constructions (you are given 30 min.)
a) The units of living matter cover a wide range of sizes. A few sorts of cells are large enough to be seen by the unaided eye; to be seen, they must be at least 0.1 millimeters, or 100 microns, in one dimension. Many animal eggs, which are single cells, are this large. Among the few plant cells which are this large are the cells in the fleshy portion of the watermelon. Most cells are smaller than this and are said to be microscppic in size; that is, within, the range of an ordinary light microscope. Below this range is another into which fall the viruses. Particles of known viruses generally occupy the size range 100 to 1 000 A or 10 to 100 millimicrons. Thus, viruses are individually invisible in the light microscope and are said to be submicroscopic, although they can be visualized with the electron microscope. Within this same size range fall several aggregations of molecules which make up the structural components of many types of cells. Between the viruses which have not been demonstrated to have cellular organization, and the bacteria, which have been demonstra- demonstrated to have a characteristic type of cellular organization, falls a group of organisms known as the Rickettsias. The Rickettsias are considered by some investigators to be cellular; by others to be noncellular and perhaps similar to the viruses.
b) The chemical analysis of plants is to show what the plant contains, what foodn it requires; the chemical analysis of the soil is to show what the tatter lacks; a comparison of the results of the first analysis with those of the second will give an answer as to how soil fertility \s to be raised. The result of this analysis cannot be considered complete unless it is concluded by a summarized synthesis. Besides, both the physical and the chemical analysis of. the soil are needed. But neither the one nor the other taken separately, nor both together, can solve the problem of soil fertility, still less the problem of the development of fertility, of the development of soils. This evolution can be understood only if we study soil as a developing integral whole governed by the activity of plant and animal organisms. We cannot
imagine either the origin or the ior-% mation of soil without the direct participation
of plants? Plant physiology is the principal basis of all the conclusions of agricultural science. If the soils of today are to be cleared of plants for a number of years it will rapidly lose its fertility and become barreh dust.
UNIT 9
THE STUFF OF LIFE.
In their attempts to solve the mysteries of life, scientists have given much attention to the jelly like living material of the cell. This substance is called protoplasm. They have studied it under high-powered microscopes; broken it down into its basic chemicals; treated it with dyes and electric currents; and dissected it with microscopic needles. Yet no one has succeeded in making any protoplasm. It is one of the most complicated of all substances. We have learned many facts about it, but there are still many secrets to be discovered. Scientific research goes on, because protoplasm is the key to a real understanding of life.
Under the microscope, protoplasm is an almost colourless substance. At times it is quite liquid, but it can easily change to a more solid jelly. All the living parts of the cell, including the cell membrane, the cytoplasm, and the nucleus are made of protoplasm. With a high-powered microscope we can see many small particles and bubbles floating in the jelly. These are often in rapid motion.
The chemical nature of protoplasm is not exactly known. Unfortunately, when chemists begin to analyse it, it usually dies. This brings about changes in the material they are studying. We do know that protoplasm is usually more than 75 per cent water. There are also salts and food materials such as sugars, fats, and proteins. Four chemical elements make up 98 per cent of protoplasm. These are carbon, oxygen, hydrogen, and nitrogen. More than 15 other elements have been found. All of these are the common elements of which our earth is composed.
There are no special elements that are found only in protoplasm. But such rare elements as strontium (Sr), rubidium (Rb), tin (Sn), nickel (Ni), gold (Au) and mercury (Hg) may enter into the composition of protoplasm as well. Where the soil is especially rich in certain minerals, the plants growing there may incorporate them, and they may find their way into the tissues or hard parts of animals that feed upon the plants. In some parts of the world gold is particularly abundant in the soil, and the hoofs, horns and hair of the deer living on the vegetation in these regions show relatively large accumulations of it. Radioactive elements in some regions are accumulated in the mosses and in vegetation of the region. These plants are the food for many animals and analysis shows that these animals are also accumulating radioactive particles in their tissues. The food chain is extended to people living in these regions who feed upon these animals and in turn incorporate the particles in their own tissues. As a result their bodies contain a relatively high account of radioactive particles as compared with the population in general.
As a summary it should be noted that protoplasm is a very complicated mixture of many kinds of substances. These are in constant activity, carrying on the processes of life. When the activity stops, life comes to an end.
Notes to the text:
1. to go on — продолжать
2. I am going to go on — Я собираюсь продолжать
3. at times — временами
4. to treat — подвергать действию, лечить
5. good fortune — удача
6. bad fortune — неудача
EXERCISES
