农学园艺植保专业外语-山西农业大学

1 The Parts of A Plant and Their Functions A plant is a living organism. It is made up of different parts, each of which has a particular purpose, or specialized function. If one part of the plant is not functioning properly the whole plant will suffer. But we may cut flowers off the plant or prune the roots. Such damage is only temporary and so the plant will continue to grow. Study the following statements carefully and write down whether they are true or not true according to the information expressed above. Different parts of a plant have specialized functions. Not all parts of a plant need function properly. If we cut flowers off a plant, the whole plant will suffer. The basic parts of a plant are the root system, which is below the ground, and the shoot system above. The root of a plant has two main functions. It takes in, or absorbs, water and minerals from the soil through the root hairs, which are single cells near the tip of each root. The other main function of the root is to hold, or anchor, the plant firmly in position in the soil. The shoot system of the plant is below the ground. Water and minerals are absorbed through the root hairs. The plant is anchored in the soil by the root system. Plant such as sugar beet and carrots are able to store food in their roots. In this way they can keep growing for more than one season. In addition, plants such as clover and lucerne, known as ‘legumes’, have special bacteria which live on the roots. This simple forms of life take nitrogen out of the air which is in the soil. Such leguminous plants are usually ploughed under the soil. By doing this the soil is made more fertile. Carrots can keep growing for more than one season. Special bacteria live on the roots of all plants. The soil is made fertile by ploughing clover and lucerne under the soil. The shoot system above the ground consists of the stem, the leaves, flowers and fruit. One of the functions of the stem is to support the plant. Another important function is to enable water and minerals to pass up from the roots to the leaves and flowers. Organic materials such as sugar travel down the stem to the roots. The leaves grow out of the side of the stem. Their main job is to make food for the plant by the process known as photosynthesis. For this process sunlight is necessary. Water from the soil and carbon dioxide from the air are converted into sugars and other carbohydrates. During the process oxygen is formed and released into the air. The flower contains the reproductive organs of the plant. The stamens produce the male sex cells, or spermatia, which are carried in the pollen grains. The carpel produces the female sex cells, or ovules. The fruit, the ripened ovary of the flower, encloses the seeds and protects them while they are developing. The seed itself consists of an embryo and foodstore. The embryo is the part which will develop into another plant and the foodstore is necessary to provide nourishment for the young plant while it is growing. 2 The Life Cycle of a Plant The life cycle of a typical annual plant can be divided into several stages. The first stage is germination. Seeds remain dormant, or in a resting state, if they are kept cool and dry. When the amount of moisture and the temperature level are right, the seeds germinate and start growing. Certain conditions are necessary for this to happen. An essential condition is that the seeds must be alive. Sometimes seeds are dried at a temperature which is too high. This has two effects: the water content in the seeds is reduced too much, and certain essential proteins are destroyed. As a result, the seeds die. Before a seed germinates it is in a dormant state. When the temperature level is right a seed will germinate. If seeds are dried at a too high temperature they will die. Other conditions for germination concern the amount of moisture in the soil. If dry seeds are planted in a dry soil, they will not germinate until it rains. On the other hand, if there is too much water in the soil, the seeds will not germinate either. This is because wet soils remain cold for a longer period of time than drier, well-drained soils. If the soil is too cold germination will not occur. An additional reason for seeds not germinating is that badly drained soils may lack sufficient oxygen. Dormant seeds require very little oxygen in order to stay alive, but when they start to germinate they require more. If a soil is too dry seeds will not germinate. The temperature of wet soils is lower than that of well-drained soils. Dormant seeds cannot stay alive in a badly drained soil. In the first stage of germination the primary root, or radicle emerges. Then the stem pushes its way upward until it appears above the surface of the soil. At the same time the root system grows downward, and begins to spread through the soil. In the early stages of development the seedling depends entirely on the foodstore in seed but as soon as the first leaves are produced, it is able to manufacture food for itself. The seedling begins photosynthesis. The root system forms after the stem appears above the surface of the soil. The seed contains enough food to nourish the seeding until the first leaves are produced. Next, the plant enters the stage of rapid growth. In this stage of the life cycle, the plant begins to grow to its full size. When it is mature enough, it flowers, and when this happens pollination and fertilization are ready to take place. In the process of pollination the pollen is carried by wind or insects from the stamens to the stigma of the carpel. It germinates on the stigma and grows down the style into the ovary, where fertilization takes place. 3 The Origin and Composition of Soil Soil is a residue composed of two main ingredients: mineral material and organic material. Organic material originates from dead plants and animals and materials other than this are derived from rocks of various kinds. These rocks are broken down into small particles by mechanical disintegration and chemical decomposition. This breaking down process, known as weathering, may thus be both physical and chemical. When weathering processes are largely physical - by heat or wind, for instance – the composition of the soil is very similar to that of the parent rock. In arid regions weathering is mostly by physical means. But in humid regions chemical processes of weathering are equally important. In such regions rock particles are affected by water which may contain carbonic or other weak acids. These acids dissolve some of the particles in the rocks. The mineral material that is left behind is insoluble. Consequently, the insoluble mineral residues in the soils have less resemblance to the original rocks. There are larger amounts of organic matter in the soil, too. Mineral materials in soil are derived from various kinds of rocks. Physical and chemical weathering breaks down rocks into small particles. The composition of the soil in arid regions has a close resemblance to that of the parent rock. Soil of humid regions are similar