4 Natural Factors Controlling the Plant Life

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Climate is one of the important natural factors controlling the plant life. Its study is called climatology.

The climate includes the following main factors:

(1) Light

(2) Temperature

(3) Precipitation and atmospheric humidity

(4) Air and atmosphere

1. Light:

Light is the most essential abiotic factor without which no life can exist. It is non-lethal limiting factor both at the maximum and minimum levels. The chief natural sources of light are sunlight, moonlight, star light and the light producing or luminescent organisms. Of these the sunlight has the greatest ecological significance.

The radiant energy coming from the sun in the form of visible spectrum is called light or luminous energy. Radiation that penetrates the earth’s atmosphere consists of electromagnetic waves of a wide range of wavelengths. A beam of light is pictured as a shower of particles called photons. Each photon carries a certain amount of energy called quantum. The energy varies inversely to the wavelength of the spectrum. The solar spectrum in the earth’s atmosphere has been analyzed on the basis of wavelengths into different radiations (Fig. 2.1).

The solar radiations which penetrate earth’s atmosphere consist of a band of visible (to man) light and a small proportion of ultraviolet and infrared radiations. It is not known whether the long radio waves have some ecological importance to plants. The visible light lies in the range of 400—750 mµ. When the visible sunlight is passed through a prism it is dispersed into a series of wavelengths exhibiting seven different colours—violet, indigo, blue, green, yellow, orange, and red (VIBGYOR).

The wavelength of ultraviolet (2% of the radiation reaching the earth surface) is below 390 mµ and that of infrared above 750 mµ (1 millimicron or mµ = 1/1000 of a micron). Since a micron is 1/1000 of a millimetre, a millin (mµ) is equal to 000001 mm or 10^-7 cm). In working with ultraviolet and other rays of shorter wavelengths Angstrom unit is used (10 A = 1 mµ). The photochemical activity is greatest at the violet end.

Ultraviolet (UV) thus, has the greatest killing effect on the protoplasm. It helps in the synthesis of anthocyanin in leaves and inactivates the growth hormones and thus checks stem growth. Microbes injured by ultraviolet rays are often rejuvenated by exposure to visible light. X-rays and y-rays show high degree of ionizing effects.

They cause mutation in the living systems and in high doses they are absolutely fatal. Infrared radiations are not so powerful as may stimulate biochemical reactions. These radiations have high heating effects. They exert influence on stem growth and germination. The radiations of wavelengths shorter than 290 mµ never reach to the earth surface.

Light is usually measured by an electric instrument called light meter or photometer which consists of a light sensitive photoelectric cell. The intensity of light is directly recorded on a dial of the photometer. It is measured in foot candle or lux. Photometers are never exposed to direct sunlight. Actually, light reflected by a thick white paper placed on the ground for surface of the plant is measured. For ecological purposes light intensity is measured in a number of habitats for different species. This will enable one to know whether the species are shade loving or light demanders.

Spatial Variations in Light Intensity:

Light intensity differs from place to place. The spatial variation in light intensity may be caused by the following:

(a) Effecting atmosphere:

When the light passes through the atmosphere of the earth, a small proportion of radiations of shorter wavelengths become absorbed by atmospheric gases, mainly nitrogen and oxygen. The places at high altitudes receive brighter light than those at lower altitudes (above the sea level) because at higher elevations the atmosphere is thinner than that at low elevation.

Atmospheric vapour exerts a powerful screening effect and for this reason the intensity of light is much greater in dry areas than in wet regions. In a cloudy day, the light may be reduced to 4% of the normal intensity. When the atmosphere is saturated with fogs and clouds, a relatively high proportion of light rays of longer wavelengths, such as infrared and other visible radiations, are absorbed by the atmospheric moisture and the light rays of shorter wavelengths and ultraviolet rays are absorbed by the gas molecules and vapour droplets and the light reaching the earth surface under such conditions is called diffused light or skylight.

The diffused light on overcast day comprises up to cent per cent of the total light and on clear day it may comprise about 10-15% of the total light. Latitudinal variations in light intensities due to the height of the sun above the horizons are very important. At the equator, the light is most intense and contains highest proportion of direct sunlight. Towards the poles, the intensity of direct light decreases and the percentage of diffused light goes high.

(b) Effect of water:

In water medium, the intensity of light is reduced and this decreases progressively with the increase of water depth. About 10% of the sunlight falling on the water surface is reflected and 90% of that penetrates water and is modified in respect of intensity, spectral composition, angular distribution (refraction) and time distribution. Phytoplankton, zooplankton and suspended particles either reflect or absorb the light rays. Submerged plants get weaker lights than the plants on the surface of water get.

It is so because a proportion of light falling on the water surface is reflected and the major part of the penetrating light is absorbed by upper layers of water. When light reaches on the surface of water the major proportion of rays of shorter wavelengths, i.e., violet, indigo, blue and green, are reflected and other lights are partly absorbed. This is why bodies of water appear bluish green in colour. Reflection of light is increased several times in the rough water surface. There is a selective absorption of light at various depths in water.

The rays of longer wavelengths are absorbed near the surface and in general light rays of shorter wavelengths penetrate deepest layer. Thus, infrared rays are absorbed in upper layers of water (about 4 meters); red and orange rays are completely absorbed up to the depth of 20 metres; yellow rays may penetrate up to 50 m and green and blue rays penetrate up to 80 to 100 m depth Violet and ultraviolet rays penetrate upto 80 to 100 m depth Violet and ultraviolet rays penetrate beyond 100 m depth and no light penetrates beyond 200 m depth.

Depending upon the penetration of light, oceans are divided into the following three zones:

(a) Euphotic zone (up to 50 m depth)

(b) Disphotic zone (80 to 200 m depth)

(c) Aphotic zone (below 200 m of depth)

In the oceans, algae are distributed according to wavelengths of light rays to which their colours are best suited to absorb and utilize; green algae are found in intertidal zone, brown algae descend somewhat deeper and red algae are characteristic of deep oceanic water. Light has got sufficient power of penetration. Photosynthesis may take place in some plants which are covered under as much as 40 cm thick snow.

(c) Effect of suspended particles:

Dust, smoke and other solid particles dispersed in the air or water have great screening effect. In smoky industrial cities, the smoke may reduce 90% of light.

(d) Effect of the layers of vegetation:

In complex plant community, for example forest, the tallest plants receive full sunlight, undershrub’s receive subdued sun flecks or diffused light, herbs and epigeous cryptogams grow in still weaker light. In dense forest the leaves completely check the penetration of light and less than 1% of total sunlight reaches the surface.

Temporal Variations in Light:

The light intensity during summer is much higher than that in winter. It is weak at sunset in winter and comparatively stronger during the mid-day hours. This periodical fluctuation in the intensity of light is due to the change in the angles of radiations reaching the earth. When the sun is at the horizon the solar rays travel through approximately 20 times the thickness of the air they have to penetrate when the sun is overhead in the noon.

At the equator, the day light prevails 12 hours out of every 24 hours in both the seasons, summer and winter. In the Polar Regions, the day length (photoperiod) becomes longer than 12 hours in summer and shorter than 12 hours in winter. The skylight or diffused light available before sunrise and after sunset is of great ecological importance. Moonlight plays some important role in the plant life. It sometimes satisfies light requirements of certain seeds, promotes starch hydrolysis in the leaves, affects nocturnal leaf movement in legumes, and stimulates the sexuality in certain marine algae.

Importance of Light to Plants:

Earth is a very small object in the solar system and it receives only 1—50 millionth of the total solar radiation reaching the universe. The journey of solar energy from sun to earth surface is very much moderated while it passes through stratosphere and the atmospheric envelope surrounding the earth. In the stratosphere lies the ozone regions which absorbs the harmful solar radiations of short wavelengths.

There are various devices to measure the solar energy. The unit of measure is erg or calorie and energy falling at a place is expressed in terms of cal/cm²/min. At any place on the earth surface this energy flux varies with season, time of day and atmospheric humidity and drought. In the northern hemisphere solar energy falling at the outer surface of the earth’s atmosphere is 0.485 cal/cm²/min. This is taken as hundred per cent solar radiation. Of this only 0.228 cal/cm²/min reaches the earth surface, i.e., only 47 per cent of the radiation reaches the biosphere and the rest 53 per cent is scattered and lost (Gate, 1965).

From the ecological point of view, the energy and its utilization is most important for the existence of life. Many of the environmental phenomena like movement of wind currents, rainfall, chemical action and organic behaviour are controlled by energy. There are three possible pathways for light to take when it reaches the surface of a plant or animal; it may be reflected from the surface, it may be absorbed by the surface and it may be transmitted through the material (Moen, 1973), Fig. 2.2.

Light affects many physiological activities of the plants. Light affects the following aspects of plant life:

1. Photosynthesis:

Out of the total solar energy reaching the earth, only about 2% is used in photosynthesis and about 10% is used in other physiological activities. It has been estimated that a com plant utilizes only 0.13 per cent of light energy. All wavelengths of light are not utilized by a plant. Green light is completely reflected by green parts of the plants making them look green. Red and blue green are the two maxima of absorption of light. The green plants, the producers of ecosystem, synthesize their food (carbohydrates) from water and CO₂ in presence of sunlight.

The solar radiations provide energy for this process. In this processes the radiant energy of sun available to the plants is converted into the chemical energy by chlorophylls. The chemical energy stored in food is utilized in various other biochemical activities of the plants. The rate of photosynthesis is greater in intermittent light than in the continuous light. The relationship of light intensity with photosynthesis in terrestrial as well as in aquatic plants follows the general pattern of a linear increase up to an optimum or saturation intensity followed by a decrease at high intensities (Rabinowitch, 1951; Thomas, 1955).

Light plays important role in the development of plastids and pigments. It has marked effect on the number and position of chloroplasts. The upper part of leaf which receives full sunshine has large number of chloroplasts which are arranged in line with the direction of light. At high intensity, the photo-oxidation of enzymes reduces not only the rate of carbohydrate synthesis but also that of protein synthesis. The protein synthesis is especially reduced by high intensity of light. High intensity of light, however, influences the formation of anthocyanin pigment. It is for this reason, alpine plants have beautifully coloured flowers.

2. Respiration:

There is no direct effect of light on the respiratory activity in the plant body. Indirect effect is much important because in presence of light the respiratory substrates are synthesized. Under certain conditions, such as, in shade and under water, the light becomes a limiting factor and the photosynthesis is not sufficient for effective growth. Under such conditions, the rate of photosynthesis is just sufficient to meet the need of respiration.

This is called compensation point. At this point, the dry weight of plant does not increase. The compensation point differs in different species and in different individuals of the same species at different ages. In many plants the respiratory rate increases with the increase in the light intensity. Ranjan and Saxena have studied the effect of light intensities on respiration rate in many plants and have shown that respiratory rate could increase in Canna, Nerium and Bougainvillea with the increase in light intensity.

However, in some other plants respiration rates decreased slightly in intense light. The rise and fall of respiration rate may be due to the effect of light on the permeability of plasma membrane, change in the viscosity of the protoplasm and photo-oxidation of enzymes. The permeability and viscosity increase with the increase in light intensity up to certain optimum. Light, however, has got very little effect on respiratory process of lower plants and thallophytes.

3. Opening and closing of stomata and in transpiration:

Mostly the stomata remain opened in the light and closed in the dark. Light brings about phosphorylation and conversion of starch into soluble sugars in the guard cells and thereby increases their osmotic pressure which, in turn, causes inflow of water in the guard cells. The increase in the turgidity of guard cells causes widening of gap between two guard cells. The opening of stomata increases the gaseous exchange and also increases the rate of transpiration during day period. Increase in the light intensity above the optimum shows detrimental effects because the increased transpiration in the intense light is injurious to plants.

4. Growth and flowering of plants:

Light shows many fold effects on the growth of the plants. Growth of plants depends especially on the intensity, quality, duration and direction of light. High intensity of light inhibits the production of auxins or growth hormones and consequently it influences the shapes and sizes of plants.

Plants growing in darkness or insufficient light produce maximum amount of growth hormones as a result of which they are elongated with slender pale yellow stem and small leaves. The plant growth is slow in the light of high intensity. Red light favours the growth. Lights of shorter wavelength, except violet, are detrimental to plant growth.

Duration of light is also very important:

Actual duration or length of the day (photoperiod) has been shown to be important factor in the growth and flowering of wide varieties of plants. The controlling effect of the photoperiod, known as photoperiodicity, is currently an active field of physiological ecology.

According to their response to photoperiods, the plants have been classified into three well defined groups:

(i) Long day plants:

Plants which bloom when the light duration is more than 12 hours per day, as for example, radish, potato, spinach, etc.

(ii) Short day plants:

Plants which bloom when the light duration is less than 12 hours per day, as for example, cereals, tobacco, cosmos, dahelia, etc.

(iii) Day nuteral plants:

Plants which show little response to length of daylight, as for example, cotton, balsam, tomato plants.

Recently it has been shown that photoperiodic stimulus for flowering is also controlled by thermal points, Azzi (1957) has shown for the first time that initiation of flowering in a plant occurs at certain constant which is specific for a particular species.

This constant is called Azzi’s constant which is expressed as follows:

Azzi’s constant = Total duration of light in hours + total mean temperature in °C.

Chinoy (1960) has confirmed this constant and called it Photo thermal quantum requirement of a species. Thus, an increase in temperature will decrease the duration of light required for flowering. This information can be applied to advantage in autecological studies of certain crop plants.Plants which receive direct sunlight are called heliophytes and those growing in the shades are called sciophytes.

Heliophytes exhibit the following features:

(i) Stem with short internodes and long lateral organs;

(ii) Roots numerous and profusely branched;

(iii) Thick cuticle;

(iv) Well developed palisade and weakly developed spongy tissue in the leaf;

(v) Well developed xylem with thick rays;

(vi) Small intercellular spaces in the tissues;

(vii) High respiration rate and much rapid transpiration;

(viii) Vigorous flowering and fruiting;

(ix) Early appearance of flowers;

(x) Low chlorophyll content; and

(xi) Proper development of mechanical tissues.

In the absence of light, the growth is very poor and plants show etiolation. The stem becomes tender, narrow, and long and the leaves become pale green, soft and small. Thus, light is essential for the normal and healthy growth of plants.

5. Movement:

Light affects the movement of some plants. The stems, roots and leaves show different responses to light. The effect of sunlight on the plant movement is called heliotropic effect. The stems elongate towards the source of light (positively phototropic) and the roots are negatively phototropic. The leaves grow transversely to the path of light. In order to receive maximum sunlight, the leaves are oriented on the stem in such a way that they do not overlap one another.

6. Germination of Seeds:

The seeds when moist are very sensitive to light. In some cases, the germination of seeds is retarded in light. The quantity of light needed for the stimulation of embryo varies in different seeds. In most cases, the red light promotes germination and far-red light inhibits germination. Investigations regarding the mechanism of seed germination in light have revealed that a pigment phytochrome is involved in this process. The pigment occurs in two reversible forms—Pr and Prf which develop under red and far-red light.

The germination depends upon the balance between two forms:

Seeds of certain plants require blue light for germination. Germination of Typha seeds is promoted in yellow light. Yellow light counters the inhibitory effect of blue light. Light is an important factor in the distribution of plants. Some plants grow in full sunlight, while others prefer to grow in shade. Bormann (1956) describes an interesting situation in certain species of pine in which young seedlings are shade adapted while older seedlings and young trees do not grow well in shades.

2. Temperature:

Temperature is a variable factor which is influenced by time, season, latitude, altitude, slope, direction, soil texture, plant cover and human activities like urbanisation and industrialization. It penetrates every region of the biosphere and profoundly influences all forms of life by exerting its action through increasing and decreasing some of the vital activities, such as the metabolism, reproductive behaviour, embryonic development and growth.

Temperature is a measure of intensity of heat. In terms of standard unit it is commonly expressed as degrees either in Fahrenheit or Celsius scale (Centigrade). Heat is a from of energy called thermal energy. Thermal energy is exchanged between organisms and environment by radiation, conduction, convection and evaporation. These four basic modes of transfer of heat energy occur within the organisms and in the interface between the organisms and their environment (Fig. 2.3).

The total amount of heat entering the biosphere from the sun is balanced by the amount lost per unit time. The flow of geothermal heat from the interior of the earth is small as compared to the amount of heat entering the biosphere from the sun. The estimate of thermal energy flow in the biosphere is referred to as heat budget (Vemberg and Vemberg, 1970).

The most influential factors in the climate are temperature and moisture. The temperature affects the vegetation either directly or indirectly.

Directly it affects in two ways:

(i) It affects the physiological processes of plants and consequently their growth and size; and

(ii) It determines which species can survive in a particular region. The different species of plants show a wide variation as regards their tolerance to temperature range and fluctuation.

On our planet, organisms can carry on their life activities over a relatively narrow temperature range extending from 0°C to 50°C and every plant has a specific range of temperature requirement. This range differs from species to species. Plants do not thrive in places with higher or lower temperatures than their specific ranges.

Generally, at 40°C the protoplasm undergoes such changes as are minimal to plant life and it dies at temperatures above 60°C. At temperatures below freezing point plants generally die because of rapid crystallization of protoplasmic water which results in mechanical injury. Air dried yeast can endure temperature as high as 114^oC. Bacteria can endure temperature between 120°C and 130^oC. A few fungi can withstand temperature up to 89^oC.

Air temperature above 32 C is most favourable for tropical plants. Existence of vegetation has been recorded between 26 C (some conifers) and 66°C (desert plants). Every organism has a definite temperature range. The temperature at which all metabolic processes necessary for life can only initiate and proceed with lowest pace is said to be minimum temperature and increase in the temperature above the minimum level increases the rate of metabolic activities until they reach the maximum level at a temperature which is called optimum temperature. Further rise in temperature beyond optimum level decreases the metabolic rate until it ceases at a temperature called maximum temperature (Fig. 2.4).

The general inability of protoplasm to endure high temperature can be ascribed in large part to the sensitivity of its enzymes to heat. Catalytic proteins in nearly all cases are irreversibly inactivated by exposure to high temperatures (usually greater than 50°C) for any length of time. They are however, able to withstand lower temperatures even below the freezing point of water.

The ability of cell to endure sub-freezing temperatures seems to depend principally upon the avoidance of ice formation. The appearance of ice crystals in cells is almost always associated with the death of cells, due in part to mechanical damage inflicted on the subcellular structures by the ice crystals themselves. Death may also be due to removal of water from the protoplasm during ice formation in the intercellular spaces, thus dehydrating the protoplasm.

According to the heat requirement of plants, Raunkiaer divided the gross vegetation into the following types:

(a) Megatherms:

These are the plants of warm habitat which require high degree of heat throughout the year. They are found in areas with tropical climates e.g., plants of deserts.

(b) Mesotherms:

These are the plants of habitat which is neither very hot nor very cold. These plants cannot withstand extreme high or low temperatures and they are found in tropical and subtropical habitats.

(c) Microtherms:

These are the plants of cold or temperate habitat and require low temperature for their growth. Such plants cannot tolerate high temperature. They may also be found in tropical and subtropical areas at high elevations where temperature conditions are less extreme.

(d) Hekistotherms:

These are the plants of cold and alpine regions. They do not thrive well in heat and can withstand long and very severe winter. Many plants are very sensitive to temperature. The sudden fall in temperature is injurious because plant tissues are badly affected by it. Forests suffer from night frost on the east side where the sun rays strike very early in the day. As an adaptation against frost, the starch of plants changes to fats or oils in the autumn. The fatty oils depress the freezing point and thus increase the power of resistance in plants against frost.

The leaves of plants in the coldest lands store fats Pentosans, mucilage and pectic substances which have high water retaining power are abundant in many plants. They decrease the danger of plants from desiccation and consequent death Dried seeds and spores are not affected by freezing because there remains no liquid in them that can freeze. Due to removal of water from seeds, the cold resistance of seeds of certain plants increases upto the extent that their exposure for 3 weeks to -190°C does not diminish their germinability.

The temperature stimulates the growth of seedlings. The optimum temperature for seed germination ranges between 20°C and 27^oC. The absorption rate is retarded at low temperature. Photosynthesis operates over a wide range of temperature. Most of the algae require lower temperature range for photosynthesis than the higher plants. The photosynthesis continues even at 80°C in some desert plants.

The rate of respiration increases with the rise of temperature up to a certain level, but beyond the optimum limit the respiration rate shows marked decrease. The rate of respiration becomes doubled at the increase of 10°C above the optimum temperature provided other factors are favourable (Vant Hoff’s law).

High temperature generally favours the growth of plants, but for some crop plants low temperature is beneficial. If the temperature ranges of winter varieties are lowered up to 0°C to 5°C, the seeds sown in the spring season will grow luxuriantly and the plants will mature and flower at normal time. The process by which temperature range of plant is lowered in order to get early crop is called vernalization. This practice is very common in cold countries.

Temperature determines the growth of many plants. Cotton prefers high temperature. Potato gives highest yield in low summer temperature. Growth of plants is retarded at high temperatures. Temperature in combination with humidity and other factors helps in the spread of diseases in plants. Low temperature and high humidity favour the rust attack. Low temperature, high humidity and cloudy weather favour the damping off, seedling blight, foot rot and root rot diseases of cucurbits, tobacco, papaya and ginger.

3. Temperature fluctuation in environment:

The environmental temperature fluctuates both daily and seasonally. Temperature in any locality is governed by the brightness of the sun. It may vary from sunlight to shade and from daylight to dark. Surface temperature in soil may be 30°C higher in the sunlight than in the shade and up to 17^oC higher during day than that in the night. In the desert the temperature may be 40°C higher during day than that during night. The Thar Desert in south Rajasthan may show diurnal change of temperature to the extent of 20 to 30°C.

Latitudes also affect the temperature cycles. With the increase of every 150 metre altitude the temperature decreases by 1°C, Different habitats such as fresh water, marine and terrestrial environments show varying response to fluctuating temperature. Temperature fluctuations are less in the aquatic environment than in the terrestrial one. The increase in depth of aquatic medium increases the temperature fluctuation.

Minimum temperature of the sea may be —3^oC while temperature of fresh water pond never goes below 0^oC. The maximum temperature of ocean generally reaches up to 36°C but it may go much higher in fresh water ponds and pools. In deep bodies of water heating or cooling is restricted to surface layer but deeper layers may also get heated or cooled as a result of vertical circulation wherein surface water is brought to the deeper regions and vice verse.

Studies on the vertical fluctuations of temperature have led to the hypothetical classification of fresh water media into the following layers:

1. Epilimnion:

This is the superficial layer of body of water which is constantly stirred by wind and is a layer of warmer water.

2. Metalimnion or Thermocline:

This is the intermediate zone between the upper and bottom layers of body of water. This layer shows vertical temperature changes.

3. Hypolimnion:

This is the bottom layer of stagnant water with little or no fluctuation in temperature. The process of differentiation of fresh water habitat on the basis of vertical changes of temperature into three strata is referred to as thermal stratification. In terrestrial environment temperature fluctuations are varied and marked. Lowest temperature recorded for any land mass is —70°C (Siberia, 1957). Higher temperatures may often reach upto 85^oC in deserts at noon. In Rajasthan the highest temperature exceeds 50°C. Water in hot springs and geysers may be as high as 100°C.

The temperature varies from place to place and likewise the vegetation’s of different areas also differ considerably. Desert plants grow in extreme heat, aquatic plants grow in low temperature range, and grasses prefer to grow in the area of moderate temperature. Temperature in combination with moisture determines the general distribution of vegetation. Northern, southern, tropical and temperate vegetation’s depend solely upon temperature and moisture.

Precipitation and Atmospheric Humidity:

Water is one of the most important climatic factors. It affects the vital processes of all the living beings. It is the plenary agent that sets in motion the nutrients of the soil and makes them available to plants. It affects the morphology and physiology of the plants. It in combination with other factors regulates the structure and distribution of plant communities.

In nature water may be found in vapour, liquid and snow or ice states. In the atmosphere, water is found in the form of vapour. The quantity of water retained in the atmosphere depends on temperature and wind. Vapour increases in the atmosphere if the temperature rises and pressure decreases. At certain temperature and pressure, the maximum water-laden air is called saturated atmosphere. At saturation point, if the temperature is lowered the water holding capacity of atmosphere is reduced which causes the condensation of water vapour in the form of ram drop, dew, frost, sleet, snow, etc. This is precipitation.

The water vapour present in unit volume of air is called absolute humidity. This is expressed in terms of percentage of water vapour present in unit volume of air at certain temperature. The amount of water vapour required to saturate the same unit volume of air under constant physical conditions is called relative humidity. Water of atmosphere reaches to the earth’s surface through precipitation and from earth’s surface it reaches to the atmosphere through evaporation and transpiration (Fig. 2.5). Thus, a continuous circulation of water from earth to atmosphere and vice versa is maintained in nature.

This is called water cycle or hydrologic cycle. It has been estimated that about 80,000 cubic miles of water from the oceans and 1,500 cubic miles of water from lakes and land surface evaporates annually. The total evaporation is equaled by total precipitation of which about 24,000 cubic miles of water falls on land surface. The main source of water on earth is ram water.

Penck using precipitation-evaporation ratio, has classified the climate as follows:

(i) Arid:

It is characterised by the condition in which evaporation is greater than precipitation.

(ii) Arid-humid:

When evaporation is more or less equal to precipitation.

(iii) Humid:

When evaporation is lesser than precipitation. The total rainfall, especially the distribution of rainfall throughout the year is one of the leading features of climate. Rainfall map of the world corresponds very closely with the distribution of great vegetational zones of the world. Sudden and heavy rains are not so beneficial as are moderate and continuous rains because in the heavy rains a large amount of water is lost from the surface of soil as runoff and the soil is eroded.

Rainfall is determined largely by geography and pattern of large air movements of weather systems. When the moisture laden winds blow from the oceans towards the high mountain they deposit most of the moisture on the ocean-facing mountain slopes with a resulting ‘rain shadow’, and produce desert on the other side. The higher the mountain the greater is the precipitation of moisture over it. This is the main reason why deserts are usually found behind high mountains. The deserts are also found along the sea coasts, where wind blows from large interior dry land areas rather than off the ocean.

The amounts of rainfall in different localities largely determine the nature of vegetation therein. The following tabulation gives a rough idea about the plant communities that may be expected in regions with different amounts of annual rainfall.

Snow, which may lie on the ground to form a valuable protective blanket and also a reserve of water is apt to limit the growing season by its late melting. Hail, a special type of precipitation during the summer season in the form of small ice pieces, may cause serious injuries, especially to young crops. Dew and sleets make a very vital contribution to precipitation in the regions of low rainfall. Dew and ground fog may be important to plants not only in coastal forests but also in deserts near the sea coasts where they provide much of surface water on which the ephemeral plants depend.

The atmospheric humidity influences directly the form and structure of the plants. It directly affects the transpiration rate of the plants. In dry atmosphere transpiration rate increases and as a result of this the water content of leaf tissues decreases and the leaves wilt temporarily. Water requirements of different plant species differ considerably. Some species, on one extreme, thrive well in the region with an annual precipitation of 10 cm while some, on the other extreme, grow only when they are submerged in water.

On the basis of their water requirements, the plants are grouped into three ecological groups:

(i) Hydrophytes:

Plants adapted to aquatic environment.

(ii) Xerophytes:

Plants adapted to grow in dry lands where water content is low.

(iii) Mesophytes:

Plants living in the habitat that usually show neither an excess nor a deficiency of water. The actual effects of water on the plants may be complicated by other conditions, such as temperature and atmospheric humidity. The combination of temperature and precipitation plays vital role in determining the broad features of plant distribution on the earth surface.

The temperature exerts more limiting effect on the organisms when the moisture conditions are at extremes (i.e., either very high or very low) than when such conditions are moderate. Likewise moisture also plays a more critical role at the extreme temperature. Some modem climatologists taking into consideration the quantitative measures, effectiveness and seasonal distribution of moisture and temperature have classified the climate into temperate, tropical, polar and high altitude climates.

The characteristic features of these climates and peculiar type of vegetation’s restricted to them are given in the following chart:

4. Atmosphere and Air:

The earth is enveloped by a gaseous layer called atmosphere. Gaseous mantle forming atmosphere extends into outer space some 1000 km or so above the earth surface. It maintains contact with all the major types of environment of earth, interacting with them and greatly affecting their ability to support life. It is a reservoir of several elements essential to life. It serves many functions including the filtration of radiant energy coming from the sun, insulation from heat loss from the earth surface and stabilization of weather and climate owing to heating capacity of the air.

Structure of Atmosphere:

There are five concentric layers within the atmosphere which can be distinguished on the basis of temperature (Fig. 2.6).

These are as follows:

1. Troposphere

2. Stratosphere

3. Mesosphere

4. Thermosphere, and

5. Exosphere.

1. Troposphere:

The lowest layer of atmosphere in which man and other living organisms live is called troposphere. Troposphere is about 20 km above the earth surface. It is thin in the Polar Regions (about 10 km thick). It is a mixture of several gases. The proportion of gases in the atmosphere is fairly constant. Troposphere is characterised by weather change and steady
decrease in temperature with increasing amplitude and it may decrease up to —60°C in the upper layers.

The composition of troposphere excepting water vapour and dust particles is presented in the table below:

The water vapour and dust particles occur in troposphere in variable concentrations. Concentration of water vapour may range from 0 to more than 4 per cent. The amount of water vapour in troposphere is maximum in the lowest level of atmosphere and it decreases gradually in the upper region and is entirely absent above 8 to 10 km.

Dust is limited to lower levels and has no specific relationship with any other feature of atmosphere. Troposphere is the layer of sulphates and is the region of strong air movements, cloud formation, lightning, thundering etc. The upper layer of troposphere which gradually merges with the next zone or stratosphere is called is called tropopause.

2. Stratosphere:

The second layer of air mass extending about 30 km above tropopause is called stratosphere. The uppermost layer of stratosphere is called stratopause. In this zone the temperature shows an increase from a minimum of about 60^oC to a maximum of about 5^oC. The increase in temperature is due to ozone formation under the influence of ultraviolet rays of solar radiation. Ozone is formed from oxygen by a photochemical reaction in which solar energy (symbolized as hv) splits the oxygen molecule to form atomic oxygen which then combines with oxygen molecule to form ozone

The above reactions are reversible. Ozone content of stratosphere is constant which means that ozone is being produced from oxygen as fast as it is broken down to molecular oxygen. The highest concentration of ozone (90%) in stratosphere approximately 20-25 km above, around the earth surface is known as ozone layer or ozonosphere. Ozonosphere is important because it absorbs ultraviolet radiations of the sun. The fact that upper region of stratosphere becomes warmer with increasing distance from the earth is due to transformation of absorbed ultraviolet rays into heat (Craig, 1968).

The absorption of ultraviolet radiation by ozone umbrella is of paramount importance in the ecosystem because these radiations are prevented from reaching the earth surface where it would be lethal to living organisms. Ozonosphere also acts as a blanket that reduces the cooling rate of earth.

3. Mesosphere:

It is the third layer of atmosphere next to stratopause. It is about 40 km in height. This region is characterised by low atmospheric pressure and low temperature. The temperature begins to drop from stratopause, goes on decreasing with the increase in the height and reaches a minimum of about —95°C at a level some 80 to 90 km above the earth surface. The upper limit of the mesosphere is termed mesopause.

4. Thermosphere:

Next to mesosphere is thermosphere which extends up to 500 km above the earth surface and is characterised by steady temperature increase with the height from mesopause. The thermosphere includes the regions in which ultraviolet radiations and cosmic rays cause ionization of molecules like oxygen and nitric oxide. This region is called ionosphere. In ionosphere, molecules of gases are so widely spaced that high frequency audible sound are not carried by the atmosphere.

5. Exosphere:

The region of atmosphere above the thermosphere is called exosphere or outer space which lacks atoms except those of hydrogen and helium. This extends up to 32190 km from the earth. Exosphere has a very high temperature due to solar radiation. The earth’s magnetic field becomes more important than gravity in distribution of atomic particles in the exosphere.