Plants can be divided into two groups, based on how they get their food. All green plants are called autotrophs. They contain chlorophyll, which enables them to capture the sunlight used in producing the food and other materials they need for growth. Other kinds of plants, called heterotrophs, lack chlorophyll and cannot make their own food. They are either parasites or saprophytes.
This section discusses the four major processes that take place in the growth of most kinds of green plants. These processes are (1) germination, (2) water movement, (3) photosynthesis, and (4) respiration. The section also discusses how a plant's heredity and environment affect its growth.
Germination is the sprouting of a seed. Most seeds have a period of inactivity called dormancy before they start to grow. In most parts of the world, this period lasts through the winter. Then, after spring arrives, the seeds start to germinate.
Seeds need three things to grow: (1) a proper temperature, (2) moisture, and (3) oxygen. Most seeds, like most kinds of plants, grow best in a temperature between 65 °F (18 °C) and 85 °F (29 °C).
The seeds of plants that live in cold climates may germinate at lower temperatures, and those of tropical regions may sprout at higher temperatures. Seeds receive the moisture they need from the ground. The moisture softens the seed coat, allowing the growing parts to break through. Moisture also prepares certain materials in the seed for their part in seed growth. If a seed receives too much water, it may begin to rot. If it receives too little, germination may take place slowly or not at all. Seeds need oxygen for the changes that take place within them during germination.
The embryo of a seed has all the parts needed to produce a young plant. It may have either one or more cotyledons, which digest food from the endosperm for the growing seedling. The seed absorbs water, which makes it swell. The swelling splits the seed coat, and a tiny seedling appears. The lower part of the seedling, called the hypocotyl, develops into the primary root. This root anchors the seedling in the ground and develops a root system that supplies water and minerals. Next, the upper part of the seedling, called the epicotyl, begins to grow upward. At the tip of the epicotyl is the plumule, the bud that produces the first leaves. In some plants, such as the many kinds of beans, the growth of the epicotyl carries the cotyledons above ground. In corn and other plants, cotyledons remain underground, within the seed. After a seedling has developed its own roots and leaves, it can make its own food. It no longer needs cotyledons to supply nourishment.
Most plants grow in length only at the tips of their roots and branches. The cells in these areas are called meristematic cells. They divide and grow rapidly and develop into the various tissues that make up an adult plant. In trees and other plants that increase in thickness, new layers of cells form between the bark and wood. This area is the cambium. New layers of cells are made as the cambium grows each year. These layers form the woody rings that enable people to tell the age of a tree.
Some kinds of plants, called perennial plants, live for many years. Most perennials produce seeds yearly. Annual plants live only about one year. Biennial plants live for two years. Most annuals and biennials produce seeds only once.
Water movement. Plants must have a continuous supply of water. Each individual plant cell contains a large amount of water. Without this water, the cells could not carry on the many processes that take place within a plant. Water also carries important materials from one part of a plant to another. Most water enters a plant through the roots. Tiny root hairs absorb moisture and certain minerals from the soil by a process called osmosis. In many plants, fungi that grow on the roots help the plants absorb water and minerals. In vascular plants-that is, plants with special conducting tissues-these materials are transported through the xylem tissue of the roots and stems to the leaves. There, water and minerals are used in making food. Water also carries this food through the phloem tissue to other parts of the plant.
Plants give off water through a process called transpiration. Most of this water escapes through the stomata on the surfaces of the leaves. Scientists estimate that corn gives off 325,000 gallons of water per acre (3,040,000 liters per hectare) by transpiration during a growing season. Some botanists believe this water loss prevents the leaves from overheating in sunlight.
Photosynthesis is the process by which plants make food. The word photosynthesis means putting together with light. In green plants, sunlight captured by chlorophyll enables carbon dioxide from the air to unite with water and minerals from the soil and create food. This process also releases oxygen into the air. People and animals must have this oxygen to breathe. Most photosynthesis takes place in small bodies called chloroplasts within the cells of plant leaves. These chloroplasts contain chlorophyll, which absorbs sunlight. Energy from the sun splits water molecules into hydrogen and oxygen. The hydrogen joins with carbon from the carbon dioxide to produce sugar. The sugar helps a plant make the fat, protein, starch, vitamins, and other materials that it needs to survive.
Some plants, called parasites and saprophytes, have little or no chlorophyll and cannot produce their own food through photosynthesis. These plants must rely on outside sources for food. Parasites attach to living plants and take the nutrients they need from these plants. Saprophytes grow on dead and decaying organisms, or use organic substances produced by living organisms for food. Mistletoe and dodder are common parasites found in many parts of the world. Mistletoe grows on the trunks and branches of many trees. It is called a partial parasite because it also makes some of its own food. Indian pipe is a saprophyte that grows near fungi. It uses organic materials produced by fungi for food. A plant called giant rafflesia is a parasite that grows on the roots and stems of other plants. It bears the largest flower of any known plant. Rafflesia flowers may grow over 3 feet (91 centimeters) wide.
Respiration breaks down food and releases energy for a plant. The plant uses the energy for growth, reproduction, and repair. Respiration involves the breakdown of sugar. Some of the products resulting from this breakdown combine with oxygen, releasing carbon dioxide, energy, and water. Unlike photosynthesis, which takes place only during daylight, respiration goes on day and night throughout the life of a plant. Respiration increases rapidly with the spring growth of buds and leaves, and it decreases as winter approaches.