PLANT GALL

Plant Galls:

Plant galls are external expression of mysterious interaction between two biological entities namely the plant and insect. The gall inducer, which are phytophagous parasites are dependent on the plant tissues not only to feed on, but also for an ideal niche to live in during a short duration of their life-cycle.  During the feeding or ovipositing efforts of the animal parasites on the plant organs, a series of action-reaction system is established between the parasites and their host plants. A compromise is reached at the end of the processes and a structure called “Gall” is produced by the plant organs.  The gall thus produced provides quality nutrition as well as safe domain to the gall-inducers and at the same time, the plants are able to localize them in space and time, thus the gall-inciting agent is physically and physiologically isolated and is driven to extreme specialization with respect to its various biological activities.

 

Insect-induced plant galls were known to humans for long, mostly for use as drugs and for extracting ink-like material used in writing and painting. Until the early decades of the 19^th century, those who studied galls and their inhabitants referred to these plant abnormalities as galls only. Friedrich Thomas first used the term ‘cecidium’ in 1873, deriving it from kekis (Greek), which means ‘something abnormal with an oozing discharge’. Consequently, the study of galls came to be known as Cecidology. One significant name in cecidology is Alessandro Trotter (1874-1967).  He founded Marcellia, a journal dedicated to cecidology, in 1902, which serviced science untiles the 1980s (Raman, 2019). 

 

Galls arise on all classes of plants, for example algae, mosses, ferns, gymnosperms, dicotyledons and monocotyledons. They may arise on underground and aerial roots, shoot axis, petioles, stipules, leaf blade and leaf veins, vegetative and flower buds, inflorescence axis, bracts, flowers and fruits. South Indian forests present more varied flora than any other tract of equal area in India, possibly in the World, a phenomenon due to the combined effects of its geographical situation and its topography (Gamble, 1935). South Indian forests comprise of five main floristic regions, namely, Sal region, Dekkan region, Semi-desert region, Wet region and Alpine region. These floristic regions are characterized by distinct floras and faunas, which are highly influenced by specific ecological factors.

 

The shola forests are mostly evergreen with shotbold and branchy trees. Trees very rarely exceed 6 m height. The crowns are usually very dense and rounded. Branches are often seen clothed with mosses, ferns and other epiphytes. Woody climbers are also quite common and canopy layers are difficult to distinguished. Western Ghats have almost a continuous physical extension close to the West coast from Nilgiris to south Kanyakumari of Tamil Nadu, but the Eastern Ghats are not only rather irregular in their extension and altitude but also are, further away from east coast of Tamil Nadu and extend between Salem, Dharmapuri, Coimbatore.  North and South Arcot districts.  The highest peaks of Western Ghats in Tamil Nadu are Dodabetta ( 2637 m) and Anaimudi (2695 m). Whereas the average height of the Eastern Ghats may be taken as about 600 m only. Nature has endowed Eastern and Western Ghats of Tamil Nadu with impressively diverse plant taxa.

 

Dispersal of plant galls whether passive mode or active mode, is greatly influenced by geographical routes or barriers. Routes i.e. plants favour dispersal and provide access to new areas.  But the barriers i.e. insects limit dispersal of galls. So the plant galls biogeography is restricted and limited to certain areas only. Dispersal, population size and reproductive potentials are parts of ecological valence of the plants or animal. The ecological valence is a measure of the reaction of the species to the environmental influences.  For the galls the ecological valence is narrow and they are stenotypes, occur under only well defined, limited set of conditions, such a species is too specialized to occur elsewhere. Even if a plant gall successfully overcome a barrier and has reached a region, it can actually established itself only if the conditions are favourable. Unfavourable conditions of temperature, humidity, rainfall and other climatic factors may wipe out the galls.  The continued existence of galls is then possible in an area is depend on thermal constant which directly influences development, growth of plant, breeding cycles of insects etc.  For example the galls in shoal forests cannot colonize area like dry-scrub forests (Mani, 2000).  In biogeography of plant galls the plants and the insects cannot be isolated from each other. They together represent an inseparable complex or community with interdependent, hierarchial levels of states and divisions of functions.  Together with non-living background thus complex makes an ecosystem, in which all members are bound together in an intricate food-chain.  So the distribution of gall insects is thus closely bound up with that of its specific plants.  Plants constitute an essential part of the natural habitats, with which all gall insect life is inseparably bound up (Mani,1964). 

 

Galls are extremely varied in size and appearance and in their location on the plant body.  They vary in consistency from relatively soft to hard and woody; many are brightly coloured.  The gall surface may be smooth, knobby, velvety or spiny. A gall, consisting exclusively of erineal outgrowths, is known as filz gall. The abnormal cell growth and multiplication, are confined wholly to the epidermal cells, thus giving rise to emergent galls of undifferentiated, excessively elongated hairy outgrowths called erineum. When the cecidogenetic centre lies in the subepidermal layers of the cortex of shoot or root, it is known as rinden galls. When cecidogenesis is on the shoot axis or petiole, the plant tissue grows around and above, so as to enclose the cecidozoen that lies initially exposed on the surface of the plant.  Such galls are otherwise called covering growth galls.

 

The simplest leaf galls are fold galls, in which the unfolding of the leaf rudiment of the developing bud is suppressed.  In the leaf roll galls, the blade rolls upward.  Many complex leaf galls are sessile or stalked, more or less swollen pouch-like outpockteings, mostly on the upper surface of the blade.  These constitute the pouch galls.  Arresting of the elongation of internode, results in a rosette-gall.

 

The flower galls, range from simple crumpling of sepals and petals to complex fusion and swelling of sepals, petals, stamens, pistil and even bracts into a composite fleshy or hard mass, in which the individual floral parts cannot be recognized.  Some galls are hollow and enclose one or more, smaller or larger gall cavities, containing the cecidozoa.  In atriate galls, there is an accessory space or atrium in addition to the gall cavity proper.  The gall cavity is lined by small, closely packed mass of cells, rich in cell contents and they constitute the nutritive tissue of a gall.

 

A special type of abnormality, of a rather distinct kind and which may be classified with organoid galls, is fasciation. These are cases, where a normally cylindrical or radially symmetrical plant part becomes flattened and elliptical in cross section to form ribbon like sometimes ring-like structures.

 

3. Plant Gall Research work done in India and elsewhere:

Man’s knowledge of plant- galls dates back to 17th century. Greek Philosophers like Hippocrates, Theophrastus, Pliny and others have called attention to the plant galls from the pharmacological stand point. In Europe, the scientific study of galls began with Marcello Malpighi who published a book, De Gallis in1686. Later, many naturalists like Kuster, Cosens, Cook, Wells, Kendell, Houard, Hough and others contributed wealth of information to enrich our knowledge of plant galls.In India, Prof. M.S.Mani initiated the scientific studies on galls which paved way for many researchers to undertake the cecidological research for Ph. D, degree. Two monographs published by Mani, (Ecology of plant galls, 1964. Plant galls of India, 2000) are time honoured treatise on cecidological science.

Ref: M.GOPI (2020) Plant Galls-A Biological Puzzle. In Sarkar A.K. (Ed) Organisms and Environment. Educreation Publication