falls.
On female trees the buds are compressed from very early stages, and consist of an enlarged receptacle enclosing the ovary and a small perianthcap consisting of five microscopic sepals, five petals (smaller than those of the male plant), usually five staminodes (though it is not uncommon to find that there are four or only three formed). These staminodes have very
short thick filaments and two anther-lobes as in the stamens of the male flower. Certain cells are seen, under the microscope, to form mother cells of the pollen-grains but in no case examined were actual pollen-grains formed.
The gynoecium in the female flower consists, as in the male flower, of two carpels each with a single ovule and two style-arms. Sometimes buds with three carpels are found. The ovary is then triangular, with slight depressions between the ridges. In the flowers with two carpels the ovary is compressed. In one tree observed the buds and fruit were mottled, green and brown, while in all others they were brown, the colour being constant for each tree.
Opening of Flower.
When the flower is about to open, one of two methods is adopted: either the perianth is shed without opening as a cap—that is almost invariable in the mottled variety—or the petals may open as in the male flowers. Enclosed within the perianth-cap thus shed are the staminodes. When the petals are spread they do not persist, the typical flower consisting only of sepals and the gynoecium. The two stigmas diverge shortly after the flower opens, and the petals are smaller than those of the male flower. In the female flower there is no scent; nor have any insects been seen visiting the flowers in search of nectar or pollen, though nectar is produced in small quantities. Occasionally there are three carpels, with the corresponding styles and stigmas. The ovary is then triangular. Intermediate stages were often noted in which there were three carpels, but only two well-formed styles, the third being nearly abortive. In others, again, only two carpels were formed, with their respective styles, occupying positions as in the trilocular ovary. This seems to bear on the gradual evolution of the Umbelliferae among the Archichlamydeae making for combined simplicity of structure and effectiveness in achieving pollination.
An article on “Floral Evolution” which appeared in the New Phytologist, vol. 10, April, 1911, p. 113, points out the importance of progressive reduction in the number of ovules in each carpel. In Umbelliferae the climax of economy is reached in the invariable production of one ovule only for each carpel. The article goes on to show the importance of floral aggregation in economy, and also of the inferiority of the ovary. In the matter of aggregation and in bicarpellarity of the ovary Nothopanax seems to mark an advance on another member of its order, Schefflera digitata. In the matter of insect-visits it does not appear to have attained any distinction.
In the female flower of Nothopanax arboreum the disc may grow up as a conical structure, but usually it remains more or less flat. Pollen-grains have been seen on the stigmas and in the axils of the pedicels and peduncles. In some cases the grains were undoubtedly those of Nothopanax arboreum, but in no case was a pollen-grain seen to germinate. No nectar is produced in the conical-disced flowers; there is, therefore, little or no apparent reason why insects should visit the flowers.
The plant is the food plant of the caterpillar of a moth, Declana artronivea. This caterpillar is mottled like the fruit among which it lives, and is therefore protected from attack, being very difficult to detect. On the trees bearing brown fruit a very similar brown caterpillar was found. Being unable to move from tree to tree, the caterpillar cannot assist in pollination.
All the flowers of the inflorescence are of the same value and structure both in male and in female plants. In this respect Nothopanax differs
from many of the Umbelliferae which have reached the advanced stage of having hermaphrodite and unisexual flowers in the same umbel. There the hermaphrodite flowers open before the unisexual (male) ones of the same umbel. In Nothopanax arboreum the outer flowers of the umbel open first. Geitonogamy is here not possible, as the pedicels separate the flowers too much.
The pollen-grains are boat-shaped, with three radiating grooves. A few experiments were made with the pollen-grains. An attempt was made to pollinate the male flowers (pseudo-hermaphrodite) with pollen from their own stamens, with pollen from stamens of adjacent flowers of the same umbels, from different umbels, and from different trees—with no result. All fell at about the same time as the other male flowers.
Several attempts were also made to germinate the pollen-grains in a sugar solution of about 10 per cent., but none was successful.
Flowers of the female tree were also protected with muslin to ensure against external pollination. Only one of these bunches remained, held by a thread of cotton, and in it the flowers had one and all withered and fallen. The other bunches had, apparently, all fallen. The flowers are evidently cross-pollinated. Their own stamens being functionless, these flowers exhibit physiological femaleness.
Some of the hermaphrodite buds found on the male tree were protected while yet unopened, several of them being under one enclosure. These opened and formed the early stages of fruit while under the muslin quite as well developed as those outside. Of these hermaphrodite buds, three or four only had withered, as had also some of those unprotected, confirming the opinion that some few were pseudo-hermaphrodite male flowers as the normal flowers on the tree, which they closely resembled. These hermaphrodite flowers all opened and spread their petals and stamens. The filaments were shorter than those of the male flowers. Both petals and stamens were cast off later, leaving the flower with the characteristic appearance of the female flower. There is the difference here, however, that pollen-grains are actually formed.
There is no definite arrangement of hermaphrodite and unisexual flowers in the umbels. In those umbelliferous plants in which there are pseudohermaphrodite male flowers and true hermaphrodite flowers—e.g., Venus's comb—the latter open first, the former not until the latter have cast their stamens and petals. Directly the petals open in these hermaphrodite flowers, a finely granular honey-secreting disc and two short styles are revealed in the centre of the flower. The stigmas mature, but the stamens are incurved like hooks.
The fruit of the previous season remains on the tree for some time after the flowers open. Later it falls in whole clusters, though the individual fruits are easily detached. The receptacle is enlarged and fleshy, and contains two seeds protected in a hard pericarp.
The anthers are large and versatile, 2-lobed, with two pouches to each lobe. Dehiscence takes place by longitudinal slits. The inner cells of each lobe are highly granular and protoplasmic. They become free from their neighbours while still enclosed in their cell-walls. Each mother cell forms a tetrad of pollen-grains, which are finally liberated
in the anther-sac. They are three-sided in cross-section, and have, when ripe, a pale-yellow colour. The external wall is marked by regular sculpturings, in addition to three prominent longitudinal ridges. The cells bordering the connective tissue and the epidermis do not form mother cells, but form the tapetum—a layer which becomes disorganized, helping in the development of the pollen-grains. Each pollen-grain consists of two cells, with clear nuclei, which can occasionally be seen through the walls. Development here is the typical development of a dicotyledon. In the male flowers the anthers are yellow, but in the female flower the staminodes never lose their greenish colour. The anthers become broken off, and often all that is left is a portion of the thickset filament alternating with the petals.
The stigmas are bifid, the outer surface of each lobe being protected by a continuous epidermis. The inner surfaces, however, on diverging, present a slightly papillate stigmatic surface (fig. 2). The cells immediately below the stigmatic surface are vertically long and narrow, with thin walls, and are ioosely packed. In many sections this tissue tended to break down, owing to its delicacy. It thus facilitates in every way the entrance of a pollen-tube; yet in no case was one seen, even when pollen-grains were observed on the stigma. The style-arms, still in their divergent position.
Fig. 2.—Stigmatic surfaces and tissue of style of mature fruit. x 175.
Fig. 3.—Longitudinal section of portion of young fruit, showing -shaped mass of thick-walled tissue at the base of the style. x 28.
persist through the formation of the fruit, but are, of course, relatively much smaller. The tissues of these arms, surrounding the central column of long cells, comprises a cylinder of loose irregular cells. In some cases the tissue below the style was of thick-walled regular cells, the thickening being of cellulose. The characteristic shape of this mass of tissue was that of an inverted stool, the disc extending towards the funicles of the ovules. This tissue was not observed till after the earliest stages in the development of the endosperm (fig. 3).
Very shortly after the male flowers have all fallen the ovary of the female flower begins to increase greatly in size, and microscopic examination showed that the cavity of the ovary is lined in the young flowers and buds
of both male and female plants by transversely elongated cells, rectangular in longitudinal section. These cells are at first from five to nine layers deep, but the number of rows increases with further development, extending almost to the axial bundle in the fully grown fruit. In the young buds and flowers the carpels are seen to be oval in cross-section. Soon, however, this shape is altered, owing to the appearance of longitudinal ridges traversing the cavity on both sides near the middle. The cavity is thus almost divided into two compartments, and presents a dumb-bell shape in cross-section. In the male flower this wall does not achieve any great thickening of its cell-walls, although the ridges are formed (fig. 4). In the female flower the ovary-wall gradually thickens its cell-walls, which later become extremely hard and wooden (fig. 5). The cells are fairly regular in shape and position, being arranged in rows. In the thin-walled stage they have protoplasmic contents, and usually a single nucleus, though occasionally two or more have been noticed. The end walls of these cells are at right angles to the lateral walls, although, at the curves in the ovary-wall, from
Fig. 4.—Longitudinal section of ovary-wall in young male and female flowers with unthickened cell-walls. x 175.
Fig. 5.—Longitudinal section of ovary-wall in mature fruit with much-thickened cell-walls. x 175.
Fig. 6.—Cross-section of developing ovary-wall, showing frayed cell-walls and multinucleate cells. x 175.
the irregular manner in which the cells here fit into each other, the ends appear to be more or less tapering. These long cells of the wall are interrupted by columns of parenchyma cells at the outer edges, and at the axis connecting the two carpels, running between the two branches of the axial fibro-vascular bundle. Near the top these parenchyma cells extend right to the cavity of the ovary, but for the greater part of the length of the ovary at least one layer of woody cells is continuous. As the cavity enlarges, the inner layers of cells become stretched to their utmost capacity, and are often frayed in the process (fig. 6). In the mature ovary the only discontinuity in this wall is seen at the origin of the funicle and arillus, where their place is taken by large-celled parenchyma.
In the female flower, even in cases where one ovule is aborted, the wall becomes thickened, though no stage has been noticed in which it has done so if neither of the ovules matured. Some of the flowers had withered and fallen, from no apparent mechanical injuries, and this pointed to the conclusion that when neither of the ovules reached maturity the flower was cast off by an absciss layer being formed at the base of the pedicels.
Coming now to the development of the ovule, we find a pendulous ovule, the funicle arising from the axial placenta, not, as at first appears, from the top of the ovary. The ovules turn from each other, as is usual. In the young buds examined the ovules had not completely achieved the anatropous condition typical of the mature form. The micropyle is in these young stages seen to be wide, and situated laterally, being almost at right angles to the funicle. Later, by unequal growth of the cells of the funicle, the ovule becomes truly anatropous. The micropyle is then almost entirely roofed over by the arillus, a structure arising from the funicle. This in the young bud is as large as the body of the ovule itself, and presents there a rounded bulgy appearance (fig. 8). Its cells are larger than those of the funicle from which it springs As the flower develops, the cells of the arillus are seen to acquire a radiating appearance, while the outer cells become papillate (fig. 16). As the ovule reaches maturity these cells collapse and the whole structure degenerates. It does not increase much in size throughout the life of the ovule. It is present in both male and female flowers, and is also seen in the hermaphrodite flowers. Here it has not the rounded character, as in the male and female flowers, formed earlier, but it is more gently sloping. In the female flower it completely overarches the micropyle of the ovule, its papillate cells seeming to lead to the micropyle.
As in Umbelliferae generally, there is only one integument. In the young ovule it is seen on both sides of the nucellus, from which it is entirely
Fig. 7.—Longitudinal section of ovule in male bud, showing crescent-shaped integument. x 175.
Fig. 8.—Longitudinal section of ovule in male bud, showing arillus and undifferentiated nucellus. x 175.
free, leaving a very wide micropyle. In some male buds sectioned the arillus formed the largest part of the ovule, the integument being scantily developed and crescent-shaped in longitudinal section (fig. 7). Later the
integument grows up, and encloses the nucellus while yet an undifferentiated mass of cells, leaving the micropyle now long and narrow (fig. 8). The integument where it borders the funicle becomes very closely connected with it The cells are angular, deeper above the nucellus than at the sides. In the young ovule the integument constitutes a large proportion of its size, but when endosperm begins to be formed, and the ovule increases greatly in size, the integument becomes thinner, and spreads over the surface of the rapidly growing ovule. The body of the ovule undergoes many contortions, forming invaginations always lined by the integument. The ovule has thus a very large surface area compared to its volume. Two chief and characteristic invaginations correspond to the ridges in the ovary - wall. There are, however, other smaller invaginations with which there seems to be no correspondence in the wall of the ovary. Soon the micropyle gives the appearance of being plugged up by the integument, though this may be due to an invagination immediately below it. The cells of the integument are small, and, in the young ovule, uniform in size. During development, however, there are seen one or two layers of much larger, thin-walled cells between the ordinary cells of the integument and the nucellus. These are interrupted at the base, and do not extend quite to the micropyle. They form, therefore, a hollow irregular cylinder open at both ends. These cells are irregular in shape. Their origin was not determined (fig. 13). The demands made upon the integument in the growing ovule by the formation of so many invaginations are quite sufficient to explain its rapidly increasing thinness (figs. 12 and 17). In ovules in which endosperm has been fully formed, the integument consists of cells of two kinds—large outer cells, usually one or occasionally two cells deep. These are turgid, and contain a diffuse green colouring-matter. The inner cells are several cells deep, regular in outline, and with dense protoplasmic contents. The large cells before mentioned (fig. 12) render the connection between the integument and endosperm easily severed, and the two can be easily separated under the dissecting-microscope when the endosperm has reached the “milky” stage. The integument in the seed is brown, and consists of a single layer of large cells (fig. 21). When the section is through the floor of an invagination the cells of the integument are seen to be large and regular. In the seed (when it has been removed from the wall of the ovary, which breaks away from the tissue of the receptacle to enclose it) there are seen to be two chief invaginations.
The nucellus consists at first of an undifferentiated mass of cells loosely surrounded by the integument. The next stage obtained in sectioning was the archesporial stage, in which a central cell of the nucellus had divided
into a tier of four cells not separated by cell-walls (fig. 10). Both ovules in the bud showed exactly the same stage in development. This stage was also seen in other buds, but all were male. No corresponding stage was observed in female buds of the same date. The female buds also showed that both ovules were at the same stage in development until the opening of the flowers, when often one is aborted. In flowers with three carpels one or two may be aborted or all may mature. In any case, all the carpels are developed, and the fruit appears normal externally.
Fig. 10.—Longitudinal section of ovule in male flower. Archesporium. x 175.
Fig. 11.—Cross-seotion of same stage. x 175.
The nucellus finally consists of a single row of cells, regular and rectangular in both cross and longitudinal sections. It is drawn up, the micropyle leaving the embryo-sac exposed. The contents of the embryo-sac in these early stages were not made out. There was, however, a possible indication of synergidae and a probable ovum, though there was no appearance of antipodal cells.
At the base of the embryo-sac, supporting it and interrupting the nucellus, was found a funnel-shaped structure consisting of cells resembling those of the nucellus, but staining less deeply with Erhlich's acid haema-toxylin. They had clear walls and large nuclei (figs. 12 and 13). This structure was identified with the hypostase which Van Teighem notes in Rosaceae and allied polypetalous dicotyledons. He says, “It consists of a small cupule of isodiametric cells which have strongly lignified but not much thickened membranes, and is found in the nucellus below the embryo-sac, its object being to arrest the longitudinal growth of the embryo-sac and endosperm towards the base of the ovule. Owing to its strong ligni-fication, it resists the various diastatic agencies at work during the formation of the embryo and endosperm, and for the same reason is incapable o-growth. Hence it appears in the ripe fruit exactly as in the pistil, but, being relatively much smaller, it is difficult to find. By arresting the basal de-
velopment of the endosperm it protects from destruction the region of the nucellus between itself and the chalaza. This in the ripe seed is intercalated between the integument and the endosperm or embryo. Hence in those cases a greater or less amount of perisperm is formed.”
Fig. 12.—Longitudinal section of embryo-sac, showing position of hypostase and large inner cells of the integument, arillus, and relative size of the ovule and ovary-cavity near the beginning of the formation of endosperm. X 28.
Fig. 13.—Lower portion of 12. X 175.
The hypostase found in Nothopanax arboreum was traced for some time after the formation of endosperm, but was lost sight of later, and could not be seen in the seed. In many radial longitudinal sections in the endosperm stage the hypostase could not be seen at all.
Many intermediate stages in the development of the embryo-sac were missed, owing to the extreme difficulty of sectioning the hard-walled ovary with the ovule in situ. Every precaution was taken. In the younger ones the base of the ovary-wall was cut immediately and before fixing. Later, when the ovule had achieved full size, it was removed from the ovary, and fixed in acetic acid, in place of osmic chromic formerly used. A vacuum process was also adopted. The results were, on the whole, better.
Fig. 14.—Longitudinal section of embryo-sac (female flower), showing early stages in development of endosperm before the appearance of an embryo. X 175.
Fig. 15.—Longitudinal section of ovule (female flower), showing continuous endosperm and no embryo. X 28.
Fig. 16.—Longitudinal section of ovule (female flower), showing early invaginations of integument, also arillus. X 28.
Fig. 17.—Longitudinal section of ovule, showing fibro-vascular strands traversing the endosperm. X 28.
Fig. 18.—Fibro-vascular strand. X 175.
The early stages in the development of endosperm were seen, in which the nuclei had become scattered through the vacuolate protoplasm. Later, cell-walls were formed about them (fig. 14). By successive division of the cells a dense endosperm is finally formed. This endosperm is continuous through a number of consecutive sections both radial and tangential, the only visible discontinuity being a very small break just below the micropyle in one section (fig. 11). There was no appearance of any embryo in any of these sections. It thus seems that here we have the formation of continuous endosperm before any pollination was observed and before the appearance of any distinct embryo.
In several cases of endosperm viewed under the dissecting-microscope there was the appearance of fibro-vascular strands traversing the endosperm. This was also confirmed in several ovules sectioned. The fibro-vascular strand is branched, and consists of annular xylem elements and phloem elements with large elongated nuclei and square ends to the cells. The fibro-vascular bundle of the funicle has annular vessels and phloem elements, but no connection was established between the two (figs. 17 and 18).
At the earliest stage at which an embryo was observed it already consisted of several cells—five to seven concentric rings of regular cells wholly embedded in the endosperm (figs. 19 and 20). It was very small, and was
Fig. 19.—Longitudinal section of ovule, showing embryo (e) immersed in endosperm. X 28.
Fig. 20.—Longitudinal section of embryo. X 175.
situated near the funicle, though whether at the micropylar or antipodal end is uncertain owing to the complete disappearance of the micropyle. In Umbelliferae generally, the hypocotyl points to the top of the ovule; in
Nothopanax arboreum the cotyledons, borne on the lengthening hypocotyl, point towards the integument where it is nearest, and away from the funicle.
A cross-section of the embryo of the seed shows the cotyledons, which are concave on their inner surfaces, and the body of the embryo showing delicate cell walls, which are not clearly defined. In a seed which had lain
Fig. 21.—Cross-section of seed with embryo. x 28.
Fig. 22.—Cross-section of embryo cotyledons. x 175.
Fig. 23.—Longitudinal section of embryo from seed. x 175.
in the ground apparently for a whole season, and was found among seedlings, the ovary-wall pericarp was softening, but the embryo had achieved no very great size. Seeds which were planted for three months showed no signs of germination.
Explanation of Lettering in Figures.
f.w. frayed walls of broken-down lining cells.
fv.b. fibro-vascular bundle.
fv.b.e. fibro-vascular bundle in endosperm.
l.i. large cells in integument.
lig. obliquely placed cells with lignified walls.
lin. cells lining cavity of ovary.
par. parenchyma cells.
st. stigmatic surfaces.
t.s. tissue of style.