The tissues and cells of a stem

In a dicotyledonous, non-woody (herbaceous) stem the epidermis is like that of a leaf: a single layer of cells perforated by stomata. The epidermis helps maintain the shape of the stem. It is covered with a waxy cuticle to reduce water loss. In woody stems of trees and bushes, the epidermis is replaced bybark consisting of many layers of dead cells. Bark is penetrated by small pores calledlenticels, through which gaseous exchange takes place. The lenticels usually appear as raised spots surrounded by a powdery and impermeable material.

Just inside the epidermis, a layer of collenchyma gives both support and flexibikity to the stem. Some collenchyma cells contain chloroplasts which make the stem appear green.

The inner parts of the stems of most non-woody plants consist of vascular bundles embedded in undifferentiated parenchyma cells. When fully inflated with water (turgid), the parenchyma cells press against the epidermis and collenchyma, strengthening the stem. The stems of trees and bushes are supported not by parenchyma but by rigid woody tissue which makes up the bulk of these stems. The woody tissue consists of xylem and associated cell such as fibres formed by a process called secondary growth. New wood is added outside the old wood each growing season to form annual growth rings, visible in transverse sections of the stems of trees and shrubs.

Vascular tissue in the stem takes the form of bundles containing phloem and xylem and reinforced with strong fibres. The xylem is located towards the inside of the stem and the phloem towards the outside. The tough rigid vascular bundles embedded in softer turgid parenchyma tissue have been likened to reinforced concrete, in which rigid steel girders are imbedded in softer concrete. This arrangement gives the stem strength and flexibility, making it well suited to resisting sideways bending in strong winds. The vascular bundles of dicotyledonous plants are arranged in a ring pattern around the outside of the stem, while in monocotyledons such as cacti the vascular bundles are scattered throughout the stem.

The stem centre is called the pith. It may consist of parenchyma cells for storage, or it may be devoid of cells, in which case it is called a pith cavity.

Louis Pasteur

Pasteur (1822-1895) began his scientific career as a chemist, but it is because of his applications of germ theory to the prevention of disease that he became known as ‘The Father of Microbiology’.

Pasteur did not create germ theory, but he proved it to be correct. Once he had achieved this, he set about finding ways to prevent germs, the microorganisms present in the air, from infecting food and people.

He completed his famous experiment proving that microorganisms were present in the air while working for a wine company. He was trying to discover why wine sometimes went bad as it was being made. Once he had found the cause – microorganisms – he began to develop the process which carries his name – pasteurization. It was perfectly possible to kill all the microorganisms in food by boiling it, a process known as sterilization, but this damaged the taste and the quality of the food. Pasteur`s process killed not all, but most, of the microorganisms, with the result that the food needed to be kept cool and eaten or drunk within a limited time. Most importantly, the quality of the food was not harmed by the process. Much of the food we eat today is pasteurized.

His next achievement was to build on the discovery of the British scientist Edward Jenner. Many years earlier, Jenner had discovered a way of giving people resistance to the deadly disease smallpox, by injecting them with a similar disease that was found among cows. The process became known as vaccination. Pasteur applied germ theory to his work and looked at samples of blood taken from healthy and infected animals. He grew bacteria in his laboratory and used it to infect animals. By chance, some of these germs failed to grow well in his laboratory; these weak germs were then used to infect some chickens. Although the chickens suffered at first, they made a complete recovery and could not be infected again. In this way he discovered a way of increasing resistance to disease. Pasteur developed vaccines for many serious diseases including cholera and anthrax. At that time, these illnesses were certain death for anyone who caught them.

Pasteur`s discoveries revolutionized work on infectious diseases. Pasteur`s vaccines were different from Jenner`s in one important way. Jenner found a weak form of smallpox and transferred it to humans. Pasteur weakened the disease in a laboratory and immunized people with that weakened form. His success allowed a colleague to develop the first vaccine for rabies, which Pasteur used to save the life of a nine-year-old boy. By this act, Pasteur`s position as a hero was assured.

Thanks to the work of Pasteur, we now live longer, our food stays fresh longer and we are less likely to die of disease. Indeed, smallpox is no longer found anywhere in the world, due to a huge vaccination programme carried out in the 20^th century. This could never have happened without the scientific achievements of The Father of Microbiology.

Extract from a lecture about immunization

Historically, being immunized against diseases is a relatively new thing but that doesn`t mean the idea hadn`t been thought before. If we go as far back as 429 BC, the historian Thucydides noted that after a smallpox plague in Athens survivors did not become infected again. This was a time before there was even recognition of such things as bacteria and viruses.

Nowadays we take it for granted that we will be vaccinated and avoid diseases like polio, but how many of us actually stop to ask ourselves what is behind the injection we have? How does vaccination work?

Basically, it is the process by which a person is exposed to an agent so that his or her immune system develops against that agent. The immune system makes antibodies which fight against infection. Once the human immune system is exposed, that is, made open to a disease, it is able to act against any future infection. Vaccination exposes a person to an immunogen – something which helps develop immunity – in a controlled way by using a weak dose so he or she doesn`t become ill while being immunized.

The good thing about a vaccination programme is that it can limit the spread of a disease among a population, reducing the risk for people who have not been vaccinated, so we have something which is known as herd immunity. That means when the number of non-immune people has dropped to a certain level, the disease will disappear from the whole population. This is how we have achieved the elimination of many diseases.

Gregor Mendel

Gregor Mendel was born on 20^th July, 1822, and died on 6^th January, 1884. He was a biologist and botanist whose scientific research showed that inheritance proceeds according to certain scientific laws.

Mendel was a brilliant student and his family encouraged him to study, but they were very poor so Mendel entered a monastery in 1843. There he taught Mathematics, Physics and Greek to his school students. Eight years later, in 1851, the monastery sent him to the University of Vienna where he was able to continue his education. In 1853, he returned to the monastery and began teaching and researching again.

Mendel`s theories of heredity based on his work with pea plants are well known to students of Biology. But his findings were so different from the accepted views on heredity at the time that his work was ignored until long after his death. His paper, ‘Experiments in Plant Hybridisation”, in which he described how traits were inherited, has become one of the most influential publications in the history of science.

Mendel was the first person to trace the characteristics of successive generations of an organism. In Mendel`s day, a number of hypotheses had been suggested to explain heredity. The most popular one was the so-called blending theory. According to this theory, inherited traits blended from generation to generation. For instance, a red rose crossed with a white rose would, over time, produce a pink rose. Another theory put forward by Charles Darwin was called pangenesis. This stated that there were hereditary particles in our bodies, and that these particles were affected by our actions. The altered particles could be inherited by the next generation. These theories were disproved by Mendel.

The first thing he noticed when he began his experiments was that traits were inherited in certain numerical ratios. This observation led him to come up with the idea of the dominance of genes and he tested it in peas. For seven years he crossed thousands of plants to prove the Laws of Inheritance. From his experiments, Mendel developed the basic laws of heredity. Those laws are the following: that traits do not combine, but are passed whole from generation to generation (which disproved the blending theory and Darwin`s theory); each member of the parental generation passes on only half of its hereditary information to each offspring (with certain traits dominant over others); and different offspring of the same parents receive different sets of hereditary information.

Mendel`s research formed the beginning of the modern science of genetics. Genetic theory has had a huge impact on our lives. Many diseases, for example haemophilia, are known to be inherited, and family histories can be traced to determine the probability of passing on a hereditary disease. Scientists can now design plants that are easier to grow, or which can produce more food. This practical side of the results of Mendel`s research is being used to improve the way we live.

Vladimir Vernadsky

Vladimir Ivanovich Vernadsky was a Russian scientist who was born on 12^th March, 1863 in St. Petersburg. His most important contributions to science were the development of the ideas of the biosphere (from the Greek word bios meaning life) and the noosphere (from the Greek word noos meaning mind).

He graduated from the Physics and Mathematics Department of St Petersburg University in 1885. From 1890 to 1911 he taught mineralogy and crystallogaphy at the University of Moscow. In 1912 he was made a full member of the Russian Academy of Sciences where he was actively involved for 33 years, until his death in Moscow on 6^th January, 1945.

Through his work in mineralogy, Vernadsky became interested in the distribution of chemical elements in the Earth`s crust, hydrosphere and atmosphere – the field known as geo chemistry. Vernadsky published many papers on the geochemistry of various elements, including the geochemistry of radioactive compounds.

Vernadsky was one of the first scientists to suggest the possibility of using radioactive elements as sources of energy, and he organized a special commissions to look for uranium ores in Russia. In 1916, the first uranium deposits were discovered. But Vernadsky was aware of the danger of putting atomic energy into the hands of man. He said that scientists carried the huge responsibility of making sure their discoveries did not lead to destruction.

However, Vernadsky is probably best known for his development of the idea of the biosphere of the Earth and his ideas on the evolution of the biosphere into the noosphere.

The biosphere is the layer of the Earth in which all life exists. The term biosphere was coined in 1875 by the geologist, Eduard Suess, but it was Vladimir Vernadsky who recognized its ecological importance in 1929. He believed that all living organisms together with their environments make up the biosphere. These environments include the air (the atmosphere), land (the geosphere), rocks (the lithospere) and water (the hydrosphere). The exact thickness of the biosphere on Earth is difficult to calculate, but most scientists would agree that it is from about 5000 metres above sea level to around 9000 metres below sea level. Thus, there is a 14-kilometre zone within which life exists.

Vernadsky defined the boundaries of the biosphere by showing that the biosphere includes all the hydrosphere, part of the troposphere – the lowest layer of the atmosphere where most weather changes take place – and the upper part of the Earth`s crust down to a depth of two or three kilometers, in short, everywhere that life exists. For Vernadsky, the biosphere had existed since the very beginning of the Earth`s history and it was constantly evolving. Our present living world is the product of a long and complex evolution of the biosphere.

Vernadsky believed that the technological activities of mankind were a stage in this evolution. He believed that human reason and combined scientific efforts could overcome the negative results of technology and could lead to a safe future for everyone. This positive evolutionary stage of the biosphere of the Earth is for him the noosphere, the sphere of reason.

In his paper, Several Words on the Noosphere (1944, the last paper he published before his death), Vernadsky outlined the conditions that were required for the creation of the noosphere: equality for all people and an end to wars, poverty and hunger. Today, Vernadsky`s vision of the world is more important than ever before.

Ivan Pavlov

Ivan Petrovich Pavlov was born on September 14, 1849 at Ryazan, where his father, Peter Dmitrievich Pavlov, was a village priest. He was educated first at the church school in Ryazan and then at the theological seminary there.

Inspired by the progressive ideas which D. I. Pisarev, the most eminent of the Russian literary critics of the 1860's and I. M. Sechenov, the father of Russian physiology, were spreading, Pavlov abandoned his religious career and decided to devote his life to science. In 1870 he enrolled in the physics and mathematics faculty to take the course in natural science.

Pavlov became passionately absorbed with physiology, which in fact was to remain of such fundamental importance to him throughout his life. It was during this first course that he produced, in collaboration with another student, Afanasyev, his first learned treatise, a work on the physiology of the pancreatic nerves. This work was widely acclaimed and he was awarded a gold medal for it.

In 1875 Pavlov completed his course with an outstanding record and received the degree of Candidate of Natural Sciences. However, impelled by his overwhelming interest in physiology, he decided to continue his studies and proceeded to the Academy of Medical Surgery to take the third course there. He completed this in 1879 and was again awarded a gold medal. After a competitive examination, Pavlov won a fellowship at the Academy, and this together with his position as Director of the Physiological Laboratory at the clinic of the famous Russian clinician, S. P. Botkin, enabled him to continue his research work. In 1883 he presented his doctor's thesis on the subject of «The centrifugal nerves of the heart». In this work he developed his idea of nervism, using as example the intensifying nerve of the heart which he had discovered, and furthermore laid down the basic principles on the trophic function of the nervous system. In this as well as in other works, resulting mainly from his research in the laboratory at the Botkin clinic, Pavlov showed that there existed a basic pattern in the reflex regulation of the activity of the circulatory organs.

In 1890 Pavlov was invited to organize and direct the Department of Physiology at the Institute of Experimental Medicine. Under his direction, which continued over a period of 45 years to the end of his life, this Institute became one of the most important centres of physiological research.

It was at the Institute of Experimental Medicine in the years 1891-1900 that Pavlov did the bulk of his research on the physiology of digestion. It was here that he developed the surgical method of the «chronic» experiment with extensive use of fistulas, which enabled the functions of various organs to be observed continuously under relatively normal conditions. This discovery opened a new era in the development of physiology, for until then the principal method used had been that of «acute» vivisection, and the function of an organism had only been arrived at by a process of analysis. This meant that research into the functioning of any organ necessitated disruption of the normal interrelation between the organ and its environment. Such a method was inadequate as a means of determining how the functions of an organ were regulated or of discovering the laws governing the organism as a whole under normal conditions - problems which had hampered the development of all medical science. With his method of research, Pavlov opened the way for new advances in theoretical and practical medicine. With extreme clarity he showed that the nervous system played the dominant part in regulating the digestive process, and this discovery is in fact the basis of modern physiology of digestion.

Pavlov's research into the physiology of digestion led him logically to create a science of conditioned reflexes. In his study of the reflex regulation of the activity of the digestive glands, Pavlov paid special attention to the phenomenon of «psychic secretion», which is caused by food stimuli at a distance from the animal. By employing the method - developed by his colleague D. D. Glinskii in 1895 - of establishing fistulas in the ducts of the salivary glands, Pavlov was able to carry out experiments on the nature of these glands. A series of these experiments caused Pavlov to reject the subjective interpretation of «psychic» salivary secretion and, on the basis of Sechenov's hypothesis that psychic activity was of a reflex nature, to conclude that even here a reflex - though not a permanent but a temporary or conditioned one - was involved.

This discovery of the function of conditioned reflexes made it possible to study all psychic activity objectively; it was now possible to investigate by experimental means the most complex interrelations between an organism and its external environment.

Subsequently, in a systematic programme of research, Pavlov transformed Sechenov's theoretical attempt to discover the reflex mechanisms of psychic activity into an experimentally proven theory of conditioned reflexes.

As guiding principles of materialistic teaching on the laws governing the activity of living organisms, Pavlov deduced three principles for the theory of reflexes: the principle of determinism, the principle of analysis and synthesis, and the principle of structure.

The development of these principles by Pavlov and his school helped greatly towards the building-up of a scientific theory of medicine and towards the discovery of laws governing the functioning of the organism as a whole.

Experiments carried out by Pavlov and his pupils showed that conditioned reflexes originate in the cerebral cortex, which acts as the «prime distributor and organizer of all activity of the organism» and which is responsible for the very delicate equilibrium of an animal with its environment. In 1905 it was established that any external agent could, by coinciding in time with an ordinary reflex, become the conditioned signal for the formation of a new conditioned reflex. In connection with the discovery of this general postulate Pavlov proceeded to investigate «artificial conditioned reflexes». Research in Pavlov's laboratories over a number of years revealed for the first time the basic laws governing the functioning of the cortex of the great hemispheres. Many physiologists were drawn to the problem of developing Pavlov's basic laws governing the activity of the cerebrum. As a result of all this research there emerged an integrated Pavlovian theory on higher nervous activity.

Even in the early stages of his research Pavlov received world acclaim and recognition. In 1901 he was elected a corresponding member of the Russian Academy of Sciences, in 1904 he was awarded a Nobel Prize, and in 1907 he was elected Academician of the Russian Academy of Sciences; in 1912 he was given an honorary doctorate at Cambridge University and in the following years honorary membership of various scientific societies abroad. Finally, upon the recommendation of the Medical Academy of Paris, he was awarded the Order of the Legion of Honour (1915).

Pavlov directed all his indefatigable energy towards scientific reforms. He devoted much effort to transforming the physiological institutions headed by him into world centres of scientific knowledge, and it is generally acknowledged that he succeeded in this endeavour.

Pavlov created a great school of physiologists, which produced many distinguished pupils. He left the richest scientific legacy - a brilliant group of pupils, who would continue developing the ideas of their master, and a host of followers all over the world.

Dr. Pavlov died in Leningrad on February 27, 1936.

From Nobel Lectures, Physiology or Medicine 1901-1921, Elsevier Publishing Company, Amsterdam, 1967.