Weed (agachha) unwanted, troublesome, or noxious plant that grows profusely without being sown. Typical weeds are competitive and aggressive and have short life cycle with high reproductive capacity. Weeds are termed as narrow-leafed or broad-leafed. Grasses and sedges are the most common narrow-leafed weeds, which belong to Monocotyledons. The broad-leafed weeds are most diverse which belong to Dicotyledons. Sometimes weeds are grouped as aquatic, terrestrial, climbers, and amphibians. Almost all weeds are autophytes but few are also parasitic. Although weeds are commonly treated as unwanted to a certain time and space yet they can be used as medicinal plants, vegetables, fodder, fuel, and organic matters recycling in the fields.

Some common weeds

In Bangladesh about 350 species are recorded as weeds of cultivated field. The number of species in an area depends on the land use pattern and its ecological conditions. Of the weeds, about one-third plants are monocotyledonous and the remaining plants are dicotyledonous. Members of the family Poaceae, Cyperaceae, Leguminosae, Asteraceae, Euphorbiaceae, Amaranthaceae, Solanaceae Scrophulariaceae and Acanthaceae are common. The most successful and common genera are Cyperus, Lindernia, Eragrostis, Panicum, Cynodon, Hygropylla, Euphorbia, Phyllanthus, Leucas, Scoparia, Croton, Celosia, Alternanthera and Solanum. In aquatic conditions Eichhornia, Potamogeton, Pistia and Monochoria spp. are the most common weeds in deep water rice fields. Three species of ferns (Marselia, Ceratopteris and Salvinia) are also recorded as weeds of rain-fed rice fields.

Flowering and fruiting are observed in the months of February to April. Many of the seeds germinate in between May and October with a peak during the months of June to July. About 20% of the present weed flora have been recognized as naturalized exotic weeds. Among these mention may be made of Argemone maxicana, Alternanthera philoxeroides, Croton bonplandianum, Nicotiana plumbaginifolia, Lathyrus aphaca, Celosia argentea and Vicia angustifolia. [Mostafa Kamal Pasha]

Aquatic Weed Scrupulous growth of a plant species grown entirely or partly in waterbodies, and influences adverse physical, chemical or biological effects on aquatic ecosystem with resultant economic and aesthic losses. These aquatic plants may be algae, bryophyte, pteridophyte, and angiosperm, and are adapted to different habitats and environmental conditions. They may grow in all aquatic habitats and according to their mode of adaptation could be classified as planktonic, floating, benthic, submerged or emergent types.

In low-lying areas, haors, baors, beels, ditches where no crop cultivation is practiced, a large number of aquatic plants grow and form thick natural vegetation and sometimes these are also listed as weeds, although they are not. In such vegetations are found plants like Aeschynomene aspera, A. indica (Shola), Trapa spp. (Paniphal), Ipomoea aquatica (Kalmi), Hygroryza aristata, species of Cyperus, Eleocharis, Hydrilla, Utricularia, Sagittaria, Monochoria, Nelumbo, Nymphaea, Aponogeton, Potamogeton, Polygonum, Ottelia, Ceratopteris, and many species of grass and other families crowd together. Such 'weed' species are the sources of food and shelter of many aquatic birds, fishes and other animals including man. When any such species invade the crop-fields, they are termed as weeds.

Many submerged or floating species may cause settling down of the soil or silt particles carried by the water currents and hasten the process of sedimentation or siltation. water hyacinth and other submerged or emergent plants when grown in large quantities disturb the navigation and transportation. In humid tropics like Bangladesh rich macrophytes may grow in haors, beels, ponds or ditches and the primary productivity is known to be very high, but the trophic levels are not followed in regular sequence, because most of the nutrients are stored in the macrophytes, which are mostly of floating and emergent types. Under this situation no phytoplankton or submerged vegetation could grow due to cut off of the sunlight and lack of free nutrients. As these macrophytes are of perennial nature, the food-chain or food-web becomes restricted. Certain aquatic plants may also interfere with the water supply project to cities and towns. Many of the aquatic plants, either algae or angiosperms, harbouring many pathogenic bacteria on their body, may help in distributing these pathogens from one place to another and create health hazards. All such plants will be considered 'weeds'.

In some cultivated fields one or two other species like Ipomoea aquatica and Ceratopteris are allowed to grow along with the crop plants. Here, species are selected in such a way that they do not interfere with the crop plants for nutrition, light, etc.

There are certain aquatic plants which are apparently without any value and cause crop damage and other problems. For example, Oryza sativa (rice plant) fields are occasionally over-grown by O. rufipogon (Jhara or chita dhan) which causes crop failure.

The names of some selected aquatic plants are given below which are commonly regarded as weeds, although individually several of them might have some useful values.

List of aquatic weeds:

Some floating aquatic plants such as Azolla pinnata (an aquatic fern containing a blue-green alga Anabaena azollae) or certain filamentous blue-green algae (e.g. species of Aulosira,Tolypothrix, Scytonema, Nostoc, Cylindrospermum) can fix atmospheric nitrogen, thereby increase the fertility of the soil or water and improve the rice production.

Floating macrophytes like Eichhornia crassipes, Lemna perpusilla, Spirodela polyrhiza absorb different heavy metals and nutrient substances from highly contaminated waters. The first one absorbs Mn, Fe, Ca, and Zn released from paper mills and tanneries and also PO₄^-3 and SO₄^-2 from fertiliser industries and tanneries. duckweed can remove 50% of N and 67% of P and nearly all heavy metals like Al, Mn, Fe, Ti, Cu, Co, etc. However, these aquatic weeds may be effectively controlled by various methods, e.g. by mechanical means (by hand or machines), by chemical applications, by biological methods or by proper utilization and periodic exploitation of these weeds for various purposes (for biogas production, cottage products, or for synthetic products).

Not all the aquatic weeds are indigenous. Several weeds are exotic and some of them were brought to Bangladesh or adjacent countries for their beautiful flowers or for aquarium plant or for other purposes, but later on these escaped from the aquaria/ small reservoirs and ultimately spread all over the aquatic habitats, the best example is the water-hyacinth, a native of Brazil. Another S. American aquatic plant (Limnocharis flava) has been found in nature around Chittagong city. Other plants like Vallisneria spiralis, Lagarosiphon alternifolia, etc are also introduced to this country. Nowadays, various species or varieties of Nymphaea have been introduced.

It will be unwise to drastically eliminate the rich aquatic vegetations of the beels, haors, wetlands, etc in order to grow only rice or any other single crop. Loss of biodiversity of the aquatic plants (weeds) has been affecting the natural equilibrium. [AKM Nurul Islam]

Seaweed  Larger multicellular forms of algae which live mainly in marine coast lines. Some live in brackish waters and a few occur in fresh water habitats. The members of three groups of algae viz Phaeophyta (brown algae), Rhodophyta (red algae) and Chlorophyta (green algae) are commonly seen at low tide along rocky shores of Pacific, Atlantic, Indian and Northern oceans. They abound in shallow waters from the midtide line down to depths of about 50m. Along damp cold-water shores, they are able to withstand several hours of exposure to the sun, and they cover rocks high into the intertidal zone. In the tropics, seaweeds are confined to the sublittoral zone; red algae predominate, especially in lagoons and around coral reefs. Although the seaweeds may reach upto 65 m in length, they are primitive plants without true stems, leaves, roots or vascular systems. However, some giant seaweeds have complex internal morphology resembling vascular systems of higher plants. They remain anchored to solid objects by the holdfasts and absorb nutrients directly from the water, and also capable of manufacturing their food by photosynthesis. The pigments of red and brown algae mask the predominant green photosynthetic pigment, chlorophyll and probably aid in photosynthetic metabolism by absorbing and transferring light energy to the chlorophyll.

Kelps are brown algae which comprise the largest seaweeds viz Macrocystis and Nereocystis- they have structures which superficially resemble leaves and stems, also large air filled bladders and strong holdfasts. Red algae are abundant in clear tropical waters, where their red pigment (phycoerythrin) enables them to carry out photosynthesis at deeper levels than is possible for ordinary green algae. More than 70 different sea weeds (mostly brown and red algae) are used for food in oriental countries and fewer in the western world as well. In Japan, red alga Porphyra tenera is extensively cultivated on bamboo screens submerged in estuaries. Agar derived from red seaweeds, is consumed as a delicacy in the orient and is used as a laboratory medium for culturing microorganisms and tissue culture technology, in packing canned foods and as laxatives. Brown seaweeds are used as fertiliser and as an ingredient for livestock meal. Alginic acid derived from kelps has wide industrial use. It can be made into a silk-like thread or a plastic material, insoluble in water, that is used to make films, gels, rubber and linoleum; and as colloid in cosmetics, car polishes and paints. Organic derivatives of alginates are used as food gums in making ice cream, puddings, and processed cheeses.

Although Bangladesh has long seashore stretching along Cox's Bazar, seaweeds are found to grow only in st martin's island, in the northeastern part of the bay of bengal. They also grow in brackish waters in the sundarbans mangrove forest, and chakaria sundarbans. Here species of Enteromorpha, Ulva, Bryopsis, Caulerpa, Codium, Halimeda and Acetabularia are the common green seaweeds. Red sea weeds are mainly Derbesia, Liagora, Galaxaura, Euthora, Gelidiella, Gelidium, Jania, Amphiroa, Melobesia, Kylinia, Hypnea, Gracilaria, Catenella, Champia, Chrysymenia, Halymenia and Caloglossa. Species of Ectocarpus, Giffordia, Dictyota, Dictyopteris, Padina, Colpomenia, Hydroclathrus, Rosenvingea and Sargassum are common brown algae growing in St Martin's Island. Some members of blue-green algae are present but they are microscopic. [ZN Tahmida Begum]

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