Are plants that live on other plants parasites? Not really, right? Especially in the tropics, you can hardly find a tree on which there isn’t a growth of another kind of plant ranging from tiny mosses via larger ferns to proper seed-bearing species. However, such ‘epiphytes’ (as these species that are using a bigger individual as a support to grow on are called) may only weaken their host by being too numerous or by becoming too heavy. They can also affect their host by intercepting some rain water and shadowing some of the host plant’s leaves and/or by providing shelter to insects and other arthropods that can be foes as well as friends. However, as long as they do not sink their own roots into the host plant’s body, they are not removing anything from their host. And that’s different in species belonging to the genus Viscum, commonly known as the “mistletoe”.
Mistletoes are referred to as hemi-parasites, because although they tap into their host’s vascular system with special adaptations known as the “haustoria”, they also retain in their leaves the green pigment chlorophyll that allows them to photosynthesize, combining CO2 with water to create carbohydrates. They remove from their hosts water, minerals and some nutrients and in nature are only found growing on trees. They get there because of their seeds, which are contained in berries that are consumed by a variety of birds that digest the fruit flesh, but not the seeds.
Groups of plants with the collective name “witchweed”, namely Striga spp. and Orobanche spp., contain species that are more truly parasitic than the hemi-parasitic mistletoe. These plants are said to have got the name witchweed, because they seem to bewitch their hosts before they themselves are visible: the hosts which can be monocots like corn, sorghum, rice, sugarcane, but also dicots like legumes, sunflowers, etc. exhibit stunted growth and weakened stems as the young Striga and the related Orobanche seedlings attack the roots of their host plants. Not only do they act as sinks for the carbohydrates that are produced by the host plant’s leaves, they also cause a significant reduction of the host plant’s essential cytokinin and gibberillin hormones and absorb trace elements like potassium, phosphorus, sulphur and iron. Species of both genera as well as numerous other root parasitizing plants, e.g. of the genera Thesium and Rhinanthus, produce flowers above ground, but while Striga spp still grow some green leaves, Orobanche spp do not. Interestingly, seed germination in these species occurs only in the close vicinity of host roots and it is known that the seedlings respond to minute quantities of chemicals like purines and flavonoids released by the host’s roots.
Undoubtedly the most spectacular and most famous fully parasitic plant is Rafflesia arnoldii, a species that occurs on the island of Borneo. It has no leaves, no stem, no green parts at all and not even a root system. Once in a while it produces an enormous, flesh coloured five-petalled unisexual flower just above the ground from a spot where the Rafflesia with its haustorium (inserted into its host’s root system) can withdraw all the nutrients it needs to produce flower and seeds. The flower, which is considered the largest flower of the plant world, can measure one metre in diameter and weigh 5 kg or a little more, but it opens for only maximally a week.
Probably too huge for any ‘ikebana’ display and certainly far too smelly, it is still a greatly admired inflorescence. It is said to possess the putrid smell of a corpse, a rotting cadaver, in order to attract flies to have its pollen distributed to female flowers nearby. Although Rafflesia arnoldii produces the largest flower of all plants on Earth, there are at least two dozen more Rafflesia species in Southeast Asian countries like Thailand, Malaysia, Indonesia, the Philippines, etc. with still impressive but slightly smaller inflorescences. To see a Rafflesia flower in the wild must be a sight and in order to find one -if you happen to visit places where it occurs- I suggest to follow your nose. It might lead you to one, but don’t be surprised if what you find is not exactly what you hoped to find!
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2019.
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