Notes on reproductive biology of parasitic angiosperms

Post date: Jul 28, 2014 4:32:53 PM

The families Loranthaceae and viscaceae represent a specialized assemblage of parasiting angiosperms known popularly as Mistletoes or Witches brooms. These groups of plants include genera Dendrophthoe, Macrosolen, Scurrula in the first family and Viscum, Taxillus in the second respectively. These have taken to a highly specialized mode of life not seen in other parasitic flowering plants. These plants parasites certain other Angiosperms as partial stem parasites.

These are obligate parasites and in order to lead their normal life they should always have close association with their host plants. They start their life as tiny seedlings and once they are in contact with their hos the seedlings develop a special type of root which is actually their primary root. This root very soon grows into a sucking root called haustoria (parasitic roots) which enters the stem of the host plant and establishes contact with the Xylem and Phloem of the host vascular tissue and starts drawing water and nutrient minerals which will be flowing in the host tissue. The parasite thus draws the valuable water and minerals from the host grows luxuriously and many times it mimics the host in ins growth and habit so well that it will be difficult tell them apart [Examples: Dendrophthoe falcate on Sapota, Dendrophthoe trigona on Banyan tree].

The parasites produce the usual flowers normal to them. The flowers are pentamerous (having 5 sepals and 5 petals and the usual stamens and ovary). The petals are all united and have a tubular shape. They will normally be unopened first. It was formerly thought that certain nectar loving birds help in the pollination of the flowers by carrying their pollen to flowers of other plants. However it is now known that the birds do not directly participate in effecting pollination but aid indirectly. These birds which love to gather nectar from the flowers actually puncture the base of the flower where in nectar would have accumulated around the base of the ovary. The birds having a long and sharp beak (e.g., Sunbirds) very dextrously puncture and make a hole at this place and draw the nectar). This act of puncturing actually makes the flower petals open with a force and this force is so violent that the pollen powder of the flower is ejected and falls of the receptive stigma of the ovary of the same flower. Thus the flowers are self pollinated and once the stigma is dusted with the pollen of the same flower. It will not be receptive to any pollen from other flowers as earlier thought. The pollination naturally leads fertilization and so. The ovules now start to develop into seeds. The ovary becomes fruit and gradually ripens into mature fruit whose outer wall becomes the fruit wall and when fully ripe, there will be a single hard seed surrounded by juicy pulp just inside the wall.

Different types of birds such a Red-whiskered Bulbul, Barbets and Flowerpeckers instinctively known that the fruits containing the juicy pulp are tasty and so love to eat the fruits. These birds regularly visit the plants and pick the fruits and eagerly gulp the entire fruits. However the pulp of fruits is very sticky and when the birds bite the fruit the sticky fruit with it seed become attached the beak and is so firmly stuck to the mouth of the bird that it does not fall off easily.

The birds with such seeds fly to other locations where the same parasites are abundant. The birds carrying these seeds feel irritated and try to get rid of the seeds by rubbing their beaks against the rough bark of a new host plant, and so these seeds get transferred to fresh host plants and once the seeds come in contact the bark of host plant start germinating into new parasite seedlings and the seeds soon send their primary roost in the form of haustoria into the host tissue and establishes contact. So in this way new host plants become infected with the parasite.This is the method (Bird aided) of how the parasiting plants are dispersed.

Author: KB Sadananda