The various metabolic activities also result in the formation of some substances (products) which are not useful for the plants. Plants do not have any mechanism to excrete such substances. Hence, they get stored in the various parts of the plants such as leaves, fruits, bark, seeds, dead cambium etc. These substances are useful for human beings such as alkaloids, gum, resins, tenins, latex, mineral crystals etc.
1. Alkaloids – They are nitrogenous excretory substance of the plants which are insoluble in water but soluable in alcohol. They are found in the leaves, fruits, seeds & bark. They may be bitter or poisonous and some have medicinal importance. Some of the alkaloids are –
- Quinine – Bark of the cinchona tree – used for treatment of malaria.
- Morphine – Papaver somniferum (opium poppy) – used to relieve pain and to induce sleep.
- Nicotine – Leaves of tobacco plants (Nicotiana tabcicum) – used to relieve pain and to induce sleep.
- Atropine – Roots of atropa plants (Atropa belladonna) – Medicine for eyes and stimulates nervous system.
- Reserpine – Roots of Raulphia serpentine – used as antihypertensivc and antipsychotic drug.
- Theine (tea caffeine or theophylline) – leaves of tea plant (Camellia sinesis) – used as stimulant.
- Taxol – Tctxus brevifolia (Gvmnosperm) – used as anticancer.
2. Gum – It is formed by degradation of cellulose of plants cell walls. Insoluble in alcohol but party soluble in water. It is extuded as sticky substance that on contact with air form homogenous crystalline structure. The stems of Acacia (babool or kikar) and Prosophis cineraria (Khejadee) commonly extude gum.
3. Resins – Resins are the derivatives of carbohydrates which are fonned by the oxidation of odourous oils. They are used in the formation of varnishes and paints. Canada balsom and Asafetida are also resins. Asafoetida is obtained from the tap roots (rhizome) of Ferula assafoetida plant. It is used as flavour enhancer (spice) and in many medicines. latex of some plants like Euphorpia, Oak, Kaner (The Vetia) can be used to make petroleum.
4. Tannins – They are bitter in taste. They are derived from cell walls, cell fibres, bark, leaves, raw fruits of many types of plants (Gymnosperms & Angiosperms). They are used in making medicines, and in leather & ink industries.
5. Latex – It is a white to yellowish brown and dense liquid. It is extuded by many types of plants (200 species) particularily Hevea brasiliensis, Ficus elctstica. The composition of latex differs from plant to plant. It is a mixture of resin, gum, stored food & other carbohydrates. The latex yielding plants are also called as petroplants. The latex of some plants like Euphorpia, Oak, Kaner (The Vetia) can be used to make petroleum.
6. Mineral Crystals – In some plants, inorganic wastes are found in the cell wall, vacuoles, plant marrow etc. They are normally crystals of silica, calcium carbonate, calcium oxalate etc.
Deposition of silica occurs on the leaves of plants of gramini family. The crystal of calcium carbonates are found attached as a bunches of grapes to the cell wall of fig and rubber plants. These crystal are called as cystolith.
Crystals of calcium oxalate are found in the plant marrow in various shapes of many plants like water hyacinth (Jal Kumbhee), Colocasia root (Arbi), Jimikand (Yams). Pistiya has needle-like crystals called raphides. In papaya, these crystals are round in shape and called spheriphides. In atropa plants, these crystals are found in the form of small granules on the leaves.
