The Life Cycle of a Plant
Although various species of plants may look very different from each other, the life cycle of a plant is consistent. All plants need water, sunshine, minerals, warmth, oxygen, and carbon dioxide in order to grow well. Green plants make food by combining carbon dioxide from the air and water from the surrounding soil in a process called photosynthesis.
Sunlight is the energy source that makes photosynthesis possible. A chemical in the plant—a green pigment called chlorophyll—traps the sun's energy and releases it. causing a chemical change. When carbon dioxide and water are combined, they produce a sugar called glucose. The plant does not need the oxygen from the water, so it is expelled from the plant as a waste product. The glucose stores energy from the sun, which the plant uses to collect salts from the soil and to grow. The glucose that is not used is converted into starch. The plant can store the starch to use later as a food source. During the growth cycle of a plant, the roots, leaves, stems, and flowers all have specific functions. The roots play a crucial role in a pianf s existence. They consist of an underground network that spreads through the soil. The roots anchor the plant firmly in the ground.
Water and dissolved minerals from the soil—such as nitrates, phosphates, and potassium—enter the rpots through the tiny root hairs on the root tips. The more roots a plant has, the more water and minerals it can take in.
Roots grow into the ground, pulled down by the force of gravity. This response to gravity is controlled by a chemical called auxin, which is made by cells in the tips of the roots. The response of plants to gravity is called geotropism. Roots grow toward the pull of gravity (positive geotropism), and the stems grow away from it (negative geotropism).
There is a vascular system of two types of small tubes in the root called xylem and phloem. These tubes are grouped in the center of the root. Water and minerals are drawn into the root through the root hairs and then up the xylem to the leaves. The plant's food—glucose (sugar)—is made in the leaves. It passes down the phloem to the growing cells at the tips of the roots where it is used to make cellulose for the new cell walls.
Roots also act as a food store. Throughout the summer growing season, the plant produces more sugar than it needs. When the sugar is subsequently turned into starch, it is stored in the underground stems and roots. It stores this starch during the dormant winter months and then uses it the follow ing spring as a food reserve. This food reserve is what provides energy for new buds to begin grow ing.
There are two basic types of roots: fibrous roots and taproots. Fibrous roots grow from the stem of the plant and branch out in a dense network of roots. Some of these are side roots, which spread out laterally in search of water and nutrients. They also function to support the plant. Grasses have a fibrous root system. A taproot is a long root that burrows down into the soil to anchor the plant and search for water during droughts. Carrots and turnips are examples of taproots.
Some plants have taproots as well as fibrous root systems. The process a plant goes through in order to produce its own food. Carbon dioxide and water combine to make glucose, while oxygen is released as a waste product. Glucose stores energy from the sun and provides food for the plant.
The ends of each root have tough skin on them for protection as they burrow through the soil. When water enters the soil, minerals from the soil particles are dissolved. The water and minerals are the nutrients the plant needs to grow . They pass through the root hairs into larger passageways. They travel through these passageways and supply nutrients to other parts of the plant.
Some specialized roots grow' above ground. For example, buttress roots are huge roots that grow7 out of the trunks of some rain forest trees.