The large cabbage white butterfly

 

       The large cabbage white butterfly is very common in early summer and can be easily recognized by its large white wings with black markings. The head of it bears a pair of large compound eyes and also three small simple eyes which are hidden among the hairs on the top of the head. It feeds on the sugary solution which is called nectar.

       The female cabbage white butterfly lays eggs in April or May on the underside of cabbage leaves. After 8 to 10 days tiny caterpillars appear. The caterpillars first eat the eggshell and then the leaf of the cabbage. It rapidly increases in size and is fully grown when it is about 3,8 cm long. After about thirty days the caterpillar moves down its leaf into the ground and proceeds to a place suitable for the development into the next pupal stage. Inside the pupa[25] there is a reorganization of structure and it takes from two to three weeks for the butterfly[26] to become fully developed.

 

THE HONEY BEE

 

       There are many kinds of wild bees, some of which are solitary while others, such as honey bee, are social and form communities. A hive of bees is a highly organized society of insects all working for common good.[27] In this society there are three kinds of individuals. In each hive there are many thousands of worker bees and two or three hundred drones, but only one queen bee.[28] It makes from fifty to eighty thousand bees in a hive. The development from egg to perfect insect takes about three weeks in all. In summer the worker bee lives for about five weeks more before dying of overwork. When its wings are worn out and it is no longer of any use to the hive, it usually flies away to die.

       If the worker has brought the nectar, it performs a peculiar little dance. Taking rapid steps, it runs in small circles, first in one direction and then in another. The worker may dance for a few seconds or for a minute. The dance is performed each time it returns with nectar and it indicates to the bees where the nectar is to be found.[29]

 

STINGING AND BITING INSECTS

 

       Mosquitoes are insects which are well known because of their biting habits. Some kinds of them not only disturb people but also there are dangerous. When an infected person is bitten the malaria parasites pass with the blood into the stomach of the mosquito. When another person is bitten the malaria organisms pass into the blood of the victim.

       There is difference in the habits of male and female mosquitoes. Only female mosquitoes are bloodthirsty,[30] male mosquitoes do not bite.

       The mosquito’s “song” is made by the rapid movements of the wings. Here are the results of measurements of the number of wing-beat per second: housefly 180 to 330; cabbage butterfly 9 to 12; honey bee 190 to 250; mosquitoes 280 to 580.

       It is said that the most dangerous animal in the world is the housefly. Scientific studies of the insects show that the housefly and its relatives may transmit about thirty diseases.

       If a honey bee has stung you, you’ll know that it’s rather painful. But you may not know that many of these insects pay with their lives for biting a person. The sting of a honey bee has several small hooks and when the sting penetrates the skin, the hooks catch and prevent the bee has used its sting, it breaks from the bee’s body and remained in the wound. This so injures the insect that it dies within a short time.

       It is considered that approximately 500 honey bee stings would kill a person.

 

SPIDERS

 

       Most spiders feed insects. They have poisonous bite with which they kill their victims. Some spiders spin webs in which other insects are caught. The webs of some tropical spiders are as much as two meters in diameter and are so strong that they can even hold small birds.

       Spider silk is among the strongest materials known. It will stretch one-fifth of its length before breaking and its tensile strength exceeds the strength of steel.

       Natives in some regions have found many uses for these webs. The complete web is often employed as a fish net and will hold a catch[31] over a kilogram.

       The common expression that the female is deadlier that the male is correct for spiders. Female spiders are often larger than males; sometimes they may even make a meal of their husbands.

       Among the different kinds of spiders there are few which are dangerous to human beings. The species with the worst reputation are the tarantulas and the “black widow”.

       The big hairy tarantulas are mainly found in tropical and subtropical areas. Not all the species of tarantulas cause death to man. People who have been bitten by various kinds of tarantulas describe the effects as similar to those of bee stings.

       The “black widow” is so called because the female is black in colour and she has the habit[32] of eating the male. The female is easy to identify: there is a red or orange-colored spot on her body.

The “black widow” is quite different from tarantulas. A biologist, who suffered no bad effects from the bites of several kinds of tarantulas, had to be taken to the hospital, when he allowed a “black widow” bite if the necessary medical treatment is not given to them.

Scorpions do not look much like spiders, but the two are related.

Unlike spiders[33] scorpions do not have a poisonous bite. They have poisonous sting at the tail. This sting curves over the back of the creature when it attacks or defends itself.

Scorpions are not sociable. If two of them meet, it is probable that one will make a meal for the other. There is a common saying that[34] two scorpions are found together only if they are mating or if one is eating the other.

During the day scorpions hide under stones or in holes and at night they go out searching for food.

There are two myths about scorpions. One is that young scorpions eat their mother soon after they are born. Another myth is that scorpions kill themselves with their own stings if in danger.[35] The truth is that many scorpions are immune to their own poison.

As one might expect the effects of scorpion stings vary according to particular species. There is also some variation in human’s sensitivity to the venom. In Mexico, there is a species that can kill an adult in less than an hour. Over a period of 35 years, it has been responsible for the death of approximately 1600 people. Dangerous scorpions also occur in Africa and several other tropical countries.

 

BOTANY

       Since ancient times people tried to find medicinal herbs and poisonous plants.[36] At that time plants were valued for their utility and their medicinal properties. They were considered to be a gift from the gods. Many centuries before the Greeks, the Egyptians who had a strong liking for flowers, recorded the names of herbs with healing properties[37] on a long papyrus.

       In the Middle Ages voyages of discovery revealed to the naturalists a lot of unknown plants and animals. Now their task was not only to describe them, but also to classify them systematically. By the end of the eighteenth century the scientists came to discover why flowers have colour, the purpose of leaf, and the structure of seeds and fruit.

       In the nineteenth century the improved microscope led naturalists deeper and deeper into the world of cellular structure[38] and then to the wonders of tissue to the growth of new substance, the working of heredity and many other wonders of Nature.

 

THE HIGHER LAND PLANTS

 

       According to their length of life plants are classified as annuals, biennials or perennials. An annual plant is one which completes its life cycle within a year, a biennial takes two years to complete it and a perennial is one that lives for many years. An annual, such as for instance mustard germinates,[39] grows up, flowers and develops seeds during spring and summer and then dies. Such plants pass the winter safely in the form of dry seeds.

       The carrot and the cabbage are examples of biennials. In the spring and summer of the first year of a biennial leafy shoots are developed and the food is stored in the thick stem. In the following spring a long leafy shoot grows up, flowers and develops seed.

       Perennials are able to live for many years, because each year after flowering and seed formation they are able to store up sufficient food to provide for the growth of new shoots in the following spring. Although most perennials flower every year, trees often do not flower until they are ten or twenty years old.

       According to their size and structure flowering plants can be classified as herbs, shrubs and trees. Shrubs and trees are distinguished from herbs by their larger size and woody structure. A tree is a woody plant with a main stem or trunk which usually does not form large branches until it has reached a certain height from the ground. A shrub, on the other hand has a stem which divides into branches at a level close to the ground.

       In the case of many trees and shrubs, the leaves drop off during the autumn; such plants are said to be deciduous. Trees and shrubs which retain their leaves during the winter are termed evergreens.

 

GENETICS

 

       The word genetics is derived from Greek word genesis which means descent. It is the science which deals with heredity. Genetics tries to discover the mechanism whereby qualities are inherited from parents to offspring. This science also attempts to predict the outcome of particular matings in certain animal and plant species. Genetics is a comparatively recent science and as yet we know relatively little about heredity in man. Genetics is also of great theoretical interest, since modern theories of evolution are based on the science of heredity.

       During the early part of the nineteenth century many investigators studied the problem of the instability of hybrids. They found that certain types of inheritance were frequent in particular hybrid matings, but their experiments were not carried out on a large enough scale to allow them to predict the proportions of the various offspring obtained by the mating of hybrids nor were their experiments numerically recorded and analysed.[40]

The first large-scale and carefully controlled experiments on heredity in plants were carried out by an Austrian named Gregor Johann Mendel (1822-1884).

Mendel’s experiments were carried out on peas. That particular kind of plant proved to be suitable[41] for his work for the following reason:

There are a number of varieties, which are easily distinguished by differences in height, shape and colour of seeds.

In his first experiment Mendel describes he chose the form and colour of the seed for study. One variety of the plant had smooth round seeds, while another variety had wrinkled seeds.

Mendel crossed these two varieties of plant and observes that only round seeds were produced. Continuing this experiment Mendel allowed the hybrid plants which formed these round seeds to pollinate themselves,[42] and produce seeds. From 253 hybrid plants 5474 round seeds and 1850 wrinkled seeds were obtained. Further experiments showed that about one-third of the round seeds behaved like the original round seeds of their grandparents and produced round-seeded offspring only, while the other two-thirds resembled those seeds which formed their hybrid parents. It appears that approximately one half of the offspring of hybrid parents resemble their parents, while the other half has equal chances of resembling either one or the other grandparent. By means of these experiments Mendel showed that inheritance of certain characteristics, at least could be predicated with reasonable accuracy,[43] and since the types of offspring of hybrid crossed appears in definite numerical proportions. The experiments on hybrids, which form the earlier part of Mendel’s work, showed that, in some cases, the inheritance of characteristics could best be explained in terms of factors or genes.

Subsequent research made by many investigators in different countries has confirmed Mendel’s ideas but showed that inheritance is often complicated by the influence of various conditions, environment in particular.

The Mendelian factors, or genes, are found in the chromosomes of the nucleus; there is evidence that nucleic acids, also found in the nucleus, are related to genes.

 

CELL DIVISION AND DNA [44]

There are three main parts in every human or animal cell. First, it has a thin outside shell[45] called a cell membrane.[46] Inside the cell there is a small, rounded body called the nucleus. Filling the rest of the cell is substance called cytoplasm.

Within the nucleus there are threads of material called chromosomes. The chromosomes are important in process of making new cells. The chromosomes make copies of themselves and then separate. The cell splits into two cells and each contains chromosomes.

All attempts to separate the nuclei failed for a long time. Work continued on cells until it was shown that the nucleus alone determined how the plant would grow.

Soon it was found that all viruses contain nucleic acid and a protein shell.

It was discovered that there are two nucleic acid “brothers” – deoxyribonucleic acid and ribonucleic acid – DNA and RNA.

It was learned that a little string of DNA hidden in the nucleus of a cell stores and then sends from one cell to another all the information necessary to create a new living thing. We know that to build even the simplest house requires pages of drawings, details and measurements. How could these little pieces contain plans for a living being? It seemed impossible, but everything indicated that DNA did the job.

By 1953, the scientists had discovered much more about DNA. They knew that DNA was a huge molecule. They knew that it contained sugar molecules and phosphate molecules that were joined to each other. In addition to the sugars and phosphates, there were four bases that are called A, G, C and T. There were thousands of these six different pieces, each with its own shape and size, in the DNA molecules.

Some scientists tired to build a model of a DNA molecule. They began to build it using wire and many pieces of metal. Each piece of metal represented a piece of the DNA molecule, either a sugar, a phosphate, or one of the bases. The wire was used to hold the metal pieces to each other. They tried many times to fit the pieces together. They found that the pieces would not fit where they placed them. Each failure taught them more about the arrangement of molecules within DNA. They realize that only one model would be correct.

Finally the pieces began to fit into the right places. The phosphates and sugar molecules formed long curving lines.[47] The four bases were attached to them to form a ladder. The sides of the ladder were made of the sugars and phosphates. The steps of the ladder were the bases – A, G, C and T.

But the bases were of different sizes. A and G were bigger, longer, C and T were smaller, shorter. How could there be a ladder with steps of different sizes? They discovered that two bases were required for each step. Each step had to contain one long base and one short base. Even so, there are four possible arrangements of the bases that form these steps. The whole truth then was that all DNA molecules had the same six pieces (sugars, phosphates, and four bases) and all were in the same shape (a curving ladder). Only one thing could change – the other of steps.

The DNA was found in the nucleus, the RNA – in both the nucleus and cytoplasm, the substance of the cell outside the nucleus. Experiments showed that RNA moved from the nucleus out to the cytoplasm. The DNA contains the directions for making the living material. Within the nucleus, in a way as yet unknown, the DNA passes its protein manufacturing instructions to the RNA. Then the RNA goes out into the cytoplasm to help in the manufacture of the proteins. In 1955 some RNA was synthesized in a laboratory. One year later some DNA was made.

 

A DROP OF BLOOD

 

Do you know that we have between 250 and 300 million red cells in each drop of blood?

As for the white cells there are only about 350 to 500 thousands of them in a drop of your blood so long as you are in good health. But suppose[48] you get an infection – appendicitis, for example. Almost immediately the number of white cells begins to increase until you may have as many as a million and a half in a drop of blood.

The disease germs produce certain chemicals in the body. These chemicals make the white cells divide, so that each white cell produces two new cells. The chemicals also stimulate the movement of the white cells toward the place where the germs are causing the infection. Then, when a white cell is close to a germ, it is further stimulated to change its shape. It wraps[49] the germ up. The germ is now inside the white cell, which then proceeds to digest it. So we can say that the white cells really eat up disease germs. Our blood also has proteins in it. And it has a lot of little cells called platelets, which contain a special chemical. Ordinarily, of course, this chemical stays inside the platelets. But, suppose, you cut your finger. The blood comes to the surface of the wound and the platelets break down. The chemical is released and it starts to affect the gelatin – like protein in the blood, building a network of fibres. Soon the network is big enough to form a jelly-like stopper for the wound. We say that the blood has clotted and the wound stops bleeding.

 

 

PROTEINS

 

Protein molecules are the largest and most complex of all molecules; they may be up to ten million times as heavy as a hydrogen atom[50] and be built of many thousands of atoms – mainly of carbon, hydrogen, oxygen and nitrogen. Proteins are the fundamental units of living material; among other compounds of vital importance[51] they include the enzymes, which bring about most of the chemical changes in protoplasm. Proteins are built of units called amino-acids. When a protein is treated with acid it[52] will break up into chains of molecules called peptides[53] and finally into amino-acids.

There are about twenty different kinds of amino-acids, the molecules of which vary in size from a molecular weight of 90 to one of about 250. An average protein molecule contains about 500 amino-acid molecules. In order to appreciate[54] the complexity of a protein, we may compare the molecule of a typical carbohydrate[55] (glucose built of 12 atoms), a fat (about 170 atoms), and protein. The protein lactoglobulin[56], found in milk, has about 5940 atoms in the molecule.

Carbohydrates, fats, and proteins are very important in living material, but there are many other substances, such as the nucleic acids, vitamins, etc. which play a vital part in living organisms. The human body consists approximately of 65 per cent water, 15 per cent protein, 14 per cent fat, 5 per cent inorganic salts, and about 1 per cent of other materials.

 

ENZYMES [57]

 

No living organisms are stable. They are in a continual state of chemical change.

All the time that we are alive we are continually dying and being reborn.[58] About three million of our blood cell dies while we read a sentence, and at the same time three million more cells are made. The word metabolism (derived from Greek that means change) is used to signify the chemical changes which take place in living organisms. By using of suitable measuring instruments it is possible to follow the blood cells in the body and to find out how long they live. This work has revealed that the average life of each blood-cell is about a fortnight. Even bones are much less stable than we might expect.

It is clear that since the living body is in a state of constant change, the chemical changes must be precisely controlled.

We now know that each of the many thousands of chemical reactions which take place in a living body is controlled by a particular enzyme. Enzymes are organic catalysts which take part in the chemical processes occurring in living bodies.

The change of the one form of sugar into another, a reaction which is common in living organisms, is a simple instance of the action of an enzyme

C₂H₂₂O1+H₂O=2C6H12O6

In words this means that in the presence of water a molecule of sucrose may split into two smaller molecules, one of glucose and the other of fructose. These are sugars which each have the same molecular formula C6H12O6; but the atoms are arranged differently in each sugar.

A great number of enzymes bring about hydrolysis – that is a chemical reaction in the presence of water. The process of digestion is brought about by hydrolyzing enzymes.

Another group of enzymes brings about oxidations, i.e. reaction during which pairs of hydrogen atoms are removed from molecules and are combined with oxygen atoms to make water.

It seems probable that a single cell may contain a thousand or more different enzymes which bring about a similar number of chemical reactions.

 

REGENERATION

 

Sometimes, as a result of accident or by design,[59] an organism loses a part of itself. The extent to which organisms can repair themselves[60] varies greatly. Plants can survive considerable damage. As a rule, the loss of parts is followed by rapid growth of development of similar parts elsewhere on the plant.

Among animals, however, the ability to replace lost parts is confined to[61] the lower animals for instance the hydra. The hydra is an animal of simple structure living in water. Its body is in the form of a cylindrical tube with a disc-shaped base by which it attaches itself to any moving substance. Its mouth is surrounded by tentacles by which it catches its blood. If a hydra is cut into two horizontally, it can grow a new set of tentacles on the old body, while the other half grows a new foot.

The flatworms are remarkable, for they can be cut into pieces and each piece will grow into a complete flatworm.

In higher organisms, such as man, the regeneration is limited to tissue repair, by means of which wounds are healed