Art XXXVI.—The Anatomical Structure of the New Zealand Piperaceae.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 44, 1911, Page 339

Art XXXVI.—The Anatomical Structure of the New Zealand Piperaceae. By Anne F. Ironside, M.A. [Read before the Manawatu Philosophical Society. 20th April, 1911.] The Piperaceae have formed the, subject of much research recently, by Campbell, Johnson, and Hill, to throw light on the relationships of the order, and on the phylogeny of the Angiosperms generally. Hill's preliminary account on seedling-structure in the order appeared in the “New Phytologist,” No. 3, 1904; the full account appearing in the “Annals of Botany,” April, 1906. He gives a comprehensive account of seedling-structure in Piper cornifolium and in many Peperomias, then a description of development in the Saurineae, the whole concluding with certain theoretical conclusions. Hill, in common with Johnson, looks on Peperomia as a reduced genus; but he suggests that this reduction may be due to the epiphytic character of many of the species. The object of the present investigation was the examination of the anatomical structure of the mature plant and seedling of the New Zealand representatives of the order, to see what bearing they had on the subject. The work was commenced at the Auckland University College some three years ago. My thanks are due to Professor A. P. W. Thomas for the assistance he rendered me. New Zealand Representatives. There are three representatives of the order:— 1.Macropiper excelsum is either a shrub, forming a common undergrowth, or a small tree, often 20ft. in height, aromatic, glabrous. Stem flexuose, jointed. Leaves 3–5 in. long, broady ovate-cordate, often sharply pointed; petioles 1–2 in. long, with sheathing base, which serves as a protective organ. Catkins solitary or 2 together, axillary, erect, slender, 1–4 in. long; berries yellow.

2.Peperomia Endlicheri (P. Urvilleana of Hooker, 20) is a small creeping, succulent herb. Grows in damp woods, on rocks, less often on trees. 3.A second Peperomia, which is probably identical with the Peperomia, reflexa of Australia and many tropical countries, is not very common in New Zealand, and was not obtainable for investigation. Anatomical Features. Stem. The woody stem of Macropiper excelsum shows the double ring characteristic of the woody Piperaceae. In a transverse section we find on the outside periderm; beneath is cortical tissue. A young stem shows a distinct epidermis, the cells of which are more or less oblong in outline; immediately below are from 2 to 3 rows of cells, showing tangential divisions. It is from these we have the periderm. Below the epidermis, or periderm, is well-marked collenchyma, especially thick behind the bundles. De Bary refers to a similar structure in Piper rugosum: “The hypodermal layer surrounds the whole stem as a distinct closed, multiseriate (collenchymatous) layer.” The collenchymatous layer consists of from 8 to 10 layers, narrowing to less layers with slightly wider elements between the bundles. Immediately beneath this is a discontinuous ring of sclerenchymatous elements, one or two' elements wide, abutting at places on the vascular bundles; longitudinal sections show these elements to be branched occasionally. Then comes the outer ring of bundles, each bundle separated by a wide medullary ray from its neighbour. The bundles of this ring show secondary thickening to a marked degree. On the inside of this ring is a sclerenchymatous band several layers thick, broken occasionally between two bundles (fig. 2) In the centre of the stem is medulla, in which is a ring of, separate bundles. In stems 3/4 in. to 1 in. broad there are about 10, but the number varies in accordance with the size of the stem; 3 to 4 is common in young parts, especially seedlings. On the outside of each bundle both of the outer and often of the inner ring are much-thickened sclerenchymatous elements. Then there is the inner sclerenchymatous band. De Bary, remarking on the inner sclerenchymatous band, says, “The converse case, that a continuous layer of sclerenchyma supports the whole inner side of the ring of vascular bundles, is rare in Dicotyledons. This is the case in the woody Piperaceae—Artanthe, Chavica sp.” In some stems—Ranunculus repens, for instance—we find sclerenchymatous elements on both sides of the bundle, representing a sheath. It is probable that in the Piperaceae these sclerenchymatous elements represent the bundle-sheath, which has become much thickened and extended on the inner side for mechanical purposes. There is sclerenchyma to the inside of each bundle of the medullary circle, but it does not form a continuous ring; also at the base of the stem, and in young branches coming from the base, as in fig. 6A, in seedlings the continuity of the ring disappears, all which lends to the view that the sclerenchymatous ring is an extended sheath. The phloem consists' of several layers showing deeply staining sieve-tubes, and companion cells; sieve-plates, where cut through, show very fine pits.

The cambium shows especially well ‘the tangential divisions for wood and bast formation. The cambial or merismatic region is continuous round the stem in the outer ring, though it is only where the cambial layer passes through the bundle that it gives rise to wood and bast. Between the bundles the tangential divisions increase to the medullary ray. In no instance are secondary bundles found between the primary bundles. The wood, as is seen in longitudinal sections, shows pitted and scalariform vessels and pitted wood-fibres, besides annular and spiral elements of the protoxylem. Some of the bundles show the primary elements of the wood to be distinctly scattered; longitudinal sections and oblique transverse sections show the protoxylem in many cases to be endarch, but there are frequently scattered elements to the sides of the bundle as well (see fig. 3). The question is, Is this a trace of centripetal woodformation ? Fig. 1. Outer tissues, young stem:a, epidermis; b, collenchyma; c, sclerenchyma X 150. M. excelsum. Fig. 2. Portion of sclerenchyma band: a, sclerenchyma; b, medulla at break in ring passing into medullary ray. × 44. M. excelsum. Fig. 3. Inner portion of bundle from outer ring, showing scattered primary xylem a, secondary xylem; b, wood-fibres; c, c', protoxylem to sides and to inside of bundle; d, boundary of sclerenchymatous band, × 44, M. excelsum. Fig. 4. Bundle from stem: a, phloem; b, cambial region; c, xylem. × 150. Peperomia Endlicheri. In the medullary bundles secondary-wood formation does not take place to the same extent as in the outer ring. On the inside of the wood, in the bundles of both rings, are a fair number of parenchymatous elements. At first sight they look like internal phloem, but on examination of young stems, and especially of those bundles of the inner ring where there are very few lignified elements, it is seen that these are only parenchymatous elements. Some bundles show only 6 to 8 lignified elements in a mass of tissue staining deeply with haematin. As more wood is formed, more of this becomes lignified also, but a little to the inside always remains unlignified. In bundles where the primary xylem is scattered, the elements are scattered through parenchymatous elements. The medullary rays are very wide, sometimes 5 to 6, often 10 to 12 elements wide, parenchymatous mainly; in older stems slightly lignified.

Young stems, the base of the stem, and to a less extent an older stem, show the central medulla and medullary rays thickly set with starch. Engler has said that in Macropiper heckeria, Piper chavica, the bundles are in rings, the outer bundles joined. In Macropiper excelsum the bundles of the outer ring are separated by very wide medullary rays, as already stated. Tangential sections can also be cut in which the medullary ray is continuous right through. A portion of the internode of the woody stem was boiled for some time in water, and the outer tissues stripped off. The bundles showed only an occasional join. Some of the bundles could be separated throughout almost the whole length of the piece, 2 in. There is no network of anastomosing. A join is effected by a branch few elements. A longitudinal face shows no anastomosing. Hence it must be concluded that the outer ring of bundles in Macropiper excelsum, at least, is only occasionally branched. Branching and forking of the bundle takes place at the nodes. Fig. 5. Bundle from stem, showing xylem scattered: a, phloem; b, cambium, c, xylem. X 150. P. Endlicheri. Fig. 6. Joining among central bundles in lowest internode seedling (see fig. 6A, b). X 44. M. excelsum. Peperomia Endlicheri. Peperomia Endlicheri is a rock-plant, less often an epiphyte. Its stem is succulent, its leaves store water. A cross-section through the stem shows on the outside an epidermis of narrowish elements. The cortex is succulent, and shows oil-sacs. The presence of oil is highly characteristic of the Piperaceae. Here, as in Piperaceae generally, the oil-sac consists merely of one of the cells of the cortex. Strasburger and De Bary both make reference to the ethereal oil found in Pipers; Engler says mucilage passes in the epidermis and cortex, but the walls of the passages do not differ from those of the cells around; the contents give a pungent odour to the plant. Scattered in the ground tissue of P. Endlicheri are bundles showing an arrangement as in many Monocotyledons. But in Peperomia, as Engler an others point out, the bundles have a cambium. In the New Zealand Peperomia, however, the activity of the cambium is at a minimum The amount of lignified tissue always remains small. There is no definite arrangement in this species in rings; whereas in some Peperomias there are rings. In Peperomia galioides there are two rings of five.

The petiole of P. Endlicheri shows one large central bundle and two smaller ones, one on each side of the central one. The leaf shows an epidermis consisting of several layers, as seen in many Peperomias. The mesophyll-cells are also large, contain but very little chlorophyll, and store water. Base of Macropiper. The base of M. excelsum is swollen, and stored with nutritive material (fig. 6A). This is shown even in young specimens. The nutritive material chiefly takes the form of starch, which is contained in the medulla and medullary rays, and also in the cortex. In the young branch coming from the base of the stem, the stem shows two rings of vascular bundles, as usual. But the sclerenchyma appears only on the inside of each bundle of the outer ring; there is no continuous band of sclerenchyma, though there are occasionally slight indications of extension of thickening between the bundles. The more usual number of bundles in the medullary circle in the youngest parts seems to be three. As one passes up the stem the sclerenchymatous elements gradually form a continuous band. Fig. 6A. a. Base of M. excelsum (x scars where stems have been broken off); b, young seedling, half life-size. Coming to the main stem, near the base in the region marked 1 in fig. 6A, b, there is a distinct corky layer outside with a collenchymatous band below. The structure of the rings is much the same as higher up the stem, but there is abundant starch everywhere. The bends of the sclerenchymatous ring, also, are here much deeper, and extend more into the interior of the stem. As one passes down to region 2 the sclerenchymatous ring gradually disappears, and sclerenchyma occurs only to the inside of each bundle extending over into the medulla in more or less club-shaped, rounded masses. The discontinuity of the sclerenchyma probably allows for the easy passage of food-material. Each of the central bundles is seen to be formed by the fusion of a bundle, or bundles, of the outer ring with a central bundle of the internode above. Fusion invariably starts at the xylem end, some bundles distinctly showing two masses of xylem at each side of a central line.

In region 3 the fusion of bundles of the outer ring with medullary bundles is traced, but the whole is gradually merged in the one ring typical of the structure of the subsidiary roots in M excelsum. Hypocotyl The details of transition in the hypocotyl in M excelsum were not at all rigid. There are six plumular traces, which are arranged on either side of the axis in threes. These appear to fuse together, the xylems first of all. A single cotyledonary trace enters from each cotyledon. On its entry the phloem is found in two masses on either side of the xylem; the elements travel towards the centre, and leave the protoxylem exarch, as in Peperomia amplexifolia, P. tithymaloides, and P. maculosa. The phloem masses ultimately fuse with the plumular phloem; as the xylem from the plumular traces moves also towards the centre a central plate of xylem is formed; and we find a typical diarch structure. But the plumular xylem may be very small in amount; so that, as in Piper cornifolium, the central plate is composed mainly of elements derived from the cotyledonary traces. But in one seedling examined the plate was composed mainly of elements derived from the plumular traces, while the mesarch elements from the cotyledonary traces decreased in amount. The plumular traces showed lignified tissue, and were not wholly meristematic, as in Piper cornifolium. Fig. 7. Transverse cotyledon, M. excelsum, passing through midrib: a, upper; b, lower epidermis; c, xylem; d, phloem, × 150. Figs. 8 and 9. Hypocotyl below entry of cotyledon-traces. X 150. Tiansverse petiole,. young plant of M. excelsum; b, dorsal; a, ventral side; c, bundle, × 44. Root—Macropiper excelsum. In M. excelsum the roots are adventitious. The primary root is very early lost. On uprooting the plant one finds it characteristically forked. Very often a seedling bearing only the two cotyledons has lost its primary root.

The structure was carefully examined. Secondary thickening is developed to a marked degree, and is especially noticeable near the base of the stem. The bundles form a ring, each separated by wide medullary rays, which are always densely granular. The medulla is also stored with starch. Sections through the base of the stem and the seedling show that the bundles in the root are continuous with those of the stem; the ring of the stem gradually passes over, into that of the root. The centripetal xylem in the root can be traced between the bundles, and sometimes occurs as scattered elements. Scattered elements often occur in the stem, to the sides of the bundle; so it would seem that there are traces of centripetal xylem in the stem. Again, in the peduncle, where one might expect to find ancestral traces, many of the bundles show small elements to the sides of the bundle. Fig. 11. Hypocotyl, showing endodermis. X 150. M. excelsum. Fig. 12. Shows splitting of xylem plate. × 150. M. excelsum. Fig. 13. Root, Peperomia: 6 protoxylems, c; b, cortex; a, piliferous layer, Fig. 14. Stem, Peperomia, longitudinal: a, annular vessels; b, secondary xylem;c, phloem; d, cortical tissue. X 150. Fig. 15. Stele from root, older plant; 8 protoxylems, c 8 phloem masses, b; a, endodermis. Small amount of central medulla. X 150. M. excelsum. Root—Peperomia Endlicheri. In P. Endlicheri, on the other hand, we find a small stele in the root, with 6(sometimes 6 to 8) protoxylems, but with very feebly developed xylem, so that there is no complete centripetal plate.

P. Endlicheri is a rock-plant, living on humus lodged in rock-crevices, or occurs as an epiphyte, but is never found on the ground. The roots are adventitious, and form mat-like masses at the nodes. In accordance with its habit, there is no need for great mechanical development, as in Macropiper excelsum; hence the root-structure is reduced. We may regard the structure seen in the root of Peperomia Endlicheri as a reduction from a type such as Mqcropiper excelsum (younger roots) in adaptation to environment. Conclusions. Several theories have been put forward as to the origin of Monocotyledons and Dicotyledons, and their relation to one another. One theory is that Dicotyledons have come from Monocotyledons, the connection being shown through the Araceae and Piperaceae; another that Monocotyledons have come from Dicotyledons; and, further, that Monocotyledons are diphyletic. The result of the present investigations on the New Zealand Piperaceae has been to lead to the following conclusions concerning the Piperaceae, and the relations between Mono- and Di-cotyledons: That Monocotyledons and Dicotyledons have a common ancestry, though the point of convergence lies probably far back in time, the habit and structure of Monocotyledons being adaptations from the dicotyledonous form to their peculiar conditions of life. That Peperomia is a reduced genus, compared with Macropiper; that the connection between Monocotyledons and Dicotyledons may perhaps be shown by an order such as the Piperaceae, where the one form, Peperomia, shows a reduction from the Macropiper form, the reduction being in the direction of Monocotyledons. That the Piperaceae are relatively an ancient family. They show a relation or resemblance to Aroids, more particularly among Monocotyledons; the key to the connection between Monocotyledons and Dicotyledons may perhaps be found in a connection between the Piperaceae and Araceae. Fig. 16. a. Seedling stem, showing sheath merging into stem; b, passing into lowest internode of seedling—leaf-traces passing in to outer ring, bundles passing in from outer ring to centre (diagrammatic). Hill, who “is in full accord with this view, that Peperoma is a reduced genus,” suggests “that the determining factor which has brought about

the reduction may be found in the epiphytic habit of many of the forms “; and later says, “Nothing has been said regarding the bearing of the geophilus habit exhibited by certain species of Peperomia” Macropiper excelsum comes very near a geophilous condition in having a swollen base stored with food-material; the adventitious roots near the base and for some distance from it are also stored with starch. The plant is sometimes tall, sometimes short and more shrub-like. Peperomia Endlicheri occurs both as a lithophyte and chasmophyte, less often as an epiphyte. Under these conditions it has succulent stems and adventitious roots, both stem and root showing reduced structure. Now, very many Monocotyledons are adapted to suit geophilous, epiphytic, aquatic, or saprophytic conditions. Fig. 17. Six steles passing downwards through lowermost internode of seedling (diagrammatic): a, four bundles just formed again in centre; b, c, central bundles joining to form two; c, d, e, central bundles move further and further outwards till they are finally merged in outer ring; e, slightly elongated, passing out to root; f, stele with, branch to root. All the steles are slightly eccentric with regard to rest of stem. Stem-structure. It has been shown (Miss Sargant) that in connection with a geophilous condition extra — fascicular cambium would first disappear; then the cambial zone, because the need for mechanical vascular tissue has disappeared. M. Queva has shown that a distinct cambial zone occurs with the bundles of some Monocotyledons, and traces of it in others. Now, in Macropiper excelsum, while there is no interfascicular bundle-formation, there is marked secondary thickening, necessary to its form. In some Peperomias the bundles are arranged in rings, as P. galioides. In Peperomia Endlicheri the scattered vascular system, and the fact that, though

a cambium is differentiated, still the amount of xylem formed remains constantly very small, point to reduction. As a general rule, among herbaceous, bulbous, &c., Monocotyledons the primary root disappears with the cotyledons. In M. excelsum there is early loss of the primary root, and great development of adventitious roots; there are numerous adventitious roots from the nodes in P. Endlicheri. The loss of the primary root is, in all cases, probably connected with geophilous characters. Among the Ranunculaceae, which are admitted to be primitive, Eranthus shows the primary root replaced in the second spring by a circle of roots. Leaf. The leaf-venation seen in M. excelsum somewhat resembles that seen in such Aroids as Zantedeschia, Arum. Professor Areschong has remarked that the linear leaves characteristic of most bulbous Monocotyledons are better adapted to push upwards through the soil than any dicotyledonous type; and that the bulbous plant seems in many respects the most highly specialized form of geophyte, its squat axis and pointed leaves, with broad sheathing base, being clearly adaptations to geophilous life. In M. excelsum the leaf is pointed, often sharply so, especially in very young plants. There is always a sheath to the petiole, which entirely covers the young bud or young leaf, and is clearly a protective organ. In P. Endlicheri the leaves are more pointed in the young plant than in the old; but here the epidermis is of several layers and stores water. Hypocotyl. Although details of transition in the hypocotyl are not rigid in the Piperaceae, they may still be of value. A similar type of structure has been found by different investigators in the Ranunculaceae, generally acknowledged to be primitive; in the Labiatae, Centrospermae; in all examined Papaveraceae, Capparidaceae, Resedaceae, Cruciferae; in Pinus maritima; and in many Monocotyledons. The same may perhaps in time be shown for further orders. Is it not possible, then, that the hypocotyledonary structure may be of phylogenetic value in showing a line of connection, or it may be common ancestry, for Monocotyledons and Dicotyledons ? It would seem, then, that Macropiper is a primitive form; Peperomia an advance with reduction; while Monocotyledons may have arisen as modifications and reductions of the dicotyledonous type, as more specialized forms, though earlier in time, perhaps, than the Piperaceae. The course of advance is, however, still shown by the relation of Peperomia to Piper, where the reductions arising in the former in response to environment are all in the direction of Monocotyledons, the monocotyledonous type most closely resembled being the Araceae. In the specialization and reduction of Peperomia we see tendencies which have become firmly established in Monocotyledons.