Category Biology

         

 

 

 

 

 

  The bipinnate compound leaves of Mimosa pudica, touch-me-not plant, have a swollen base called pulvinus which has two distinct halves. The lower half below the vasular strand is made of thin walled parenchyma cells with larger intercellur spaces and the upper half has slightly thick walled parenchyma cells with a few small intercellur spaces.

            Under normal conditions, the cells of both the halves remain turgid. When the touch stimulus reaches the pulvinus the osmotic pressure in the lower half of pulvinus falls. As a result they release water into the intercellur space and become flaccid. But the upper half maintains turgidity the pressure excerted by which causes the leaves to drop down.

            The leaflets also have similar swollen bases but are smaller and are called pulvimules. The touch stimulus is first perceived by these pulvimules. Here also the process occurs which results in the folding of the leaflets. When the stimulus is passed on to the stalk base the entire leaf droops down.

            The touch-me-not plant shrinks within a few minutes of being touched. This is due to the loss of turgidity by cells within the pulvini-specialized motor organs at leaf joints. Upon stimulation the leaf cells lose a potassium ion which causes water to leave the cells by osmosis. It takes about 1 o minutes for the cells to regain turgidity and the leaflets to open out.

  Crotons are ornamental plants grown for their variegated leaves. The different coloured patches in these leaves are due to the presence of chromoplasts in the leaf cells. Chromoplasts contain coloured pigments, other than chlorophyll, which can reflect or transmit light, or both.

            The colour of a pigment depends on its selective absorption of certain wavelengths of light and its reflection of others. Carotenoids are a group of red, orange, and yellow pigments and contain many catalytic members. Some carotenoids act as accessory pigments in photosynthesis, transferring the light energy they absorb to chlorophyll for conversion to chemical energy.

            Chemically, pigments fall into a number of minor groups, arbitrarily divided into 2 major groups. The first group comprises pigments that contain nitrogen; it includes chlorophyll and dark coloured pigments called melanin.

            Related to melanins are the indigoids, of which the well known plant pigment indigo is an example. Riboflavin, also known as vitamin B12, is one of a number of pale yellow to green pigments produced by several plant groups.

            The second group is formed of pigments without nitrogen. Carotenoids are members of this group, as are the important plant pigments called flavonoids. In leave, flavonoids selectively admit light wavelengths that are important to photosynthesis, while blocking out UV light, which is destructive to cell nuclei and proteins.

            Bright colours are produced by the conversion of colour less flavonoids, called flavonols, into coloured forms, called anthocyanins. Quinones provide many yellow, red and orange pigments.

            Venus fly-trap, an insectivorous plant, normally grows in swamps and moist soils characterized by lack of sufficient nitrogen (as nitrates). Their root system is also not so well developed. As a result these plants tend to trap insects and ‘digest’ them to augment their nitrogen supply. These carnivorous plants do not have any special mechanisms or honey secretions to attract insects but only modified leaf traps (Dionaea muscipula), vase-like leaves (Nepenthes Khasiana), leaf hairs having glue on their tips (Drosera) and leaf surface having a sticky coating (Pinguicola alpina) to trap them. In Venus fly-trap plant, the two halves of the leaf blades can swing upward and inward as though hinged.

            Inside the hinged portion of each leaf are several long trigger hairs. As the insect walks along the leaf surface and touches these hairs, it stimulates a hydraulic response in the leaf-cells and makes them lose water rapidly. This causes the leaves to close. Long projections along the leaf margins help in trapping the insect.

            Once an insect is trapped, digestive enzymes are secreted by the hairs which ingest the insect and absorb the contents. After a meal, the trap opens again only after several days. Generally each modified leaf is used to trap only 3-4 insects before it falls.

            These plants also have chlorophyll by which they can photosynthesis to cater to their energy (food) requirements. Hence these plants are not obligatory carnivorous forms. But they can grow exuberantly to produce flowers and seeds, if insects are available, as they supplement their nitrogen supply.

           Plant tissue culture (PTC) is the art and science of multiplying plants or plant parts (such as organs, tissues, cells, pollens, sores and embryos) under controlled conditions of light, temperature and humidity in an optimal nutrient medium under aseptic conditions in a glass vessel.

            Even a single cell has the potency to perform all the metabolic activities to form an independent plant. This phenomenon is known as totipotency. This is successfully used in tissue culture. Infinite number of plants can be produced from single explants in a span of time, irrespective of natural conditions. The seedlings will be genetically ‘true copies’ of the mother plant. That means, genetic purity can be maintained as far as required, in every seeding, which is almost impossible in conventional means of propagation.

            PTC has been successfully tried in almost all plant varieties. Some plants like orchids produce millions of non-endosperm us seeds in a single fruit, which cannot be cultivated in natural conditions. When grown through PTC, they have more than 75 percent germination. In the medium in which the plants are cultivated all the required micro and macro nutrients and vitamins are added. Since the growth inside the glass vessel is heterophic, a carbon source in the form of sucrose is added to the medium. PTC has been commercialized and is a lucrative business.

            In PTC, a very small tissue from a parent plant called as explants is placed in a test tube in a nutrient medium. The tissue may be taken from any part of the plant, that is, root, stem, leaf, anther or embryo. This is because all plant cells possess totipotency meaning a single cell can give rise to an entire plant.

            The nutrient medium used in tissue culture consists of sucrose apart from mineral salts and vitamins. Plant hormones such as Auxins are used to help growth and cell division. The solidifying agent, agar makes the medium semi solid otherwise the culture is done suspension. The inoculated tubes are kept in an incubator to maintain sterile conditions and controlled temperature and light. After 2-3 weeks of incubation an irregular mass of cells called callus develops, which on sub culturing gives rise to small plantlets. These are potted and maintained in a green house and subsequently transferred to the field. PTC is aimed at engineering crop plants for good traits.

Abscission is a physiological process whereby plants shed a part, such as leaf, flower or fruit, and retard their vegetative growth. It is a survival mechanism adopted by plants to live through adverse conditions and is promoted by a plant hormone called abscisin (also abscisic acid) produced by leaves and fruits. Extreme temperatures limit the metabolic activities such as respiration, of plants. Such a reduction consequently necessitates only a low level of photosynthetic activity.

The reduction in the requirement of energy can be to such a level which could be got from the photosynthesis activity of a few green cells, present in the terminal regions, after all the leaves fall.

Sometimes, in winter, ice crystals begin to form in the extra- cellular spaces and the viscosity of the cell protoplasm increases. To counter this, compatible osmotic such as betaine (an alkaloid) begins to build up. This process, osmo-regulation, helps plants to overcome the stress due to frost.