Chapter 3 — Transportation – The Circulatory System

Every cell needs a steady supply of oxygen and food and a steady removal of carbon dioxide and other wastes. In a single-celled organism like Amoeba, or in a thin flat animal like a planarian, every cell is close enough to the surface for simple diffusion to do the job. Diffusion, however, is fast only over very short distances; across a millimetre it takes minutes, across a centimetre hours. A large animal or a tall plant therefore needs a system that moves materials in bulk from where they are taken in to where they are used, and this is the transport system. In animals it is the circulatory system of heart, blood vessels and blood; in plants it is the vascular tissue of xylem and phloem.

Until the seventeenth century the movement of blood was not understood. The ancient view, from the Greek physician Galen, was that blood was made in the liver, flowed outward through the veins to be consumed by the tissues, and that the arteries carried a separate blood mixed with air. In 1628 the English physician William Harvey published the results of years of careful experiment and reasoning. He measured that the heart pumps out about 70 mL at each beat; multiplied by the number of beats in an hour this came to far more blood than the body could possibly make, so the same blood must be used again and again. He tied a bandage (ligature) tightly round an arm and showed that the arteries above it swelled while the veins below emptied; when the bandage was loosened a little, the veins below it swelled, proving that blood flows out through the arteries and back through the veins. He showed by pressing on the veins of the forearm that their valves allow flow only towards the heart. From these observations Harvey concluded that blood circulates: it is pumped by the heart into the arteries, returns by the veins, and is pumped round again. He could not see how blood crossed from arteries to veins; the capillaries were discovered under the microscope by Marcello Malpighi in 1661, completing the picture.

Harvey's work is a model of scientific method: measurement, experiment, logical argument and the willingness to overturn an authority that had lasted fourteen centuries. It also founded modern physiology, the study of how the body works.

In this chapter we study the human circulatory system in detail and then the transport system of plants, which works on entirely different principles because a plant has no pump.

Blood is a red fluid connective tissue that circulates in the vessels. An adult has about 5 to 6 litres, roughly 7 percent of body weight. It is slightly alkaline (pH 7.4), slightly heavier than water, and consists of a straw-coloured liquid, the plasma (about 55 percent of the volume), in which float the blood cells or corpuscles (about 45 percent). When blood is spun in a centrifuge the cells settle and the plasma floats on top.

Plasma is about 90 to 92 percent water. Dissolved in it are plasma proteins (about 7 percent: albumin, which keeps water in the vessels; globulins, which include the antibodies; and fibrinogen and prothrombin, which are needed for clotting), nutrients (glucose, amino acids, fatty acids, vitamins), mineral salts (sodium chloride, bicarbonate, calcium, potassium), waste products (urea, uric acid, carbon dioxide), hormones and dissolved gases. Plasma carries all these round the body, distributes heat, and by its buffers keeps the pH steady. Plasma from which the clotting proteins have been removed is called serum.

Red blood cells (erythrocytes, RBCs) are the most numerous cells, about 5 million per cubic millimetre in men and 4.5 million in women, making 25 trillion in the body. Each is a biconcave disc about 7 micrometres across, thinner in the middle, which gives it a large surface for gas exchange and lets it bend through capillaries narrower than itself. Mammalian red cells have no nucleus and no mitochondria when mature, leaving more room for their one important content, haemoglobin, the red iron-containing protein that carries oxygen from the lungs to the tissues and part of the carbon dioxide back. Red cells are made in the red bone marrow of the ribs, vertebrae, skull and the ends of long bones, at about 2 million a second, and each lives about 120 days before being broken down in the spleen and liver. The iron is recycled and the rest of the haemoglobin becomes the bile pigment bilirubin. Making red cells needs iron, vitamin B₁₂ and folic acid, and a shortage of any of these causes anaemia, a low haemoglobin count that leaves the person pale, weak and easily tired. Iron deficiency anaemia is very common among adolescent girls and pregnant women in India, and green leafy vegetables, jaggery, pulses and iron tablets are the remedy.

The oxygen-carrying capacity of blood depends on the haemoglobin content, normally 13 to 16 g per 100 mL. People living at high altitude and trained athletes have more red cells. The spleen, besides destroying old red cells, stores blood and releases it in an emergency.

White blood cells (leucocytes, WBCs) are the defenders of the body. They are far fewer than red cells, about 5,000 to 10,000 per cubic millimetre, but their number rises sharply during infection, which is why a doctor orders a blood count. They are larger than red cells, colourless, irregular in shape and each has a nucleus. They are made in the bone marrow and in lymph tissue, and most live only a few days. White cells can squeeze out through the walls of capillaries into the tissues, a movement called diapedesis, and crawl like an Amoeba towards the site of infection.

There are several kinds. Neutrophils, the most numerous (about 65 percent), and monocytes, the largest, are phagocytes: they engulf and digest bacteria and dead cells, exactly as Amoeba engulfs food. Pus at a wound is mostly dead neutrophils and the bacteria they have killed. Lymphocytes (about 25 percent) are made in the lymph glands and bone marrow; some of them produce antibodies, proteins that fit on to the surface of a particular germ or its toxin and neutralise it, and others attack infected cells directly. Antibodies give us immunity, and vaccines work by stimulating lymphocytes to make them in advance. Eosinophils increase in allergies and worm infections; basophils release histamine in inflammation. A count of the different kinds, the differential count, helps a doctor tell a bacterial infection from a viral one or an allergy.

Platelets (thrombocytes) are not whole cells but small fragments of large bone-marrow cells, about 250,000 per cubic millimetre, without a nucleus, living about a week. Their job is to start the clotting of blood when a vessel is damaged.

Blood clotting (coagulation) stops blood loss from a cut and prevents germs entering. The sequence is: (1) The damaged tissue and the platelets that stick to the torn vessel release an enzyme, thromboplastin (thrombokinase). (2) In the presence of calcium ions and vitamin K, thromboplastin converts the inactive plasma protein prothrombin into the active enzyme thrombin. (3) Thrombin converts the soluble plasma protein fibrinogen into insoluble threads of fibrin. (4) The fibrin threads form a mesh across the wound in which blood cells are trapped, forming the clot; it shrinks, squeezing out a clear yellow fluid, the serum, and dries into a scab under which the wound heals. Normal clotting time is 3 to 8 minutes. Blood inside undamaged vessels does not clot because there is no thromboplastin and the smooth vessel lining and an anti-clotting substance, heparin, prevent it.

In the inherited disease haemophilia one of the clotting factors is missing and even a small cut bleeds for hours. Blood taken for transfusion is kept from clotting by adding sodium citrate, which removes the calcium ions. A clot forming inside an intact vessel is a thrombus; if it blocks an artery of the heart it causes a heart attack, and if it blocks an artery of the brain it causes a stroke.

The heart is a hollow muscular pump about the size of the owner's closed fist, weighing about 300 g, lying in the chest between the two lungs, slightly to the left, with its pointed lower end (apex) resting on the diaphragm. It is enclosed in a double-walled bag, the pericardium, whose pericardial fluid protects it from shock and friction. Its wall is made of a special kind of muscle, cardiac muscle, which contracts rhythmically on its own, never tires, and is under involuntary control.

The heart has four chambers. The two upper thin-walled chambers are the atria (singular atrium, also called auricles), which receive blood; the two lower thick-walled chambers are the ventricles, which pump it out. A muscular wall, the septum, divides the heart lengthwise into a right and a left half, and there is no opening between the two sides after birth; the right side handles deoxygenated blood and the left side oxygenated blood, and the septum keeps them from mixing. The wall of the left ventricle is about three times thicker than that of the right, because it has to push blood to the whole body while the right ventricle pushes it only to the nearby lungs.

Valves allow blood to flow in one direction only. Between the right atrium and the right ventricle is the tricuspid valve, with three flaps; between the left atrium and the left ventricle is the bicuspid or mitral valve, with two flaps. These flaps are anchored to the ventricle walls by tendon-like cords (chordae tendineae) which stop them turning inside out when the ventricles contract. At the openings of the two great arteries are the semilunar valves, each of three pocket-shaped flaps: the pulmonary valve at the start of the pulmonary artery and the aortic valve at the start of the aorta. They open when the ventricles contract and snap shut when the ventricles relax, so that blood cannot fall back.

Vessels attached to the heart. Two great veins, the superior vena cava from the head and arms and the inferior vena cava from the lower body, bring deoxygenated blood into the right atrium. The pulmonary artery leaves the right ventricle and divides to carry deoxygenated blood to the two lungs. Four pulmonary veins, two from each lung, bring oxygenated blood into the left atrium. The aorta, the largest artery, leaves the left ventricle, arches over and runs down the body, giving branches to every organ. The heart muscle itself is supplied by the two coronary arteries, the first branches of the aorta; blockage of a coronary artery starves a region of heart muscle and causes a heart attack.

Note the exceptions to the usual rule: the pulmonary artery is the only artery carrying deoxygenated blood and the pulmonary veins are the only veins carrying oxygenated blood. Arteries are defined by carrying blood away from the heart, veins by carrying it towards the heart, not by the kind of blood.

In humans and all mammals and birds, blood passes through the heart twice in one complete circuit of the body. This is called double circulation, and it has two parts.

Pulmonary circulation (pulmo = lung). Deoxygenated blood from the body collects in the right atrium, passes through the tricuspid valve into the right ventricle, and is pumped through the pulmonary valve into the pulmonary artery, which divides into a right and a left branch, one for each lung. In the capillaries around the alveoli the blood gives up carbon dioxide and takes up oxygen. The oxygenated blood returns through the four pulmonary veins to the left atrium. This is the short circuit: heart → lungs → heart.

Systemic circulation. Oxygenated blood from the left atrium passes through the bicuspid valve into the left ventricle, which pumps it with great force through the aortic valve into the aorta. The aorta branches into arteries to the head, arms, liver, intestine, kidneys, legs and every other organ; the arteries branch into arterioles and then capillaries, where oxygen and nutrients pass to the cells and carbon dioxide and wastes are collected. The capillaries join into venules and veins, which return the deoxygenated blood by the superior and inferior venae cavae to the right atrium. This is the long circuit: heart → body → heart.

The two circuits form a figure of eight with the heart at the crossing. Within the systemic circuit there are special portal systems: the hepatic portal vein carries blood from the intestine, loaded with absorbed food, to the liver before it joins the general circulation, so the liver can store, process or detoxify what has been absorbed; and the renal circulation takes about a quarter of the blood output through the kidneys for cleaning.

Why double circulation? In fish, blood passes through the heart once: the single ventricle pumps it to the gills, and from there it flows on, at low pressure, to the body and back. In double circulation the blood that has lost pressure passing through the fine capillaries of the lungs is returned to the heart and pumped again, so the body receives it at high pressure and fast flow. This supports the high metabolic rate needed to keep a warm body temperature. Equally important, the complete separation of the two sides of the heart means oxygenated and deoxygenated blood never mix, so the tissues receive fully oxygenated blood. Amphibians and most reptiles have a three-chambered heart with a single ventricle in which some mixing occurs, an intermediate arrangement; crocodiles, birds and mammals have the four-chambered heart and complete separation.

The whole circuit takes about one minute at rest; the heart pumps about 5 litres a minute, the total volume of blood in the body, and up to 25 litres a minute during hard exercise.

The heart works by contracting and relaxing rhythmically. Contraction of a chamber is called systole and its relaxation diastole. One complete sequence of contraction and relaxation of all four chambers is a cardiac cycle, and it corresponds to one heartbeat. At the resting rate of about 72 beats per minute a cycle lasts about 0.8 second.

The cycle has three phases. (1) Atrial systole (0.1 s): both atria contract together and push their blood through the open tricuspid and bicuspid valves into the ventricles, which are relaxed. (2) Ventricular systole (0.3 s): both ventricles contract. The rising pressure snaps the tricuspid and bicuspid valves shut so that blood cannot return to the atria, and then opens the semilunar valves; the right ventricle sends blood into the pulmonary artery and the left into the aorta. (3) Diastole (0.4 s): all four chambers relax. The semilunar valves close as the ventricles relax, preventing backflow from the arteries, and the atria fill with blood from the veins; towards the end of the phase the atrioventricular valves open and blood begins to flow passively into the ventricles even before the atria contract. Then the cycle repeats.

Heart sounds. Placing a stethoscope on the chest one hears a two-part sound, lubb-dupp. The first sound, lubb, is longer and lower and is produced by the closing of the tricuspid and bicuspid valves at the start of ventricular systole. The second, dupp, is shorter and sharper and is produced by the closing of the semilunar valves at the start of diastole. Abnormal valve sounds, murmurs, tell a doctor that a valve is leaking or narrowed.

Origin of the heartbeat. The heart does not need a signal from the brain to beat; a heart removed from the body goes on beating for a while in a suitable solution. The beat starts in a small patch of special muscle in the wall of the right atrium called the sinoatrial (SA) node or pacemaker. It generates an electrical impulse about 72 times a minute; the impulse spreads through both atria, making them contract, reaches a second patch, the atrioventricular (AV) node, and is conducted from there by a bundle of fibres (bundle of His and Purkinje fibres) through the septum to the ventricles, making them contract from the apex upward. The delay at the AV node ensures the atria finish contracting before the ventricles begin. The electrical activity can be recorded from the skin as an electrocardiogram (ECG). When the natural pacemaker fails, an artificial electronic pacemaker can be implanted.

The rate is adjusted by the nervous system and hormones. Sympathetic nerves and the hormone adrenaline speed it up during exercise, fear or excitement; the vagus nerve slows it during rest and sleep. The rate is higher in children (about 100 per minute) and in small animals (a mouse's heart beats 500 times a minute), lower in trained athletes (around 50), and rises with fever.

Each time the left ventricle contracts it forces about 70 mL of blood into the aorta, which is already full. The elastic wall of the aorta stretches to accommodate it and then recoils, and this wave of stretching and recoil travels along all the arteries faster than the blood itself. Where an artery lies just under the skin over a bone the wave can be felt with the fingertips as the pulse. The usual place is the radial artery at the wrist on the thumb side; others are the carotid artery in the neck, the temporal artery in front of the ear, and the artery on the top of the foot. The pulse is never felt in a vein, because the pressure in veins is steady. Since each pulse corresponds to one ventricular contraction, counting the pulse for a minute gives the heart rate: about 72 in a resting adult, 60 to 100 being the normal range. A doctor also notes whether the pulse is regular, strong or weak, which reflects the condition of the heart and the blood volume. A fast pulse accompanies fever, exercise, anxiety, anaemia and blood loss; a very slow pulse can mean a block in the heart's conduction system.

Blood pressure is the force that blood exerts on the walls of the arteries. It is highest in the aorta when the ventricle contracts and lowest just before the next contraction. The pressure during ventricular contraction is the systolic pressure, normally about 120 mm of mercury in a young adult; the pressure during relaxation is the diastolic pressure, normally about 80 mm Hg. It is written 120/80. It is measured with a sphygmomanometer: an inflatable cuff is wrapped round the upper arm and pumped up until it stops the flow in the brachial artery; a stethoscope is placed over the artery below the cuff; as the cuff is slowly released the pressure at which the first tapping sound is heard (blood beginning to squirt through) is the systolic pressure, and the pressure at which the sounds disappear (flow becoming smooth) is the diastolic pressure. Electronic meters do the same automatically.

Blood pressure depends on the force of the heartbeat, the volume of blood, the elasticity of the arteries and the resistance of the small arterioles, which can narrow or widen under nervous and hormonal control. It rises with exercise, anger and fear and falls during sleep, and it rises with age as the arteries stiffen. Persistent pressure above 140/90 is hypertension (high blood pressure). It usually causes no symptoms, which is why it is called a silent killer, but over years it damages the arteries and causes heart attack, stroke, kidney failure and loss of vision. Salt-rich diet, obesity, smoking, alcohol, stress and lack of exercise raise it; regular checks after the age of 30, a low-salt diet, exercise and, when needed, medicines control it. Very low pressure (hypotension), as after severe bleeding, dehydration or shock, means the tissues are not being adequately supplied; the patient feels faint and the pulse is weak and rapid.

Blood travels in a closed system of tubes of three kinds. Arteries carry blood away from the heart, veins bring it back, and capillaries connect the two in every tissue. All three have the same inner lining, a single layer of flat cells called the endothelium, but they differ in the layers outside it, and each difference fits the job.

Arteries receive blood at high pressure in spurts from the ventricles. Their walls are therefore thick and strong, with a middle layer of smooth muscle and elastic fibres and an outer layer of tough connective tissue. The elastic tissue stretches with each spurt and recoils between beats, smoothing the flow and maintaining pressure; the muscle can narrow the vessel (vasoconstriction) or widen it (vasodilation) to control how much blood reaches an organ. The lumen (bore) is relatively narrow. Arteries have no valves, except at their origin from the heart, because the high pressure keeps blood moving forward. They lie deep in the body for protection, and a cut artery bleeds bright red in spurts. Arteries branch into smaller arterioles, whose muscular walls are the main regulators of blood pressure and distribution.

Veins receive blood from the capillaries at low pressure, about 10 mm Hg or less, in a steady flow. Their walls are thin, with little muscle and elastic tissue, and their lumen is wide, so that they hold a large volume; at any moment about 65 percent of the blood is in the veins. Because the pressure is too low to push blood up from the legs against gravity, veins have valves, pocket-shaped flaps that open towards the heart and close against backflow. Blood in the veins is moved along by the squeezing of surrounding skeletal muscles during movement (the muscle pump) and by the suction of the chest during inspiration. Standing still for a long time lets blood pool in the leg veins, which is why soldiers on parade sometimes faint; when the valves give way the veins become swollen and twisted, a condition called varicose veins. Most veins lie near the surface, appear bluish, and a cut vein bleeds dark red in a steady flow. Small veins are venules.

Capillaries are microscopic vessels, about 8 micrometres wide, just enough for red cells to pass in single file, and their wall is the endothelium alone, one cell thick. There are so many of them that no cell of the body is more than a few cells away from one; laid end to end the capillaries of one person would stretch about 100,000 km. Their thinness and enormous total surface allow rapid exchange: oxygen, glucose, amino acids and hormones pass out through the wall into the tissue fluid that bathes the cells, and carbon dioxide and wastes pass in. Blood flows through them slowly, giving time for exchange. Some fluid leaks out of the capillaries into the tissues and is returned by the lymphatic system.

The sequence in every organ is artery → arteriole → capillary network → venule → vein. The pulmonary vessels reverse the usual contents but not the structure: the pulmonary artery has a thick arterial wall although it carries deoxygenated blood.

As blood flows through the capillaries under pressure, some of its plasma is forced out through the thin walls into the spaces between the cells. This fluid, called tissue fluid or interstitial fluid, is like plasma but with fewer proteins and no red cells; it bathes every cell and is the medium through which oxygen and nutrients actually reach the cells and wastes leave them. Most of the tissue fluid returns into the capillaries at their venous end, but about 10 percent, some 3 litres a day, does not. If this were left in the tissues they would swell, so it is drained away by a second set of vessels, the lymphatic system, and once inside them the fluid is called lymph.

Lymph is a colourless or pale yellow fluid. It contains water, dissolved salts, glucose, some proteins, waste products and many lymphocytes, but no red cells and no platelets. Lymph from the intestine after a meal is milky because of absorbed fat and is called chyle.

Lymph vessels begin as blind-ended lymph capillaries in the tissues, join into larger vessels with valves like those of veins, and finally empty through two large ducts, the thoracic duct and the right lymphatic duct, into the large veins near the heart. So the lymph eventually rejoins the blood. There is no pump; lymph moves slowly, pushed by the squeezing of muscles and by breathing, and the valves keep it flowing one way.

Along the lymph vessels are bean-shaped swellings, the lymph nodes or glands, found in groups in the neck, armpits, groin, abdomen and chest. Lymph passing through a node is filtered: phagocytes in the node engulf bacteria, dead cells and other particles, and the lymphocytes made in the node multiply and produce antibodies against any germ they meet. This is why the glands in the neck swell and become tender during a throat infection; they are working. The tonsils, adenoids, spleen and thymus are also lymphoid organs. The spleen, besides destroying old red cells, filters the blood as the nodes filter lymph.

The lymphatic system therefore has three functions: it returns escaped tissue fluid and plasma proteins to the blood, it absorbs fats from the intestine through the lacteals of the villi, and it defends the body by filtering out germs and producing lymphocytes and antibodies. When lymph vessels are blocked, fluid accumulates and the part swells (oedema); in filariasis, spread by mosquitoes in parts of Andhra Pradesh and other states, the parasitic worm blocks the lymph vessels of the leg, which swells to the enormous size called elephantiasis. Cancer cells too may spread through the lymph vessels, which is why surgeons examine the nearby nodes.

Blood and lymph together form a two-way system: blood delivers, tissue fluid distributes, and lymph collects what does not return directly.

When one person's blood is given to another, a transfusion, the red cells of the donor may be clumped and destroyed by the plasma of the recipient, with fatal results. Karl Landsteiner discovered in 1900 that this depends on substances on the red cell surface called antigens and matching antibodies in the plasma, and he classified human blood into the ABO groups. Group A has antigen A on its cells and antibody b in its plasma; group B has antigen B and antibody a; group AB has both antigens and no antibodies; group O has no antigens and both antibodies. An antibody clumps cells carrying the matching antigen, so a person can safely receive only blood whose antigens the recipient's plasma does not attack. Group O, having no antigens, can be given to anyone and is called the universal donor; group AB, having no antibodies, can receive any group and is the universal recipient. A second system, the Rh factor, divides people into Rh positive (about 95 percent of Indians) and Rh negative; an Rh-negative person must not receive Rh-positive blood, and an Rh-negative mother carrying an Rh-positive baby may need an injection after delivery to prevent problems in the next pregnancy. Before every transfusion the blood is cross-matched in the laboratory and screened for HIV, hepatitis and malaria. Donating blood is safe; the body replaces the plasma within a day and the cells within a few weeks, and healthy adults between 18 and 60 are encouraged to donate every three months.

Circulatory disorders are now the leading cause of death in India. Atherosclerosis is the deposit of fatty material, mainly cholesterol, in the lining of arteries, narrowing them and making them rough and stiff. It is promoted by a diet rich in saturated fat, smoking, high blood pressure, diabetes, obesity and lack of exercise. When a coronary artery is narrowed the heart muscle is short of oxygen on exertion, causing the chest pain of angina. If a clot forms on the roughened wall and blocks the artery completely, the muscle beyond it dies: this is a heart attack (myocardial infarction), felt as crushing chest pain spreading to the left arm and jaw, with sweating and breathlessness, and needing immediate hospital care. Blockage of an artery in the brain, or bleeding from a weakened one, causes a stroke, with sudden weakness of one side of the body, loss of speech or vision. Hypertension both causes and results from arterial damage. Heart failure is the state in which a weakened heart cannot pump enough blood, and fluid accumulates in the lungs and legs.

Prevention is mostly a matter of habit: a diet low in saturated fat and salt and rich in vegetables, fruit and fibre; no tobacco; limited alcohol; at least 30 minutes of brisk exercise most days; a healthy weight; control of blood pressure and diabetes; and management of stress. Treatment ranges from medicines that lower cholesterol and blood pressure, through angioplasty in which a narrowed artery is widened with a balloon and held open by a stent, to bypass surgery in which a vein from the leg is grafted to carry blood round the block.

A plant has no heart and no blood, yet water absorbed by the roots of a tall tree reaches leaves a hundred metres above the ground, and sugar made in those leaves reaches the tips of the roots. The plant does this with two kinds of conducting tissue running together in vascular bundles from root to leaf: xylem, which carries water and dissolved minerals upward, and phloem, which carries dissolved food in both directions. The story begins in the soil.

Root hairs. Just behind the tip of every young root the epidermal cells grow out into thousands of fine tubular root hairs, each a single cell about a millimetre long that lives only a few days and is replaced as the root grows. Root hairs push between soil particles and are surrounded by the film of water that clings to them. Their thin walls, enormous number and large total surface make them the main absorbing organ of the plant; a single rye plant was found to have about 14 billion root hairs with a surface of over 400 square metres.

Absorption of water by osmosis. The cell sap of a root hair is a fairly concentrated solution of sugars and salts; the soil water is a very dilute solution. The cell membrane is semi-permeable, so water moves by osmosis from the dilute soil solution into the more concentrated cell sap. The root hair cell now has a more dilute sap than the cortex cell next to it, so water passes by osmosis from cell to cell across the cortex, through the endodermis, and into the xylem vessels at the centre of the root. Some water also flows along the cell walls without entering the cells. A plant in a waterlogged soil cannot absorb well because the root hairs are starved of oxygen, and a plant in soil that has been over-fertilised or salted actually loses water, because the soil solution becomes more concentrated than the cell sap and osmosis runs the wrong way; this is why too much fertiliser scorches a crop and why plants cannot grow in sea water.

Absorption of minerals. Mineral salts are dissolved in the soil water as ions: nitrate, phosphate, potassium, calcium, magnesium, sulphate and traces of iron, zinc and others. They are often more concentrated inside the root than in the soil, so they cannot enter by diffusion; the root hair cells take them up by active transport, using energy from respiration to pump the ions in against the concentration gradient. This is another reason roots need oxygen. Once inside, the ions are moved across the cortex and loaded into the xylem, and their presence in the xylem sap is one of the things that keep water moving in by osmosis. Fungi living in partnership with the roots of many trees (mycorrhiza) and the nitrogen-fixing bacteria in the root nodules of pulses improve the supply of minerals.

Water and minerals thus arrive together in the xylem at the centre of the root, ready for the upward journey.

The upward movement of water and minerals from the root through the xylem to the leaves is the ascent of sap. The xylem is a system of dead, hollow, lignified tubes, the vessels and tracheids, running without interruption from the finest root to the finest vein of the leaf; being dead, they cannot pump, so the forces that lift the water come from elsewhere. Three forces are recognised.

Root pressure. The continuous osmotic intake of water by the roots, together with the active pumping of minerals into the xylem, builds up a pressure in the root xylem that pushes water up the stem. It can be shown by cutting the stem of a well-watered balsam or tomato plant a few centimetres above the soil: sap oozes from the cut surface, and if a glass tube is fitted over the stump the sap rises in it several centimetres. Root pressure is also the cause of guttation, the drops of water seen at the tips and edges of the leaves of grasses, balsam and Colocasia on cool humid mornings, when transpiration is slow and root pressure forces water out through special pores called hydathodes. Root pressure, however, is small, seldom more than 2 atmospheres, enough to lift water about 20 metres in theory and far less in practice, and it is absent in many trees and in summer. It cannot explain the ascent in a tall tree.

Capillarity. Water rises in a narrow tube because of the attraction between the water molecules and the walls (adhesion) and between the water molecules themselves (cohesion). Xylem vessels are very narrow, but capillarity alone would raise water only a metre or so.

Transpiration pull and the cohesion theory. The main force is a pull from above. Leaves lose water vapour continuously through their stomata by transpiration. As the mesophyll cells lose water to the air they become more concentrated and draw water by osmosis from the cells next to them, and those from the xylem in the leaf veins. Water in the xylem forms an unbroken column from leaf to root, held together by the strong cohesion between water molecules and clinging to the vessel walls by adhesion. When water is drawn out at the top, the whole column is pulled upward, like a rope being hauled, and the tension is transmitted right down to the roots, where it draws in more water. This is the cohesion-tension theory put forward by Dixon and Joly in 1894. The pull generated by a transpiring tree is enough to lift water well over 100 metres, and the column does not break because water's cohesion, in a narrow tube free of air bubbles, is remarkably strong. Evidence: a cut shoot with its stem in water continues to draw water up as long as it transpires; the diameter of a tree trunk shrinks slightly at midday when the tension is greatest; and if a bubble of air enters a vessel that vessel stops conducting.

The ascent of sap therefore needs no energy from the plant itself; it is driven by the evaporation of water from the leaves, powered ultimately by the sun. The price the plant pays is the loss of huge amounts of water, over 90 percent of all the water it absorbs.

Transpiration is the loss of water in the form of vapour from the aerial parts of a plant, mostly through the stomata of the leaves (stomatal transpiration, about 90 percent), and to a smaller extent through the cuticle of the epidermis and the lenticels of the stem. A single maize plant loses about 3 to 4 litres of water a day, a large tree several hundred litres, and a hectare of crop many tonnes.

Mechanism. Water in the mesophyll cells evaporates into the air spaces of the leaf, which are almost saturated with water vapour. When the stomata are open, this vapour diffuses out into the drier air outside. The rate is therefore controlled mainly by the opening and closing of the stomata, which in turn is controlled by light and the water status of the plant. Stomata open in light and close in darkness and when the plant is short of water.

Factors affecting transpiration. Light opens the stomata and warms the leaf, so transpiration is high by day and almost nil at night. Temperature: higher temperature speeds evaporation. Humidity: dry air draws vapour out faster; in humid weather transpiration is slow. Wind removes the moist air near the leaf and increases the rate, though a strong wind may make the stomata close. Soil water: when the soil is dry the stomata close and transpiration falls. Plant factors include the number and position of stomata, the thickness of the cuticle and the leaf area.

Demonstration. Enclose a leafy branch of a potted plant in a transparent polythene bag tied at the stem, with a similar bag round a leafless twig as control; after an hour in sunlight the first bag has droplets of water inside and the second does not. A more precise measurement uses a potometer, in which a cut shoot draws water along a graduated capillary tube; the movement of an air bubble measures the rate of uptake, which is very nearly the rate of transpiration, and the effects of wind, light and humidity can be compared. Cobalt chloride paper, which is blue when dry and turns pink when moist, clipped to the two surfaces of a leaf shows that the lower surface of a dorsiventral leaf transpires faster because it has more stomata.

Significance. Transpiration is sometimes called a necessary evil. On the one side it is the main force that pulls water and minerals up from the roots; the evaporation cools the leaf, which would otherwise overheat in the sun; and it keeps the cells turgid and the mineral stream flowing. On the other side it wastes enormous amounts of water, and on a hot dry afternoon a plant may lose more than its roots can absorb, so that it wilts. Plants of dry places (xerophytes) reduce transpiration with a thick cuticle, few or sunken stomata, small or needle-like leaves, hairs, or by shedding their leaves in the dry season; the cactus stores water in its stem and has only spines for leaves. Farmers reduce water loss by mulching the soil, shading nurseries, planting windbreaks, and irrigating in the evening rather than at noon.

The sugar made in the leaves must reach every living cell that cannot photosynthesise: the roots underground, the growing tips, the flowers and fruits, and the storage organs. The transport of the products of photosynthesis from the leaves to the other parts is called translocation, and it takes place in the phloem. Unlike xylem, phloem is living tissue. Its conducting cells are the sieve tubes, long cells joined end to end with perforated end walls, the sieve plates, through which the contents flow; each sieve tube has lost its nucleus and is kept alive by a companion cell alongside it, which supplies energy and controls loading and unloading.

What is transported. Glucose made in photosynthesis is converted into sucrose, the sugar of sugar cane, and it is sucrose that travels in the phloem sap, at concentrations of 10 to 25 percent, along with amino acids, hormones and some minerals being moved from old leaves to new ones. Sucrose is chosen because it is very soluble, chemically stable and not used directly by the cells it passes.

Direction. Xylem transport is one-way, upward. Phloem transport is in both directions, and in any direction at need: downward from leaves to roots, upward from leaves to the growing shoot tip, flowers and fruit, and upward from storage organs to the shoot when a stored root or tuber sprouts in spring. Movement is always from a source, where sugar is made or released, to a sink, where it is used or stored.

Mechanism: the pressure flow hypothesis. At the source, the leaf mesophyll, sucrose is actively loaded into the sieve tubes by the companion cells, using energy from ATP. This makes the sap in the sieve tube concentrated; water enters it by osmosis from the neighbouring xylem, and the pressure inside rises. At the sink, sucrose is actively unloaded into the root or fruit cells, where it is used or converted to starch, so the sieve tube sap there is dilute; water leaves by osmosis and the pressure falls. The difference in pressure between source and sink drives the sap in bulk through the sieve tubes from the high-pressure end to the low-pressure end. Because loading and unloading require energy, translocation stops if the phloem is poisoned or deprived of oxygen, in contrast to xylem transport which continues in a dead stem.

Evidence: the girdling (ringing) experiment. If a ring of bark, which contains the phloem, is removed from around a woody stem, leaving the xylem intact, the leaves above the ring stay fresh, since water still rises through the xylem, but after some weeks the bark just above the ring swells because the sugar moving down from the leaves accumulates there, unable to cross the gap. The roots below the ring, starved of food, eventually die and the tree with them. This is why goats and rats that gnaw a ring of bark kill a tree, and why fruit growers sometimes ring a branch to make it store more sugar in its fruit. Aphids feeding with their stylets in a sieve tube, and radioactive carbon tracing, give further evidence.