ADVERTISEMENTS:
In this article we will discuss about:- 1. Characters of Loranthaceae 2. Distribution of Loranthaceae 3. Affinities.
Characters of Loranthaceae:
Aerial parasite usually, cup-shaped receptacle; inferior ovary and the absence of distinct ovules.
A. Vegetative characters:
ADVERTISEMENTS:
Habit:
Herbs, perennial, aerial parasitic or semi-parasitic on trees.
Root:
Modified adventitious roots in the form of haustoria, Nuystia (W. Australia) rooted in earth.
ADVERTISEMENTS:
Stem:
Herbaceous, soft, branched, branching dichotomous, swollen nodes.
Leaf:
Simple, opposite, entire, exstipulate, thick and leathery, ever-green, often reduced to mere scales.
B. Floral characters:
Inflorescence:
Flowers are solitary or in racemes and cymes.
Flower:
Hermaphrodite or unisexual, e.g., Viscum (when so, plants dioecious), actinomorphic or slightly irregular, often brightly coloured, dimerous or trimerous, apetalous, epigynous with cup-shaped or disc-like receptacle.
ADVERTISEMENTS:
Perianth:
Biseriate, the two whorls similar and 2-3 merous, both green and sepal-like (Viscum) or both large, brilliantly coloured and petaloid, e.g., Loranthus (no apparent differentiation into calyx and corolla), an irregular rim, called calyculus, is sometimes present below the perianth and is interpreted as calyx.
Androecium:
Stamen 4—6, opposite to the perianth members, and borne on them (epiphyllous) or at their base; anthers 2-celled, dehisencing by longitudinal or by transverse slits or by pores (in staminate flowers a rudimentary pistil may be present).
ADVERTISEMENTS:
Gynoecium:
Carpels 1-3 or 5, syncarpous, inferior, unilocoular, ovules not differentiated from placenta, basal placentation, style simple or absent, stigma 1 and often sessile.
Fruit:
Berry or drupe, often viscid.
ADVERTISEMENTS:
Seed:
Fleshy endospermic.
Pollination:
Entomophilous.
ADVERTISEMENTS:
Floral formulae:
Distribution of Loranthaceae:
Mistletoe family consists of 36 genera and 1300 species, mostly distributed in tropical and temperate regions. About 10 genera and 52 species have been reported from India.
Affinities of Loranthaceae:
ADVERTISEMENTS:
The family is closely related to Santalaceae but can be distinguished by its aerial parasitic habit; cup-shaped receptacle; inferior ovary, lacking a clearly defined locule and the absence of distinct ovules.
The Loranthoideae and Viscoideae show great variations in the floral structure. Moreover, the mode of development of the embryosac, endosperm and embryo and location of the viscous part of the fruit are different in the two sub-families.
ADVERTISEMENTS:
The Loranthoideae contains bisexual or unisexual flowers generally associated with calyculus. The ovary is characterised by a collenchyma pad and contains a mamelon which may or may not be lobed.
The viscid zone of the fruit is located outside the vascular bundles of the corolla. Other features peculiar to the Loranthoideae include the Polygonum type of embryosac, protrusion of tip of the embryosac into the style, formation of composite endosperm and vertical division of the zygote.
The Viscoideae contains only unisexual flowers where the calyculus in invariably absent. The viscous layer of the fruit is situated within the vascular bundles of the perigone. The bisporic embryosac with a slow but steady curvature, endosperm developing from a single embryosac and embryo without a suspensor are unique to the Viscoideae.
ADVERTISEMENTS:
In view of the above differences, Johri and his associates suggested that the Loranthoideae is distinct from the Viscoideae. These differences, according to them, warrant the elevation of the status of the sub-families to that of families – Loranthaceae and Viscaceae.
Common plants of the family:
1. Arceuthobium minutissimum Hook, f., parasitic on the twigs of Pinus wallichiana, is the smallest known dicotyledonous plant, known in the world, is endemic in India.
2. Dendropthoe falcata – Parasite on mango and other trees.
3. Viscus album – True Misletoe- was used as ceremonial plant by the Druids and other early Europeans.
4. Loranthus cordifolius – Abundant on Oak (Quercus dilatata) in Giri valley, Central India.
Division of the family and chief genera:
The Loranthaceae is divided into two sub-families:
Sub-family I. Loranthoideae:
Flowers bisexual or unisexual. Calyculus present. Examples: Loranthus, Struthanthus, etc.
Sub-family II. Viscoideae:
Flower unisexual. Calyculus absent. Examples: Arceuthobium, Viscum etc.
