In this article we will discuss about:- 1. General Features of the Casuarinaceae 2. Position and Affinity of the Casuarinaceae 3. Number and Distribution 4. Commonly Occurring Plant 5. Economic Aspects.
General Features of the Casuarinaceae:
Habit — Evergreen shrubs or trees.
Stem — Woody, grooved, jointed, verticillate.
Leaves — Whorled, scale-like, linear or lanceolate, united at base to form a toothed sheath.
Inflorescence — Male flowers in spikes and females in spherical heads.
Flowers — Unisexual, perianthless, standing at axil of a bract and surrounded by 2 bracteoles.
Androecium — Stamen 1 (in male flowers), central; anthers 4-celled, basifixed, dehiscence by vertical slits.
Gynoecium — Carpels 2 (in female flowers), ovary superior, originally bilocular but ultimately unilocular by suppression at anthesis; style 1, very short; stigmas 2, linear.
Fruits — 1-seeded winged nut, enclosed by woody bracteoles to form a dry cone-like structure; embryo straight; endosperm absent.
Position and Affinity of the Casuarinaceae:
The Casuarinaceae is an isolated taxon under the Apetalae. It was thought by Engler as the most primitive of dicots, constituting the A, shoot with male inflorescences and fruits; B, portion of male inflorescence showing two whorls; C, diagram of a whorl of male flowers ( p, perianth; a, bracteole; b, bract); D, male flower before elongation of filament; E, female inflorescences with protruding stigmas; F, female flower (a, bracteole); G, diagram of female flower; H, fruit; I, lower portion of fruit with L.S. of seed (r, radicle; c, cotyledon); J, portion of L.S. of ovary showing functional ovule and course of pollen tube; K, T.S. of a branchlet (J and o, inner and outer rings of bundles; s, sclerenchyma; p, palisade tissue). (A, after Poisson; C, D and G, after Engler; J, after Treub; B, E, F & H-K, after Rendle).
Verticillatae:
It also formed the eighth family of Rendle’s Monochlamydeae and the first family of Core’s Archichlamydeae.
At first, Bessey (1897) took the Casuarinaceae as a member of the Sapindales; later (1915), he made it look like a “leafless representative of and derived from the Hamamelidaceae within the Rosales”. Hallier (1905) treated it as a part of his Amentacees. Anatomically, according to Tippo (1938), the family illustrates a highly advanced element originating from the Hamamelidaceae.
Hjelmqvist (1948) held the view that the family is not a component of the Amentiferae, though it is a primitive dicot. The primitiveness of the family is ascribed to the presence of apparent catkin-like male inflorescence, simple anemophilous flowers and large rayed wood. Studying the wood anatomy of 30 different species of Casuarina, Moseley (1948) concluded that the family is “moderately rather than highly specialized”.
This is evident from the lack of libriform fibres and predominance of simple perforation of vessels (which are thin walled and circular in outline), metatracheal xylem parenchyma and alternate vascular’ pitting. Cytological data indicate the heterogeneity in the number of chromosomes in the different species of Casuarina.
Hutchinson (1959) derived the family from Hamamelidales, considering it as a climax family. Eames (1961) favoured a relationship of the Casuarinaceae with the Betulaceae. This is based on the similarity of pollen structure, two trace stamen character and other floral aspects.
Lam (1961) went to the extent of removing Casuarina from the angiosperms to be included in the protoangiosperms along with Ephedra, Welwitschia and Gnetum. The monotypic genus Casuarina provides a rather mysterious chemical correlation in producing biflavonyls—compounds atypical of angiosperms but similar to many conifers, ginkgo and cycads.
It has an extremely low relative proportion of syringyl subunits, found only in certain conifers like Podocarpus and not in angiosperms. While taking into account the natural affinities of Casuarina, it is difficult for a taxonomist to ignore these interesting chemical features of the plant. Whether such chemical parallelisms form valid phylogenetic links are far from clear.
Cronquist (1968) opined that there is nothing obvious in the floral or vegetative morphology that cannot justify derivation from a generalised fagalean ancestor, but entertained the possibility that the similarities reflect parallel changes from a common ancestry in the Hamamelidaceae.
Chanda (1969) stated that the Casuarinaceae is a stenopalynous family with little pollen morphological variation: pollen grains betuloid, aspidote, oblate to suboblate, three porate (sometimes four to five porate). On the basis of palynotaxonomy involving 40 species and subspecies of Casuarina, he asserted that the genus is neither primitive nor a derivative of the hamamelidaceous ancestors. There is also a striking palynological resemblance between the Casuarinaceae and Juglandaceae.
Hutchinson (1969) conceived the Casuarinaceae “as an extreme reduction adapted mostly to dry climatic conditions, but with no very relations still living. The cone-like infructescence resembles that of the Pandanaceae in the Monocotyledons, but should not be regarded as more than a parallelism, nor has it anything to do with the cones of the Pinaceae, the resemblance being entirely superficial”.
The habit of Casuarina recalls that of Ephedra in the branches bearing alternating whorls of scale-like leaves. In both genera, the male and female flowers are simple and reduced. The equisetoid appearance of the branchlets of Casuarina is also remarkable, but other dissimilarities are so basic and many that no concept of a connection between the genus and Equisetum can be upheld.
Number and Distribution of the Casuarinaceae:
The Casuarinaceae is a monotypic family, containing a single genus (Casuarina) with about 65 species. It is mainly developed in northeast Australia but also distributed in Fiji, New Caledonia, Malaysia and Mascarenes Islands. “Casuarinas”, in the words of Humphries (1978), “are well adapted to very dry habitats in the region of high temperature and low rainfall and are thus remarkably xeromorphic.”
Commonly Occurring Plant of the Casuarinaceae:
Australian Pine or Beef-wood Tree (Casuarina equisetifolia J.R. and G. Forst.) is a big tree, usually planted in avenues.
Economic Aspects of the Casuarinaceae:
C. equisetifolia is grown on the coasts of India in order to reclaim sandy dunes. Valuable timbers are produced by C. cunninghamia and C. stricta. Various species yield fine grained wood suitable for furniture, gates, shingles, staves, turnery and wagons and the bark of some is utilised for tanning.
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