In this article we will discuss about the classification of Santalales. According to Engler, Santalales consists of three families:- 1. Santalaceae 2. Loranthaceae 3. Balanophoraceae.
Family # 1. Santalaceae:
Santalaceae are herbs or shrubs, or small trees; hemiparasites. Leaves of Santalaceae are simple, opposite or alternate, exstipulate, entire. Inflorescence various, often flowers solitary and axillary.
Flowers of Santalaceae is bisexual or rarely unisexual, accinomorphic. Perianth of one whorl, sepaloid or, petaloid; segments 3-6, usually 4-5, valvate. Stamens as many as perianth segments and opposite and adnate to them; filaments short; anthers 2-celled ultimately 1-celled, and opening by vertical slits.
Nectar secreting disc is often present. Carpels 3-5, rarely 2, united to form a unilocular ovary; ovary inferior or semi-inferior or superior; ovules usually 3 but only one is fertile, pendulous from the upper part of the central placenta, orthotropous or anatropous, with 1 integument only. Fruit a drupe or an achene. Seeds without testa and with fleshy endosperm and straight embryo.
Plants of Santalaceae are semiparasites on the trees or on roots. Flowers are bisexual or unisexual and monoecious; but Osyris pyrularia is dioecious. In the pentamerous flowers of Leptomeria the ovary is five-locular at base becoming unilocular above.
In Anthobolus, Exocarpus, etc. ovule is solitary and basal. In Exocarpus the ovule shows no differentiation from nucellus and embryo sac, and the pollen chamber resembles that of Gymnosperms and therefore treated as a Gymnosperm by many.
Floral formula of a bisexual flower may be shown like this:
P(4-5) A(4-5) G(3-5).
A peculiar bent tube-like structure is formed by the elongation of the embryo sac at the chalazal region. This is called chalazal caecum which encloses the antipodal cells. It is a kind of haustoria in the embryo sac. The family consists of 26 genera and about 600 species occurring in the temperate and tropical countries of the world.
Only a few species are found in India, the Sandal wood tree—Santalum album Linn, growing in S. India while the rest are Himalayan. The Sandal wood tree is cultivated in India and Malaya while species of Buckleya and Pyrularia are cultivated as ornamental plants.
The Santalaceae family is closely related to the other families of the order Santalales, viz., Olacaceae, Opiliaceae, Octonemataceae, Grubbiaceae, Myzodendraceae, Loranthaceae and Bala- nophoraceae. Santalaceae appears to be more allied to Loranthaceae both having mucilaginous seeds and chalazal caecum in the embryo sac among other common characters.
Family # 2. Loranthaceae:
Loranthaceae are hemiparasitic herbs or shrubs with sympodial stem, attached to the branches of trees by means of haustoria; the growth of the stem is often dichasial, e.g. Viscum. Leaves of Loranthaceae is persistent, simple, coriaceous, exstipulate, entire, opposite or rarely alternate; sometimes leaves reduced to opposite scales which are united. Inflorescence axillary, usually a biparous cyme which may be of a mixed type, or flower solitary.
Flower of Loranthaceae are regular or almost so, bisexual or unisexual, monoecious or dioecious, epigynous; bracts present, often adnate to the pedicel. Perianth in 2 whorls with 2-3 segments in each whorl, all similar and petaloid or sepaloid, or in one whorl with 6 segments; segments free or connate in a tube arising from the rim or ring-like outgrowth of a cup-shaped receptacle. The inner members are valvate, the outer valvate or imbricate.
Stamens of Loranthaceae is as many as perianth segments, opposite and adnate to them; anthers, bithecal or unithecal, or transversely multithecal, dehiscing longitudinally, or by transverse slits or by apical pores.
Carpels 3-4, united to form a unilocular ovary; ovary inferior, with an indistinctly differentiated chamber; ovules several, indistinct on a large central placenta and only 1-3 attaining maturity. Fruit a pseudocarp or drupaceous, 1-3 seeded; seed mucilagenous, without testa, with a large straight embryo and fleshy endosperm.
The ring-like outgrowth of the receptacle is found in the tribe Loranthoideae where the perianth is uniseriate. This is called ‘calyculus’ Some authors consider that the calyculus is the remnant of the calyx while others think it to be the part of the receptacle. The embryo sac development is like that in Santalaceae; it grows to an elongated structure at the base containing the antipodals and forms the chalazal caecum.
The plants of Loranthaceae are parasites on the branches of the hosts but -the Australian genus Nuytsia consists of erect trees. The seeds in this genus have 3 cotyledons. A few species are leafless and the work of photosynthesis is performed by the green stem, e.g., Viscum articulation Burm. f. and species of Arceuthobium.
Arceuthobium minutissimum Hk. f. parasitising on Pinus excelsa Wall, is the smallest plant among the Dicotyledonae. This is found in West Himalaya. The ovary of the flower in this family remains apparently solid at first but later on a chamber is formed. In Lysiana the ovary is 4-celled.
The Loranthaceae family consists of over 1100 species under 30 genera and are distributed in the tropical as well as temperate countries of the world. The family is of no economic importance.’ Viscum album Linn, is Misletoe much used for decoration in European countries during Christmas time when most other plants are leafless.
Viscum articulatwn Burm. f., V. monoicum Roxb., V. orientale Willd., several species of Dendroptho, Scurrula, etc. are found in India.
Family # 3. Balanophoraceae:
Balanophoraceae are small leafless total parasites on roots of other plants; the plant-body consists of a tuber-like underground rhizome; scale leaves seldom present.
The inflorescence is a dense-flowered spike often a more or less globose head; the spike is formed within the rhizome and ultimately comes up breaking through the rhizome; the cylindrical axis, or the peduncle is covered by sheathing bracts; the inflorescence is a panicle in the genus Sarcophyte.
Flowers of Balanophoraceae are unisexual, actinomorphic, monoecious or dioecious. The staminate flowers with one whorl of perianth; segments 3-4, valvate. Stamens as many as perianth segments and opposite to them; anthers 2-8-chambered, opening by pores or slits.
Pistillate flowers with or without perianth. Carpels 1-3; ovary unilocular, inferior; styles 1-2; ovules 1-3, without integument, sometimes reduced to embryo sac and often united with ovary wall.
In Balanophora and some other genera the gynoecium is merely a slender emergence. There is a single archesporial cell without a proper placenta. The fruit is a nut with one seed containing oily endosperm and a small undifferentiated embryo.
The Balanophoraceae family is closely related to the other families of the order Santalales. Reduction in the development of the gynoecium to some extent is noticed in all these families and extreme condition is attained in the family Balanophoraceae.
The family with about 120 species under 18 genera occurs in the tropical countries of the world. A few species of Balanophora are found in India in the forests and savannah formations. Rhopalochnemis phalloides Jungh of E. Himalaya and Assam has a tuberous root stock as big as a man’s head. The family is of no economic importance.