Gardening-Plant-How plants grow
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 of
between 18 °C and 30 °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 to the seedling. 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
maize 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. Other species, called annuals, live for only
about one year. Still other species, called biennials, 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 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 maize gives off more than 3,000,000 litres 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 90 centimetres 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.
Factors affecting plant growth. A plant's growth is shaped by both its heredity
and its environment. A plant's heredity, for example, determines such
characteristics as a flower's colour and general size. These hereditary factors
are passed on from generation to generation. Environmental factors include
sunlight, climate, and soil condition.
Hereditary factors. Within the nucleus of all plant cells are tiny bodies called
chromosomes that contain hereditary units called genes. These bodies contain
"instructions" that direct the growth of the plant. As the cells divide and
multiply, the "instructions" are passed on to each new cell.
Substances made within a plant also play a part in regulating plant growth.
These substances, called hormones, control such activities as the growing of
roots and the production of flowers and fruit. Botanists do not know exactly how
all plant hormones work. But they have learned that certain hormones, called
auxins, affect the growth of buds, leaves, roots, and stems. Other growth
hormones, called gibberellins, make plants grow larger, cause blossoming, and
speed seed germination. Still other hormones called cytokinins make plant cells
divide.
Environmental factors. All plants need light, a suitable climate, and an ample
supply of water and minerals from the soil. But some species grow best in the
sun, and others thrive in the shade. Plants also differ in the amount of water
they require and in the temperatures they can survive. Such environmental
factors affect the rate of growth, the size, and the reproduction of all plants.
The growth of plants also is affected by the length of the periods of light and
dark they receive. Some plants, including lettuce and spinach, bloom only when
the photoperiod (period of daylight) is long. Such plants are called long-day
plants. On the other hand, asters, chrysanthemums, and poinsettias are short-day
plants. They bloom only when the dark period is long. Still other plants, among
them marigolds and tomatoes, are not affected by the length of the photoperiod.
They are called day-neutral plants.
Plants also are affected in other ways by their environment. For example, a
plant may display a bending movement called a tropism. In a tropism, an outside
stimulus (force) causes a plant to bend in one direction. A plant may have
either a positive or a negative tropism, depending on whether the plant bends
toward or away from the stimulus. Tropisms are named according to the stimuli
that cause them. Phototropism is bending caused by light, geotropism is caused
by gravity, and hydrotropism is caused by water.
A plant placed in a window exhibits positive phototropism when its stems and
leaves grow toward the source of light. Roots, on the other hand, display
negative phototropism and grow away from light. However, roots demonstrate
positive geotropism. Even if a seed or bulb is planted upside down, its roots
grow downward--toward the source of gravity. The stem of the same bulb shows
negative geotropism by growing upward--away from the source of gravity.
Hydrotropism occurs chiefly in roots and is almost always positive.
Some plants are affected by being touched. When the sensitive plant, Mimosa
pudica, is touched, its leaflets quickly fold and its branches fall against its
stem. A change in pressure within certain cells of the plant causes this action.
After the stimulus has been removed, the plant's branches and leaflets return to
their original position. |
