My Science School

The main difference between trees and shrubs is their size. Trees are much taller than shrubs and their root balls (system of roots with attached soil that supports the health of the tree) are also bigger. Unlike the thin and flexible stalks or stems of the shrubs, trees have thick stems to bear the load of the heavier and bigger leaves, branches, and fruits. Since trees are taller, they need to transport water from the soil much further up than the shrubs. This is done by several thin hair-like pipes in the stem and the branches.

The chloroplast inside the green leaves of plants is the ‘workshop’ where photosynthesis takes place. Water is supplied to the leaves by the roots, and carbon dioxide  is absorbed from the air through tiny pores on the lower side of the leaf. Both these are broken down into the elements hydrogen (H), carbon (C), and oxygen (O) with the help of chlorophyll and sunlight. Plants build ‘glucose’ (grape sugar) from these elements and oxygen is released in the atmosphere as a by-product. Photosynthesis has been the source of oxygen in the air since the origin of life on the Earth.

 

Unlike animals, plants produce their own food and except for a few carnivorous plants, do not eat other organisms. Green plants use water, nutrients, and a green matter in their leaves called ‘chlorophyll’ to produce their food. Normally water and nutrients are absorbed from the ground through their roots. Some plants, however, have developed other methods for obtaining water and nutrients. Many tree-dwelling plants make funnel-shaped ‘containers’ with their leaves to catch water. Carnivorous plants use their digestive juices to convert the insects, caught in their folding or sticky traps, into nutrients. 

Almost all plants and animals need air, water, light, and nutrition to grow. Green plants use a complicated chemical process, called photosynthesis, to create energy for survival and growth. Since plants create their own energy, they are the first link in the food chain. Animals need to drink water and eat plants or other animals to get energy. Therefore, they are present at the next step in the food chain. There are some animals, like camels, that can survive for days without drinking water. Since they live in deserts with very little water, they have adapted to the special conditions of their habitat. 

Most of the beverages contain alkaloids which act as mild stimulants. Caffeine, for example, is found in coffee, tea, cocoa, milk chocolate and also in cola drinks. Tea has a trace of theophylline. Cocaine is found in products. These alkaloids are collectively referred to as methyl xanthenes. They share a number of pharmacological actions of much therapeutic interest: they relax, smoothen the muscles (notably bronchial muscle), and stimulate the central nervous system and cardiac muscle. They induce the kidneys to excrete more urine, stimulate mental activity, and quicken the reflexes, increase vigilance and decrease motor reactions time in response to both visual and auditory stimuli. They increase stamina and reduce fatigue. They give the users a feeling of confidence and power. They even induce euphoria in some users. These stimulant effects are short lived: they last for about an hour. They are then subsequently followed by depression. Overdose or repeated use may lead to paranoia, psychosis.

            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.