My Science School

            Fragrance in flowers, fruits and spices is due to a wide variety of essential oils (volatile liquids) present in them. They are mostly insoluble in water but freely soluble in alcohol, ether and vegetable mineral oils. They are not oily to touch.

            The oils may be grouped into five classes, according to their chemical structure alcohols, esters, aldehydes, ketones, lactones and oxides.

            The fragrance may be in leaves (as in sage, thyme and mint), in bark (as in cinnamon and cassia), in wood (as in cedar and sandalwood), in flower petals (as in rose and violet), in seeds (as in anise and caraway), in roots, in fruit rind (as in orange) or in resinous gums secreted from the tree (as in camphor and myrrh).

            The oils are formed generally in the green parts of the plant, and with plant maturity, transported to other tissues particularly to flowering shoots. The exact function of an essential oil in a plant is unknown – it may be to attract insects for pollination, or to repel harmful insects, or it may simply be a metabolic intermediate.

            Dr. Palaniappan of Pudukkottai, TN, writes: aroma associated with cinnamon, vanilla and cuminum are due to carbonyl group of aldehydes and ketone. Aromatic aldehydes such as cinnamaldehyde and vanillin are found in cinnamon and vanilla respectively. Cumaldehyde (p-isopropyl benzaldehyde) is found in the volatile oil of cuminum. Aliphatic esters namely methyl n-butyrate and ethyl n-butyrate are found in apples and pineapples. Benzyl acetate, an aromatic ester imparts fragrance to jasmine. Spices and condiments contain monoterpenoids with two isoprene units and sesquiterpenoids with three isoprene units. Eugenol in clove, linalool in coriander, zingiberene in zingiber, menthol in mint, cineol in cardamom and anethole in feoniculum are a few examples. Sandalwood contains a terpenoid called santol in the wood cells.

            Plants use many methods to distribute their seeds and succulent fruits such as lemons are not necessarily designed to be eaten. Many, such as blackberries and plums, are bitter until the seeds are ‘ready, while others, apples and many tropical fruits are designed to encourage pecking by birds, which scatters the seeds. Another group including figs and senna pods encourage animals to eat the fruits without digesting the seeds, allowing the seeds to pass undamaged through the animals, be it a mouse and elephant.

Yet other fruits remain unappetizing to animals until they drop to the ground, where they are eaten or scattered when fully rips or rotten.

Lemons come into this category, though it is possible that monkey, baboons or other animals may be fascinated by the bitterness and attack them earlier, giving  themselves an unexpected dose of vitamin C and, of greater benefit to the tree, subsequently spitting out the pipe. 

            Many citrus trees that are natives of and regions have sour fruits to discourage animals from eating it. The flesh of a lemon is there for three main reasons: to add weight so that it will roll a long way after it falls from the tree. To dissuade foraging animals from eating the seeds before they can develop and to supply water and nutrients as the flesh rots around the germinating seeds. The main aim of any seed is to propagate the species, not to feed the local animals. Animals benefit only as a side effect of plants wanting to use them as a form of transport for their seeds.

            The trouble with citrus fruits is that they have been cultivated for so long that nobody knows what their original seeds dispensers actually were. In cultivation, however, they do seem to be eaten by monkeys. May be monkeys like acid tastes more that more than people do. Many tropical fruits are dispersed by becoming over ripe, falling to the ground and being eaten by animals. May be the acid in citrus fruits was meant to act as a deterrent to these foraging animals  so that the fruits and the seeds the contained were left to grow where fell. 

            Plants can prevent pollution of environment in many ways. However, the answer is restricted to prevention of air pollution by trees.

            The major components of atmosphere are nitrogen (78.08 per cent) and oxygen (20.95 per cent) (major) with minor components are argon and carbon dioxide (0.0314 per cent) and many trace elements such as neon, helium, nitrous oxide, methane, carbon monoxide, sulphur dioxide, ozone, ammonia and aerosols (colloidal sized particles) are also present.

            The ratio of these components is changing very fast due to increased human activities like fossil fuel burning, afforestation and changes in land use. They result in the liberation of tones of carbon dioxide, carbon monoxide, methane and aerosols into the atmosphere. The server human interference over the last century is said to have strained the buffering capability of nature.

            Trees help reduce the pollution in more than one way. First, they act as sink for carbon dioxide. Through photosynthesis they synthesize carbohydrates using carbon dioxide, water and sunlight. This way thousands of tonnes of carbon dioxide are trapped by the trees. By the same process, trees release oxygen, which is needed by other living organisms. They also help in cooling of the atmosphere by transpiration, a process in which water is given up by plants as vapour. I addition, aerosols and dust particles (components of atmosphere pollution) settle on the dense foliage of trees. Thus trees, especially the tall ones with dense foliage around houses and industrial establishments, reduce aerosol and dust pollution by acting as barriers or curtains.

   Bitterness is cucumber and other cucurbitaceae vegetable is due to the presence of compounds called cucurbitacins. Chemically these are tetra-cyclic triterpenes having high oxidative levels. They occur in nature as free glucosidesor as complicated mixtures, at high concentrations, in fruits and roots, for example in a wild variety of cucumber called Cucumis hardwikii.

            High temperatures above 92 degrees have been implicated in the increase f bitterness in fruits, although there is no evidence to support this. Conversely more bitter cucumbers are seen growing during the cooler growing season.

            Betacyanin is a group of phenolic pigment present mainly in eight plant families. Betanin is one type and is present in Beta Vulgaris beetroot. Betanin occurs in hydrolyzed with sugars as betanidin, a reddish pigment in beets.

            Structurally this pigment is not a vitamin or provitamin or a cofactor to act as an extrinsic factor in the formation of blood cells. But recent studies have shown many phenolics and flavonoids present in the plant kingdom have anti-oxidant properties and prevent free radical reactions from taking place in our body.

            Free radical reactions like free oxygen, superoxide, peroxide are one cause for the ageing of cells. These phenolics and flavonoids scavenge the oxidants and prevent the free radical reaction and hence stop the ageing of cells.

            Since Betanin present in beetroots is also a phenolic compound might have these anti-oxidant properties and stop free-radical reaction and prevent ageing of our body cells.

     In plants which show secondary growth the outer appearance of the stem differ in different species f plants. This difference results from the manner of growth of the periderm, the structure of the phellum and the nature and amount of tissue that are separated by the periderm from the stem.

            The periderm consists of three parts: the phellogen which is the cork cambium, the phellum which is the cork produced centrifugally by the phellogen and phelloderm which is parenchymatous tissues produced centripetally by the phellogen.

            In trees which produce successive periderms by the formation of successive phellogens up to the depth of phloem, there will be many cork layers.

            All the cork layers together with cortical and phloem tissues external to the innermost phellogen are termed rhytidome. In such trees the colour of the stem will be dark brown and never green.

            In plants like citrus, eucalyptus, acer and acacia the development of periderm commences only after the production of the secondary vascular tissue has reached considerable dimensions. In such cases the circumference of the epidermis increases together with secondary and other tissues on the outer side of the cambium.

            In viscum cork tissue is never formed and the epidermis increases in circumference and persists on the stem throughout the life of the plant. In all these plants stem surface looks green even after secondary growth.

            In plants like solanum, guava, pyrus and nerium the first phellogen is formed in the epidermis itself and iln plant like populous, jugulans and ulmus the first periderm is formed in the outer most cortical layer next to the epidermis.

            In such cases the subsequent periderms are not formed to the full circumference of the stem similar to the formed one. But they develop in the form of scales. So in these plants absence of well marked rhytidome give the stem a green appearance even after the secondary growth.

            The timing of leaf loss varies with species, site and season. Day length and temperature are the two triggers for colour change and leaf loss.

            The timing is usually species-specific but is also related to site conditions. For example, a fairly dry season would result in some trees leaves dropping early, before they had turned, in a reaction to the drought stress; leaves may also die on the tree but hang on until much later. Species variations are also important. Norway maples normally have green and fully functional leaves that keep on photosynthesizing until two or three weeks after leaves of sugar maples have turned. If both are on a cramped site, Norway maples, with extra weeks of energy storage, may outgrow and outlive sugar maples.

            Oaks keep their leaves much longer than many other species because a layer of cells that forms where the leaf stem is attached, called the abcision layer, does not form a complete barrier. In the beech trees, which are in the same family, an incomplete layer is seen in younger trees, but mature beeches, 25 to 30 years old, form a complete layer. There are also sex differences; leaves of female ginkgo trees usually colour and drop earlier than those of males. And trees near street lights may be affected by the longer light exposure and keep their leaves longer.

    The red portion in the stem of cane is due to a fungal disease called red-rot caused by the organism Glomerella tucumanensis. The organism attacks during the conidial stage (imperfect stage) when it is known as Colletotrichum falcatum.

            The pathogen infects the host mainly through the leaf scars at abscission or immediately thereafter, enters the parenchyma, grows intracellularly in the early stages, and forms an intercellular mycelium in the later stages. The fungal hyphae penetrate the host’s cell wall during the progressive stage of the disease forming minute penetration pegs. These pegs expand to the normal hyphal diameter immediately after reaching the other side of the cell wall. This mechanical pressure causes the dissolution of the tissue. Thus the tissue dissolution is not due to enzyme action, but due to mechanical pressure.

            But hydrolyzing enzymes are produced at a later stage when the tissues begin to die and the pathogen grows on the dead cells of the host, that is, in the saprophytic phase of the fungus. Only at this time reddening of the stem vascular tissue occurs followed by the formation of lysigenic cavities. At this stage when the affected canes are split open, the tissues of the internodes which are normally white or yellow-white will become red in one or more internodes usually near the base.

            The reddening is conspicuous in the vascular bundles and progresses towards the pith. When such diseased shoots appear in the field, secondary infection is caused by conidia which are produced in aierouli (asexual reproductive bodies) and transmitted through insects, wind and water.

 

         

 

 

 

 

 

  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.

         

 

 

 

 

 

  Most of the mushrooms have a cap called pileus and a stem called stipe. The cap on its underside consists of gills which bear the spora producing structures. Important to the identification of a species are the properties of cap, the shape and colour of the gills, the way in which they are joined to the stem, presence or absence of sheath, scales and annulus ring etc. The most poisonous mushrooms are species of Amanita which come under the family Amanitaceae and the most delicious edible mushrooms are species of Agaricus (Button mushroom) which come under the family Agaricaceae. In general the fruit bodies of Amanita species can be distinguished from the Agaricus species by the following characters.

In Amanita species the pileus on its upper surface bears the scales and the stipe bears at its base a sheath called Volva. These scales and sheaths are absent in the fruit bodies of Agaricus species.

Volvariella (Paddy straw mushroom) is also having Volva at the base of the stipe as in Amanita. But it is an edible mushroom and also commonly cultivated. The Oyster mushroom namely Pleurotus is another edible one. This can be identified by its stem at the side of the cap and gills on the under surface of the cap. Boletus (Penny bun mushroom) and Lactarius (milk cap) are also edible members which grow in wild condition but not cultivable. Among these Boletus can be identified by its dense layer of tubes instead of gills on the underside of the cap.

The familiarity in distinguishing the poisonous and nonpoisonous mushrooms is needed only when we collect the wild fungi from the field for our diet table. But this problem will not arise in the case of edible fungi which are cultivated for this purpose.

   Ripening may be regarded as a special case of sequence. During ripening, a number of enzyme-assisted reactions take place inside the fruits. The list includes softening of tissues, hydrolysis, changes in pigmentation, flavour and respiration rate, and conversion of carbohydrates and organic acids into fruit sugars. These changes are induced by ethylene which is also called as ripening hormone.

            It has been found that during ripening, ethylene production goes up. In fleshly fruits like mango significant amount of ethylene may be present some time before ripening, but the fruit’s response to ethylene is inhibited till the fruit is harvested. In banana, a presumably effective ethylene concentration may be present in the unripe fruit, but the fruit is insensitive to that concentration at that stage. Only as it matures, it becomes sensitive and begins to ripen.

            Generally, an ethylene-forming mechanism and breaking of the insensitiveness to ethylene are attained only fruits reach a certain physiological age.

            When unripe fruits are kept inside a sack or tin of rice, the time needed to attain this critical physiological age is shortened. It could be that the fruit is totally cut off from light which promotes yellowing. (It is not known whether there is any increase in the temperature of the fruit.) The ethylene produced in the fruit also diffuses rapidly through the fruit’s tissues.

            If the fruits are placed in an airy place, this ethylene may be immediately lost in the air. When confined in rice or sack, its flow is restricted and there is always a layer of ethylene surrounding the fruit which accelerates ripening.