Age of the Earth

During the early part of the present century the Earth was thought to beat least 100,000,000 years old, possibly as much as 500,000,000 years old. The.former estimate of the minimum age of the earth was determined from а study of the ocean salts. Chemical analyses of ocean water indicate that salts are present there in the following proportions:

These various salts in the oceans are carried there in solution by the streams, along with other land materials that are carried as sediments. All materials except the salts tend to become sediment at the ocean bottoms, while the salts accumulate in the ocean water. So if we can determine the total amount of these salts in the oceans and divide that by the total quantity contributed annually by the streams, the quotient willbe the time, in years, that this process has been going on, in other words, it will be the age of the oceans.

There are several chances for slight errors to be made in this computation since one cannot be sure that the conditions in the past were always the same as now. For instance, the area of the land regions was probably less in the past than now. If less land was above water the streams from it were probably not very swift so they carried less sediment and mineral matter in solution. Besides, some of the salt of the oceans may have come from the rocks on the shore lines and at the bottom, some of it we know was lost to the oceans when the rock salt beds were formed. Allowing for the variable factors as best we can - the age of the oceans has been estimated to be about 360,000,000 years. If this is the age of the oceans we know at once that the earth must be older since it had to come first to make а place for the oceans.Some scientists have argued that this first estimate is still too small since salt is being dissolved much faster now than in the past; hence they would make their estimate of the probable age of the earth four or five times greater.

10. Controlled Environments.

Man cannot quietly adapt to the existing environrnenta1 conditions of his agricultural areas. He has altered these conditions by clearing the land of unwanted species, controlling weeds, increasing soil fertility and providing water by means of irrigation. Man has also been actively developing crop varieties which are better adapted to various environments.

With the ever-increasing demand for food, knowledge of plant response to the environment has become more and more important. Greenhouses or glasshouses, plant-growing chambers and phytotrons are used to obtain answers to many problems now facing the agricultural science, the most important of them being the determination of maximum yield that is possible and optimum conditions for maximum growth.

The controlled-environment rooms have become very important in biological research. They are important in obtaining biological knowledge to improve agricultural practices and produce.

Controlled environments for plant growth serve several purposes:

1) to determine how environmental factors affect plant development; what factors should be controlled and when such controls are useful and/or necessary during the life-cyc1e of the plant;

2) to establish the maximum yield that farm crops can be expected to produce;

3) to supplement field research in plant breeding and the introduction of new crops;

4) to provide conditions useful for biochemical and physiological investigations which can give necessary information for solution of agricultural problems;

5) to understand how environment affects pathogenesis, and some others.

Much investigation of environment control is stil1 carried out in the field through irrigation, weed control, fertilisation and frost prevention.

Plant-growing chambers and phytotrons provide most ideal conditions for plant growth, but now they are not

Date: 2015-10-19; view: 333; Нарушение авторских прав