The earliest forms of life on earth that we know dateback to about 3.5 billion years ago. These organisms were called bacteria andblue-green algae. Photosynthesis is said to most likely have first appeared inbacteria. The earliest land plants appeared on earth more than 400 millionyears ago.
The first ever seed plants appeared about 350 million years ago. Usually,they are called seed ferns because they have large fernlike leaves.Plants have four types of growth responses to thethings around them.
These responses are called tropisms. There are types oftropisms: phototropism, geotropism, thigmotropism, and chemotropism.Phototropism refers to its response to the directionof light, geotropism refers to its response to gravity, thigmotropism refers toits response to water, and chemotropism refers to its response to a certainchemical. One of the broad characteristics of plants is that they tend to growcontinuously throughout their lives. There are two sides of plant growth:primary and secondary. Primary growth takes place in young organs, which givesthe result of the increase in length of the plant.
Secondary growth refers to theincrease in the width of the plant.Radiation refers to the transmission of energy inthe form of waves. Any kind of energy that is released through the form ofwaves is called electromagnetic radiation. Each kind of electromagneticradiation follows the same physical laws as the rest. All of them travel in thespeed of light, and when they fall on a surface they release a pressure. There are three types of radiation: alpha, beta, andgamma.
Alpha particles interact with matter due to theircharges and combined mass. Normally, alpha particles can only penetrate a fewmillimeters of materials with low density, meaning that alpha particles cannot penetratethe outer layers of dead skin cells and live tissues. There are a few high energyalpha particles that are made up about 10% of cosmic rays. They can penetratethe body and even thin metal plates.Beta radiation can penetrate more than alpharadiation can. Beta radiation fromradioactive decay can be stopped by a few centimeters of plastic or a fewmillimeters of metal.
Sometimes, beta radiation can be used in radiotherapy togive treatment to external tumors. Gamma radiation works to get rid of an unstablenucleus of excess energy. Both alpha and beta particles have an electric chargeand mass, and can interact with other atoms along their way. However, gammaradiation is made up of photons, which do not have mass or electric charge,which makes it have the ability to penetrate much further through matter thanalpha and beta radiation. Gamma rays can be stopped by a thick or dense layerof material, and it doesn’t matter whether the material has high or lowdensity. At low levels, radiation can have a positive effecton plant growth, and at high levels, very harmful effects. Radiation can affectthe way a plant develops and germinates. The amount of damage is determined by theamount radiation that is received by the plant.
Becauseof radiation, soil can become compact and can also lose the nutrients that areneeded for a plant’s growth. Intense radiation kills plants but in a differentway. Trees and shrubs change in the way they react to radioactive substances.