An alien plant also referred to as exotic, introduced, foreign, non-indigenous or non-native, is one that has been introduced by humans intentionally or otherwise through human agency or accidentally from one region to another. An alien plant that has escaped from its original ecosystem and is reproducing on its own in the regional flora is considered a naturalized species. Those naturalized aliens that become so successful as to spread in the flora and displace native biota or threatens valued environmental, agricultural or personal resources by the damage it causes are considered invasive In India there are a number of invasive weeds which have caused havoc with native biodiversity (Koh li et al., 2006). It is very important to know and understand their biology, ecology and factors imparting invasiveness in order to manage them.

      Ageratum conyzoides (family; Asteraceae) is one such rapidly colonizing invasive alien species that has become a troublesome weed over a wide range of ecosystems in tropical and subtropical countries (Batish, 2008 ). It is one of about 30 species of the genus Ageratum, all of which originated in America (Okunade, 2002). The literal meaning of Ageratum is non-ageing (referring to the longevity of its flowers or of the whole plant), with its origins in the Greek word ageras, whereas the species name conyzoides is derived from konyz, the Greek name for Inula helenium L., which it resembles (Kissmann and Groth, 1993). The English names of the plant, goat weed or billy goat weed, derive from its peculiar odour like that of male goat (Okunade, 2002). Johnson (1971) divided the taxon A. conyzoides into two subspecies, i.e. latifolium and conyzoides, of which latifolium is found throughout the Americas whereas conyzoides has a pantropical distribution.

The plant Ageratum conyzoides is now found as a weed of over 36 crops  in 46 different countries . It has been ranked as 19th of the world’s worst weeds (Holm et al., 1977) and 15th most troublesome weed of South-east Asia and the Oceanic Pacific (Waterhouse,1993).

In India,  the weed A. conyzoides has been reported as existing prior to 1882 in The Flora of British India . It was probably introduced as an ornamental plant in the 1860s (National Focal Point for APFISN, India, 2005), later attained a weedy habit and turned harmful to mankind. Its invasion and spread has caused ecological havoc to indigenous floristic composition in various regions of India, including the north-west Himalayas, eastern Himalayas, central India and Western Ghats (Yoganarnarasimham, 2000; Silori and Mishra, 2001) .

A. conyzoides has become an invasive weed in a wide variety of natural and manmade ecosystems ranging from forests and grasslands to farmland. It is a prolific ephemeral herb with great morphological variations and easily adapts to different ecological conditions (Sauerborn and Kock, 1988). The weed has been recognized as one of the most serious pests of modern agriculture, one that has encroached on almost all other types of lands, such as wastelands, natural forest, crop fields,  orchards, range/grasslands, riparian zones, ruderal/ disturbed sites, shrub/shrub lands, tea orchards, vegetable garden, meadows, water courses, fresh landslides or areas with deep gullies and wetlands, and also along roadsides (Kunwar et al.,  2001; Sankaran, 2007; Batish et al.,  2009a, b; GISD, 2010). Hence, This research work is aimed to provide a holistic perspective on  morphological and anatomical variations among A.conyzoides  accessions selected from four different geographical regions of Tamil Nadu, India..

Materials and Methods

Adult plant specimens of Ageratum conyzoides L., were collected from different eco-type regions such as 1.Coromandel coast (plain and agricultural field); 2.Pachaimalai (moderate altitude); 3.JawaduHills (excessive rainfall), and 4.Gudalur (high altitude),) in Tamil nadu, India (Fig.1) for assessing the morphology and anatomy of the species. 

Transverse sections of fresh leaf, petiole, stem, root and nodal region of stemwere cut into small pieces for taking free hand sectioning then soaked in distilled water for a day and warming subsequently to expel air bubbles. For the epidermal studies the peels of fresh as well as preserved leaves were used and the sections were applied withsafranin, fastgreen, double staining procedures and permanent mounts prepared using Canada balsam and studied under a microscope. All the sections were photomicrographed.

The sections were transfered to safranin stain till itattains dark red colouri.e. 10-15 minutes. They were then passed successively through 30%, 50%, 70%, 90% and absolute alcohol. In each dilutions were kept for one minute. Just when still visibly over stained, the sections were transferred to fast green in clove oil. Here the section loses the red stain rapidly. At the critical moment it was shifted to absolute alcohol and xylol (1:1) solution. In this solution detaining is arrested and the colour of the safranin would be fixed over the section. Then the section was treated with pure xylol twice. Finally the sections were transferred to mountant Canada balsam on a slide and cover slip was placed over it and set aside to dry Johansen et al,(1940).Anatomical observations and photographic documentation were made with a light microscope (model AX70TRF, Olympus Optical) equipped with U-Photo system.Descriptive terms used for the anatomical features are asgiven in the standard anatomical reference books.

Foliar epidermal morphology was investigated in all four A.conyzoides accessions with following details. Shape, Size, type and nature of the stomata. Epidermal appendages like hairs, trichomes etc. and  Structure of epidermal cells. The epidermal peels of both upper and lower surfaces of the leaves were made mechanically. These peels were stained with 2% aqueous safranin solution, mounted on 5% glycerin solution and the slide for observations. The collection process and slide preparation was done following the standard methodsprescribed by Shah (1968), Shah and Kothari (1973), Hickey (1973, 1979), Leelavathiet al. (1980), Sharma (1990) and Croxdale (2000). Finally the prepared slides were observed under compound microscope (15x40x).

The shape, size, type and nature of the stomata have been studied.  And their subsidiary cells and epidermal cells were also been assessed. Number of stomata and epidermal cells were counted and stomatal frequency and stomatal indexes were calculated by using the following formula.

           Where S= Average number of stomata and E= Average number of epidermal cells in the same unit area (Salisbury, 1927). The length and breadth of stomata were measured with ocular micrometer under high power magnification (15x40x). Thirty readings for each parameter per species were taken. The mean values were calculated, statistically analysed and the results were tabulated. Microphotographs of both upper and lower epidermal peels were taken with the help of Olympus Camedia C5060 model microphotographic camera. Terminologies for different epidermal structures as proposed by Van Cotthem (1970), Leelavathi and Ramayya (1983), Croxdale (2000) have been followed. The stomatal index and frequency were calculated by the formula proposed by Gupta (1961) and Salisbury (1927) respectively.

Results and Discussion

             A. conyzoides. generally  it is erect branching some times prostrate  and it  grows to approximately 2-5 ft. in height in Jawadhu hills  and 1-3 ft from Coromandel coast. The young stems are often red and pubescent; leaves are opposite,o vate or rhombic-ovate, acute at the apex, acute to obtuse or sub cordately rounded at the base, rarely cordate, the margins crenate. Pubescent on surfaces, glandular dorsally, 2 – 10 cm long and 1 – 6 cm wide., aromatic or unpleasant smelling, The stems and leaves are covered with fine white hairs .The petiole is pubescent, 1 – 5 cm long; It has a shallow tap root system.

    The flowers  are small, pretty  at the top of its hairy stems The inflorescence is pale  purple and white head, respectively for Jawadhu hills  and Coromandel coast and flowers are  less than 6 mm across and arrange in close terminal corymbs of 8-15 heads. Involucres are campanulate, the bracts are 2-3 seriate, linear, sub equal, acute, sparsely pilose outside; corollas are all tubular, 1-1.5 mm long, the limb 5-cleft.    In both the accession, the fruits are linear-oblong black achene  with 5-angled and are easily dispersed via air water and soil. The pappus is 5 short triangular scales, which are often serrate below ending in a long awn. Transverse sections (Fig.-2 & 3) of A. conyzoides through the internode of the stem shows following arrangement of the tissue from periphery towards center. Outer most layer, single cell layered thick, compactly arranged, without any intercellular spaces, rectangular outline, thick walled; outer surface cuticularized; hypodermis

            Petiolar anatomy shows the arc shape, adaxial side concave, abaxial surface convex. Epidermis: Uniseriate. Cortex: Compose of 1-2 layers of polygonal angular collenchyma cells and 11-14 layers of polygonal parenchyma cells. Vasculature: Vascular bundles, 3-5, bicollateral, arranged in form of an arc. Pith: Consists of thin walled parenchyma cells. Trichome type: Simple, uniseriate and multicellular. Rootanatomyshows following arrangement of the tissue from periphery to centre Epidermis: Outer most layer, single cell layered thick, compactly arranged, without any intercllulal spaces, rectangular outline; cortex present below epiblema.

         Vascular bundles were  conjoint, collateral and closed. Xylem is present towards the upper epidermis, while the phloem towards the lower epidermis. Vascular bundles are surrounded by a compact layer of parenchymatous cells.  Protoxylem vessels are present towards the upper epidermis.  Phloem consists of sieve tubes, companion cells and phloem parenchyma. Phloem fibres are absent.  Xylem consists of vessels and xylem parenchyma. The morphological and anatomical variations observed in the plants  might be determined by the edafic and climatic factors of the  invasive alien species  A.conyzoides.

Acknowledgement

 Authors convey their sincere thanks to University Grants Commission, New Delhi, India for providing financial assistance under Major Research Project Scheme (F.42-954/2013(SR), Dt.;14.03.2013) Authors are also thankful to the, Prof  and Head, Department of Botany, and authorities of Annamalai University, Annamalai Nagar, Tamil Nadu, India for providing good infrastructure facilities during the course of work.

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