HOW PLANTS WORK AND GROW

All life depends upon the successful trapping and subsequent utilization of the .radiant energy from the sun. Green plants are an essential component of this process because they are the primary energy receptors on which all other life depends. The method whereby plants are able to trap and use light energy is called photosynthesis. Light energy is utilized to synthesize energy-storing carbohydrate from carbon dioxide and water vapour in the air. The reaction is represented as by the following diagrammatic equation:

Plants contain special structures within their cells called chloroplasts which control and contain the photosynthetic reaction taking place.

There are two parts to the overall process. First the light energy has to be ‘trapped’ by the chloroplast and second the trapped light energy is used to ‘drive’ the synthesis of carbohydrate.

Trapping light energy

Green plants appear green because they reflect green light and absorb the other colours that make up ‘white’ light. The pigment which gives plants this green colour is called chlorophyll and it is mainly this substance which is used to trap light. The chlorophyll molecules are arranged in layers within the chloroplast where they act as a receiving array for the light energy.

After the light energy is trapped by chlorophyll it is stored within the plant in the form of ‘high energy substances’. These substances are used for transferring energy from one place to another in the chloroplast and they ‘drive’ the synthetic reactions which make carbohydrates from carbon dioxide and water. During the process of trapping light energy, oxygen is ‘split’ from water and released into the atmosphere, eg in water plants bubbles of oxygen may temporarily accumulate on the leaf surfaces.

The synthesis of carbohydrate

The formation of carbohydrate involves the reduction of carbon dioxide gas which is an oxidized form of carbon. The energy needed for this process comes from the ‘high energy substances’ produced in the chloroplasts. The synthesis of carbohydrates requires a complex sequence of actions which takes place within the chloroplasts, but the products, such as sugar and starch, are transported and used elsewhere in the plants.

The efficiency of photosynthesis in energy conversion

Since all of life finally depends on the conversion of light energy to usable chemical energy it is worthwhile to estimate the actual energy conversion efficiency which is achieved in practice. When all of the relevant factors are-taken into account (such as percentage of total solar radiation received by the plant, light which is reflected or transmitted by the leaves and light absorbed but converted into heat), the final efficiency of energy conversion into usable carbohydrate is something like 0.2 percent. Even though this may seem to be a very small figure it still means that an average of 87 x io9 tons of sugar are produced by photosynthesis per year! (The annual mining and manufacturing product of the earth is about ix io9 tons.)

The utilization of carbohydrate Plants use the carbohydrates that they produce by photosynthesis in order to grow. Some of the carbohydrates are used directly in order to make cell walls and the main structural components of the plant, some are kept for future use in special storage organs or as components of seeds as food reserves and others are utilized directly to provide energy for the multitude of synthetic processes which the plant employs in vegetative growth. The plant releases energy stored in carbohydrates through the process of respiration. This process is essentially the opposite of photosynthesis.

The energy released is ‘trapped’ in the form of ‘highenergy substances’ which the plant then uses to manufacture essential compounds needed for growth. Just as plant cells contain special structures concerned with photosynthesis, they also contain special structures (mitochondria) concerned with respiration.

Basically, the process of respiration involves the cleavage of carbohydrates such as glucose into smaller units, which are then transported into the mitochondria where they are broken down further into carbon dioxide. During this process the chemical bond energy originally held in the glucose molecule is released and trapped in the form of ‘highenergy substances’ ready for use at other sites in the plant cell.

The efficiency of respiration in producing usable energy

It has been calculated that respiration is about 40 percent efficient in terms of conversion of stored energy into usable energy. This is a relatively high degree of efficiency as compared with energy conversion in general. The unconverted energy appears as heat, which is used in animals to maintain a stable body temperature, but it is wasted in plants, being dissipated to the surroundings.

The cycle of photosynthesis and respiration

Photosynthesis and respiration represent two processes, one of which is the reverse of the other. Consider an overall cycle in which these processes take place, the purpose of which is to enable the plant to trap the energy of the sunlight, store it and then release it in a usable form for growth.

Primary production—the key to all life

All the energy which a plant succeeds in trapping from the sun is ultimately used for its growth and survival. Basically growth depends on the production of all of the molecules required for processes such as building up the plant structure, maintaining the structure once built and production of seeds or other mechanisms for the propagation of the species. Plants, given the abundance of energy from the sun, are able to produce all of their needs from quite simple mineral requirements —man cannot produce all he needs and thus depends on plants for survival.

The most fundamental process involving growth is cell division. In plants the cells which give rise to all other cells are called meristematic and exist only in the tips of roots and stems, in very young leaves and where there is secondary growth such as in trees.

The basis of the cell division process depends upon a duplication of the hereditary material known as DNA (deoxyribonucleic acid) contained in the chromosomes of the cell nucleus, followed by the division of this material and its containment within two new daughter cells formed when the original cell divides into half. Continued growth of the two new daughter cells then allows a repeat performance of the whole sequence.

The importance of water and its movement in the plant

Between about So and 95 percent of the weight of plants is water. The continued growth of a plant is totally dependent on water which in a typical herbaceous species is obtained from the soil. Of the water taken up by the roots about 98 percent is lost again by diffusion through specialized pores in the leaves (called stomata) and to a much lesser extent through the leaf outer skin or cuticle.

The diffusion of water through stomata in the leaves occurs by a process called transpiration. Stomata, although variable in size, are very small (from about 5 microns X3 microns to 40 microns xio microns) but are still large in comparison to the size of the water molecules that pass through them. Leaves contain large numbers of pores on both their upper and lower surfaces but there are usually more on the lower than the upper leaf surface.

Stomatal openings in leaves can open or close depending upon a number of environmental factors. A closed pore prevents the escape of any water from the plant but will also stop the entry of carbon dioxide into the leaf thus stopping photosynthesis. A wide open pore, while allowing ingress of carbon dioxide and thus permitting photosynthesis, will also permit an easy loss of water from the plant which, unless replaced by further uptake from the roots, will cause the plant to wilt and droop. In practice progressive stomatal closure tends to reduce water loss more than photosynthesis and so many plants partially close their stomata enabling them to minimize water loss while having little effect on photosynthesis.

The main factors that change transpiration rates are temperature, atmospheric humidity, availability of soil water and light. Light has an indirect effect since it can alter both the temperature and the extent of stomatal opening. The other factors influence transpiration through their effects on changes in the water vapour pressure gradient between the leaf and the atmosphere. The steeper the gradient, the greater the transpiration rate, and vice versa. Thus on a warm dry day with a slight breeze transpiration rates would be high, provided that there was no shortage of soil water available to the plant.

Water uptake using cohesion forces Any loss of water through transpiration has to be made up by a simultaneous uptake of water through the roots. Most plants have extensive and deep root systems which enable them to extract the water from a large volume of soil.

Not only are plants able to take up this water but they can raise it to the upper parts of the plant—a height of perhaps loom (328ft) in the tallest trees such as the redwoods of the Cali-fornian forests. How is it that water can be carried to such heights which are many times that which the atmosphere will support through its own pressure (about 1,000cm — 394m)?

There is no completely satisfactory explanation but the best idea seems to be that of the so-called cohesion theory. This idea is based upon the small tubes (called xylem elements) which make up part of the plant structure and provide a continuous ‘pipeline’ from the root tips to the top of the plant. When water is lost through transpiration at the top of the plant more is drawn up through the xylem from the roots which in turn take up more from the soil. The plant acts like a wick with the water columns being held together by cohesion (the attraction of like molecules for each other). The existence of root pressure There is evidence that under certain conditions instead of water being drawn up the xylem it can be pushed up by the roots acting as a pump. If a plant is healthy and well-nourished its roots can accumulate large quantities of mineral salts. The presence of these salts in the plant cells increases the ability of these cells to take up water which is then forced up the xylem elements as part of the transpiration stream. Even though such root pressures can be experimentally observed recorded pressures are not high enough to account for water movement to the top of the tallest plants.

The mineral requirements of a plant

Xylem elements not only carry water but also various minerals which the plant obtains from the soil. A growing plant requires most or all of sixteen chemical elements; nine in large amounts and seven in small. Of the former (called macronutrients), carbon, hydrogen and oxygen are derived from carbon dioxide and water. The others, nitrogen, phosphorus, potassium, sulphur, calcium and magnesium are obtained from the soil (except in a few species, such as clover, which obtain their nitrogen from the air).

The other seven nutrients which are required for plant growth are termed micronutrients or trace elements since they are only required in relatively small quantities. These are iron, manganese, boron, zinc, molybdenum, chlorine and copper.

Plants take up mineral nutrients through their roots from a solution in soil water. The required nutrients must either be in, or capable of being transferred into, a soluble form ready for transport into the plant otherwise the plant cannot use them. For example, it is quite possible for cereal crops to starve in soils which are amply supplied with phosphorus and potassium simply because the latter are in an insoluble form unavailable to the plant.

Although mineral nutrients might be used in many ways to sustain the growth of a plant, the absence of any one required mineral nutrient results in the production of certain characteristic growth deficiency symptoms. These include a reduction of chlorophyll synthesis giving the plant a pale green or yellow colour (chlorosis); localized death of tissues such as buds, leaf tips or edges (necrosis); production of red colours in stems or other structures where it is usually absent; stunted growth; unusually thin and woody stems; poor reproductive development; and sometimes the complete failure of fruit or seed development. Phloem: the plant’s arteries Not only do water and minerals have to be moved upwards through the xylem pipeline but also material made in the leaves by photosynthesis has to be transported about the plant to the various places where it is required for growth. Plants contain very special cells whose job it is to perform this function. These cells are called sieve-tubes and occur in the phloem tissues. Sieve-tubes are only found in angio-spcrms. Other cells of a very similar nature perform the same job in the gymnosperms and lower vascular plants.

Sieve-tube elements contain no nucleus, but instead they are associated with other cells called companion cells which lie parallel to the sieve-tubes and which are interconnected in such a way that the loss of the nucleus is overcome. The ends of sieve-tube cells are perforated to form sieve plates and are connected up longitudinally so that a ‘pipeline’ is formed through which organic material can be carried. The transported material is mainly sucrose which occurs in the phloem as a 10 to 20 percent solution. Other materials are also transported, but in smaller quantities. Thus amino-acids, minerals, plant growth hormones and other molecules may all be found in the translocation stream. There is, however, a mystery concerning the way that sieve-tubes work.

The most accepted idea, although not completely satisfactory is known as the Munch ‘mass’ flow hypothesis after the botanist who proposed it. He suggested that the ‘producer’ cells in leaves which contain a high concentration of material are caused to take up water from the surrounding tissues. This causes a flow of material through the phloem tissues to the ‘receptor’ cells where it is then used. The excess water passes into the xylem tissues where it rejoins the transpiration stream. It is a kind of circulatory system. The integration of structure and function in leaves

The leaf as the main site of production of material for plant growth is obviously a very important part of a plant. The main function of leaves appears to be that of photosynthesis and their structure is well adapted for this purpose. Leaves are usually thin and flat presenting large surface areas for maximum ability to absorb light energy. In certain cases the shape may be modified for specific environmental reasons. For example, succulent plants from desert areas very often have thick fleshy leaves with a thick waxy cuticle designed for maximum water storage with minimum water loss; many conifers have needle-shaped leaves designed to minimize water loss during the cold season when the ground is frozen and also to minimize snow accumulation thus reducing the snow burden on the branches.

The internal structure of the leaf is also designed to allow for maximum photosynthctic efficiency. There are large spaces within the leaves adjacent to stomata which give a high surface area for absorption of the carbon dioxide required for photosynthesis. The large surface area also allows for the easy removal of oxygen produced as a by-product ol photosynthesis.

The products of photosynthesis are carried away to other parts of the plant where they are needed for growth by a very extensive vascular (or transport) system. This system is made up of large numbers of veins which permeate the entire leaf blade. ¦n^

The veins not only remove the products of photosynthesis from the leaf but also furnish the leaf with necessary water, essential minerals and certain organic materials manufactured in the roots and older leaves. It is an extensive two-way transport system ensuring the efficient and rapid utilization of photosynthetic ability for the maintenance and further growth of the plant.

Growth and the formation of meristems Growth implies an increase in size and volume. It can also be taken to mean an increase in complexity and then it is perhaps more properly described as differentiation. In plants these two aspects of growth are seen quite separately but both are ultimately dependent upon the activity of special regions called meristems. These are areas of embryonic tissue from which all of the new tissues of a plant are generated. All plants have mcristems which are located at the tips or apices of the shoots and roots. For many plants, particularly herbaceous forms (ie plants having little or no woody tissue), they are the only meristems present and because of their position these are known as apical mcristcms.

In woody plants other meristems are present also, known as secondary meristems. The cells produced in these regions are responsible tor increase in girth, for example the increase in width which takes place when a tree grows. Growth in different directions— elongation growth

Elongation growth in plants is achieved through cell division followed by cell enlargement. Growth rates vary enormously; for a plant growing at its maximum rate an increase in height of about 1-5cm (o.4-2.oin) per day is normal but values as high as 60cm (24111) a day have been recorded, eg 111 some species of bamboo.

Growth is not uniform over the whole plant.

In most plants elongation growth takes place in regions just behind the tips of shoots and roots. In shoots this region of elongation growth extends some distance down the shoot but in roots is completely restricted to the area immediately behind the tip. Growth in different directions— increase in girth

Plants not only grow upwards, but outwards. Outward growth occurs by the production of lateral branches (giving a plant its characteristic shape) and also by the thickening of the stem. The mechanism whereby this latter growth takes place is more easily understood if we consider in a little more detail the structure of the stem in a woody plant which contains secondary meristems.

A stem is made up of several concentric layers of cells surrounding a central region of older tissue. All of these different layers are composed of cells which have different functions. The innermost layer is composed of xylem tissues through which water is translocated to the leaves. Adjacent to this layer is a meristem known as the vascular cambium. This is a region of dividing cells some of which give rise to new xylem cells and some of which produce cells for the next outer layer which is composed of phloem tissue. Adjacent to this there is sometimes another mcristcm known as the cork cambium and from this the outermost protective ‘skin’ or bark of the plant is produced.

Increase in girth is quite simply the result of successive cell division in the vascular cambium giving rise to new xylem and phloem tissues. As this increase in lateral size takes place, the layers of cells split apart as they are pushed outwards. The vertical gaps so produced are filled in by the activities of the cork cambium and cambial tissue within the stem. Growth rings in woody plants In temperate regions there is a marked seasonal periodicity of growth, both in the primary growing points at the apices and in the secondary growth from lateral meristems. Growth is usually very rapid once initiated in the spring but at the apex slows down quite quickly where it has usually stopped by the summer. Secondary growth or increase in girth also slows down but continues until the autumn when die plants finally become dormant.

It is this periodicity of growth that causes the production of growth rings, the study of which can provide considerable information on the climatic conditions and local environment experienced by a tree. Usually only one ring is formed per year but under certain conditions such as insect infestation or a late frost causing premature defoliation and a subsequent interruption of growth, two rings per year may occur. This latter situation can usually be detected since the rings so produced are likely to be unusually narrow. Support by woody tissue The increase in girth provided by the activities of the lateral meristems is the result of a thin cylindrical layer of cells continually being created in the outermost layers of the stem. What happens to the older cells?

The older cells of the phloem eventually die and can no longer be used for transporting organic solutes. They become incorporated into the outermost corky or bark layers of the tree. The older cells of the xylem which form the sapwood of the tree eventually become nonfunctional and generally become impregnated with a variety of organic substances such as tannins, gums, oils and resins. These cells then form the hcartwood of the tree. The primary function of the heartwood is as a mechanical support and the changes which occur in its formation contribute towards increased com-pressional strength and confer an increased resistance to decay.

The control of growth—plant hormones Plants are very sensitive to changes in their environmental conditions and a change in external surroundings can have a profound influence on the rate or type of growth. Despite this, however, growth does not take place in a haphazard way but is controlled very closely by the plant itself. Growth does not occur uniformly all over a plant but in certain specific regions. Growth between specific regions can be correlated too—for example, a root never completely outgrows a shoot or vice versa. One obvious explanation for this is a nutritional one. Roots and shoots are interdependent because shoots need the materials extracted from the soil by roots while roots need the photosynthetic products of shoots for their continued growth.

The situation is more complex than this and it is now known that rates of growth in plants are controlled by special messenger molecules or hormones. These compounds are generated at various places within the plant and transported in very small quantities to their sites of action where they can profoundly influence the type of growth taking place.

The discovery of plant growth hormones initiated entirely new areas of experimentation which are still being pursued and although hormones are now produced and used in quantity for various commercial purposes, such as seed set, fruit ripening and breaking of dormancy, their modes of action are still imperfectly understood.

Day length and other influences

Perhaps the two main influences the environment has upon plants are those on flowering and movement. Quite often these effects are mediated through the action of growth hormones.

In angiospcrms, flowering is very often controlled by the length of the daily period of illumination. This response is known as photo-periodism and normally day length has to fall within certain limits for flowering to occur. Three main groups of plants can be identified. Plants which flower if the day length is less than a certain critical value are called short-day plants, eg chrysanthemums, xanthium and strawberries. Plants which flower if the day length exceeds a certain critical value are called long-day plants, eg henbane, spinach, lettuce, wheat and beet. The third group of plants are called day-neutral plants and these can flower at any time regardless of the day length, eg tomatoes, dandelions and sunflowers.

The way in which flowering is controlled by daylight is not fully understood. It seems however that plants contain a pigment called phytochrome which changes its nature in the presence of light.

This pigment is able to control the movement of a hormone or group of hormones from the leaves to the apex of the plant where the flowering process is initiated.

Movements in plants are brought about cither by the unequal distribution of growth or by changes in cell water content. The direction of movement may be related to that of the application of the stimulus, in which case it is called a tropism, or it may be dependent on the plant alone, known as a nastic movement.

The two main tropic movements which can be observed are those of phototropism and geotropism. Phototropism is the bending of a shoot towards a light source. The magnitude of the response depends upon the amount of light received and thus the same effect can be produced by low intensity light given for a long period of time or by high intensity light given in a flash. The effect is caused by the unequal distribution of a hormone, called auxin, following the application of light. The auxin concentrates on the shady side of the plant where it causes greater cell growth and hence a bending of the plant towards the light.

Auxin is also involved in geotropism. Geotropism is the response of plants to the earth’s gravitational field, which causes shoots to grow upwards and roots to grow downwards. Since they grow directly towards the stimulus (the earth) roots are said to be positively geotropic whereas shoots, growing in the opposite direction are negatively geotropic.

The other sorts of movement in which the direction of movement is independent of the direction from which the stimulus is received are the so called nastic movements or nasties. The principal nastic movements are those caused by changes in light intensity (photo-nasties), temperature changes (thermonasties), touch (thigmonasties or seismonasties) and alternation of day and night (nyctinasties).

The rapid opening of a flower when it is brought from a cold room to a warm one is an example of a thermonastic response, while the opening of tobacco flowers in the evening is a photonasty resulting from a decrease in illumination intensity. Some of the most common examples of nyctinasties are the ‘sleep movement’ of plants such as the raising or lowering of the leaves of a bean plant during the day and night to give a horizontal or vertical orientation respectively.

Perhaps the most dramatic responses, however, are the thigmonasties. When an insect settles on the leaf of the Venus’ fly trap (Dionaea muscipula), the blades fold together, the spines on its margin interlocking to imprison the hapless victim. Another example is the sensitive plant Mimosa pudica. When this plant is stimulated water escapes from special cells at the base of the leaf stalks causing the leaflets to fold up in pairs and the main leaf stalk to drop. If a very strong stimulus is applied to one leaflet it can be transmitted throughout the whole plant at the rate of about 3cm (I.2in) per second.

Plant shape: making the best use of space

All plants have characteristic shapes which have been determined throughout the course of evolution to best fit them for survival in the particular environmental niche which they occupy. There are many environmental factors which contribute towards the shape that a plant takes; eg desert cacti have evolved their peculiar and characteristic shapes in response to the need to conserve water and yet be able to photosynthesize. Thus their leaves have been reduced in the course of time to no more than needles or spines, while the main body of the plant, where all the photosynthesis now takes place, has become swollen for purposes of water storage.

In a mixed deciduous forest the shapes of the trees are considerably modified by the pressure of space. Close packed trees tend to be tall with few lateral branches in the effort to obtain as much of the sunlight as is available by outgrowing all their neighbours, whereas well spaced out trees assume a more symmetrical and well proportioned shape being able to spread their lateral branches out to a greater extent.

The problem of obtaining sufficient light for photosynthesis is a very important factor in determining plant shape and leaf arrangements play important roles in determining the photosynthetic efficiency. For example, the mode of growth of a cereal crop enables a good photosynthetic efficiency to be obtained. In cereals the leaves grow from the base of the plant assuming a nearly vertical angle of growth. In this way all of the leaves receive some light and are able to make a direct contribution to photosynthetic efficiency, although the effective light intensity is reduced because the leaves are not orientated at right angles to the sunlight.

Many plants which have horizontally placed leaves have evolved a shape which places the leaves in a spiral pattern. This gives the maximum chance of intercepting light with a minimum of shading. In addition, other plants are able to either move their stems or their leaves so that a maximum leaf area is presented to the incident light.

Light is not the only environmental factor which influences shape. Temperature, wind, soil conditions, salt spray and even grazing animals all play their part. For example, an oak tree growing in a sheltered forest appears very different to the stunted and tortured structures which are assumed on windswept and colder moorland areas, and many trees growing in coastal areas are bent and forced to grow in a direction away from the prevailing winds.

However, whatever variety of external shape and form is forced upon a growing plant by the environmental conditions it experiences, it is always recognizable as a particular species because the fundamental aspects of its structure are still determined by its inherited genes in a way not properly understood and which forms one of the great mysteries of plant science today.

Helleborus niget Christmas Rose

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