Tuatara: Volume 14, Issue 3, December 1966
Unusual forms of Secondary Thickening in New Zealand Trees and Shrubs
Unusual forms of Secondary Thickening in New Zealand Trees and Shrubs
TheVascular Cambium, which is responsible for secondary thickening in Angiosperms, must be one of the most familiar meristems. However, there are a number of variants of it, some of which are less familiar. As several of these occur in native New Zealand plants, it may be useful to bring together information about them.
The Storeyed Cambium
In most cambia the initials when first formed are arranged irregularly and their future increase, by oblique division and intrusive growth, serves to emphasize this random arrangement. In some plants, however, increase of the original initials is by vertical walls, with little, if any, intrusion of cells between one another. This process leads to the formation of horizontal blocks of initials which are known as storeys. The derivatives of such cambia may retain the storeyed arrangement, or some or most of the elements may lose their regularity as they mature. New Zealand woody plants with storeyed cambia include the species of Hoheria (fig. 1), Plagianthus, Corynocarpus, Sophora, Carmichaelia, Chordospartium, Corallospartium, some species of Senecio and Olearia and probably other members of the Compositae (see Carlquist 1960 and 1962).
Cambium in Medullary Bundles
Vascular bundles that run in the pith may include a cambium which augments the xylem and phloem of these bundles. The increase is usually slight as the confined position does not permit indefinite extension. A New Zealand example, Macropiper excelsum, has been recorded by Balfour (1958).
Successive External Cambia
Fig. 1: Storeyed cambium in Hoheria angustifolia.Fig. 2: Wood of Avicennia resinifera, showing bands with included phloem. Fig. 3: TS of stem of Pyrosforgia venusta (not native to New Zealand), showing four sectors of ‘interrupted’ cambium. Fig. 4: TS old stem of Tetrapathaea tetrandra with six sectors of ‘interrupted’ cambium.
The Interrupted Cambium
In lianes belonging to several unrelated families, the mass of secondary xylem comes to be broken into longitudinal strands by a modified action of the cambium. The cambium arises in the normal position and functions normally for a longer or shorter time. The best known examples of this type of cambium occur in the Bignoniaceae and for the sake of clarity it will be described with reference to Pyrostegia venusta (Ker-Gawl.) Miers (fig. 3) although this plant is not native to this country. The greater part of the cambium continues to function normally, but short arcs come to produce xylem very slowly. These arcs, therefore, lag behind the outer edge of the woody cylinder. The furrows produced in the wood in this way are filled with secondary phloem produced in great abundance by the outer sides of the anomalous arcs of cambium. In this way the outer surface of the stem remains entire and unfurrowed. It will be appreciated that this mode of thickening results in the outward movement of phloem tissues relative to the xylem which forms the edges of the furrows. This shearing movement disrupts cells, so that there is a discontinuity in the stem tissues along the sides of the furrows: this can be seen clearly in the section photographed in fig. 3.
This type of anomalous secondary thickening was recently described for some members of the Passifloraceae in America (Ayensu and Stearn, 1964) and Africa (Obaton, 1960). Examination of old stems of the N.Z. Passion Flower, Tetrapathaea tetrandra (Sol.) Cheesem., revealed the incipient stages of this anomaly (fig. 4: Studholme, 1966).
Persistence of Medullary Rays
In very few woody plants do the interfascicular portions of the cambium continue to produce indefinitely secondary tissue which is distinct from that of the fascicular cambium. A fine example of a woody shrub with this feature is provided by Macropiper excelsum Forst. (Balfour, 1958). Here the medullary rays which separate the stem bundles in the primary state are perpetuated as large rays running vertically throughout the internodes. This condition is more frequent though less striking in herbaceous plants. It also occurs in some lianes as, for example, Clematis. In this genus the interfascicular portions of the cambium, which produce the secondary parenchyma that extends these larger rays, are so ill-defined that the cambium appears to be confined to the fascicular sectors.
The Uni-Directional Cambium
The secondary thickening of Heimerliodendron brunonianum (Endl.) Skottsb. is distinctive (Studholme & Philipson, 1966). The cambium of this plant is not related to the primary vascular bundles as is the case in a normal dicotyledonous stem, because it arises by divisions of cortical cells. From this meristem secondary xylem and phloem are derived. However, the action of this lateral meristem differs from that of a normal cambium because the phloem as well as the xylem mature to the inner side of the meristem. The banded appearance of the mature stem is due to the alternate production of lignified cells (fibres and vessels) and of parenchyma, in which the phloem strands are embedded (fig. 5) The phloem forms in an anomalous manner: cells within the meristematic zone divide to produce groups of randomly orientated cells which mature to form the phloem elements. There are, therefore, no files of derivatives from the outer face of the cambium, as in the normal formation of phloem. This type of secondary thickening is typical of other woody members of the Nyctaginaceae, and also of the Amaranthaceae and Chenopodiaceae and Phytolacca (Balfour, 1965). It has also been reported in a few South African composites and one Australian member of the Stylidiaceae (Philipson and Ward, 1965). Few of the N.Z. Chenopodiaceae have been examined by us but Salicornia, Sueda, Atriplex, and Chenopodium all have a undirectional cambium.
The Cambium of Monocotyledons
Fig. 5: Wood of Heimerliodendron brunonianum showing bands of parenchyma with included phloem. Fig. 6: TS stem of Cordyline australis with cortex at the left and successively older secondary bundles towards the right.
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Balfour, E. E., 1958. The development of the vascular systems of Macropiper excelsum Forst. II. The mature stem. Phytomorphology, 8: 224-233.
Balfour, E. E., 1965. Anomalous secondary thickening in Chenopodiaceae, Nyctaginaceae and Amaranthaceae. Phytomorphology, 15: 111-122.
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Obaton, M., 1960. Les lianes ligneuses à structure anormale des forěts denses d'Afrique occidentale. Ann. Sci. Nat. Bot., ser. 12, 1: 1-220.
Philipson, W. R. and Ward, J. M., 1965. The ontogeny of the vascular cambium in the stem of seed plants. Biol. Rev., 40: 534-579.
Studholme, W. P. Studies of the anomalous cambia of selected dicotyledons. University of Canterbury, N.Z. M.Sc. thesis.
Studholme, W. P. and Philipson, W. R., 1966. A comparison of the cambium in two woods with included phloem: Heimerliodendron brunonianum (Endl.) Skottsb. and Avicennia resinifera Forst.f. N.Z. Journ. Bot. (in press).