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Learning About Plants 1
Learning About Plants:
Seeds, Plant Parts, & Life Cycle
Rosalyn Han
Jessica McGlinn
Penn State University
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In today’s world, plants are as different as people. There are thousands of different kinds
of plants. All plants are living things that grow and reproduce; most plants can also make their
own food.
Plant Reproduction
Springtime is one of the most beautiful times of the year because flowers are blooming
but most importantly, because flowers help a plant make seeds. These seeds grow into new
plants, which is called reproduction. To form a seed and reproduce, the pollen (the male cell
from a plant) must mix with a female cell. The male parts of a flower are called stamens and the
female parts are called pistils.
Female
The pistil is made of a long thick tube with a sticky section at the top and a pouch at the
bottom. The rounded pouch at the bottom of the pistil is called the ovary and it is a very
important part of the flower because the seeds are formed in the ovary as well. When the pollen
travels down the long tube of the pistil, it combines with the female cells (Tokarz, 1987).
Standing out of the center of the ovary is a tiny stem known as a style. The tip of the style carries
the stigma, which is sticky, so that the pollen grains will stick to it (Van Dersal, 1977).
The anthers are the male parts of the flower and they contain the pollen grains. Inside the
ring of stamens, petals, and sepals, there is a tiny body called the pistil and it is at the end of the
stem that carries the flower.
Male
The male part of the plant is known as the stamen. Inside of the petals are the stamens,
which consist of a tiny little steam called a filament. Filament is a composed of a long supporting
stalk, and at the upper end of the filament is an enlargement known as the anther. When mature,
the anther bears thousands of the male reproductive structures called pollen grains. The pollen
grain is actually the immature from which will develop into the sperm (Tant, 1992).
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Fertilization
Fertilization takes place when a pollen grain lands on the stigma. If the pollen grain came
from the same kind of flower, it will reproduce a tube, which grows down the style to the ovary
and will then develop sperm (Tant, 1992).
In the meantime, the ovule starts to mature and the changes in the ovule then produced an
embryo sac. Special cells then divided to form eight structures called nuclei. A sperm that was
developed from the pollen will fertilize one of the eight structures. This will then develop into
the new plant embryo, while a different sperm will fertilize another one of the nuclei and develop
into a material called endosperm (Tant, 1992).
The fertilized egg is called a zygote and it begins to divide and produce a large number of
cells that are in the early stages of the embryo. Within a few days, the embryo develops the
important parts of the plant that it will become. These parts include the beginning of the root
system, stem, and leaves. The embryo usually has a curled up position very similar to that of a
human body in its early stages of life (Tant, 1992).
During this process, which can only be found under a microscope, the pollen grains
sprout and is sent down the tubes into the ovary. A tube grows into one of the ovules, breaks
open, and the contents fuse with the egg contained in the ovule. The fertilized egg now begins to
develop into a seed (Van Dersal, 1977).
As the seeds develop, they could remain covered by the wall of the ovary that later
becomes dry and tough, or the ovary wall may enlarge to some degree on the outer side and
become a fleshy fruit. Besides these scenarios, dozens of other things can and do happen,
depending on the plant. (Van Dersal, 1977).
Germination
In order to begin the growth process, germination, the embryo needs to receive water and
oxygen. Different types of plants have evolved various methods to help insure that growth
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conditions are suitable for the delicate plantlet as it starts its life. If the climate is favorable,
specifically the temperature, and the embryo has absorbed sufficient water and oxygen, it will
begin the growth process of germination (Tant, 1992).
As this process starts, omitted words one can see omitted word the breaking of the seed
coat that results from the swelling caused by the absorption of water. The next event is the
appearance of the first root at the base of the seed (Tant, 1992).
The primary root will almost immediately begin to show a strong positive gravitropism,
growing downward in response to the force of gravity. This will occur regardless of the position
of the seed. The root may curl around the seed to accomplish its downward growth, or in some
plants, it may pull the seed around, thus making it easier for the shoot to begin its upward growth
(Tant, 1992).
The process described above is called sexual reproduction, which is based on the union of
sperm (from the pollen tube) and the egg in the ovule.
Seed Dispersal
When the flower is only a bud, it is usually wrapped up in the calyx. The green bracts
enclosing the bud are the sepals. Later when the flower opens, the sepals fold back and the petals
are displayed. A bee may go searching around at the base of the petals for nectar, and in the
process the bee rubs off some of the pollen from another anther sac, which may open about the
same time as the petals. As the bee visits another flower, some of the pollen (already on the bee)
might be brushed onto the stigma. This process is called pollination, and while the bees and other
insects pollinate many flowers, the wind is also a pollinator (Van Dersal, 1977).
Seeds and their associated parts have all sorts of devices to help them get distributed as
widely as possible. Very tiny seeds will blow in the wind, while other seeds have wing-like
structures that help it be carried in the wind. Other plants develop hooks on its seeds, or on the
whole ovary. These seeds hook onto the fur of passing animals and are thus carried about (Van
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Dersal, 1977). Wind, bees, and birds help carry the pollen from the male parts of the flower to
the female parts of the flower.
Photosynthesis
In plants, photosynthesis occurs mainly within the leaves. Since photosynthesis requires
carbon dioxide, water, and sunlight, all of these substances must be obtained by or transported to
the leaves. Carbon dioxide and oxygen are obtained through tiny pores in plant leaves. Water is
obtained by the plant through the roots and delivered to the leaves through the stem. Sunlight is
absorbed by chlorophyll, a green pigment located in plant cell structures called chloroplasts.
Chloroplasts are the sites of photosynthesis. Chloroplasts contain several structures, each having
specific functions.
Photosynthesis occurs in two stages. These stages are called the light reactions and the
dark reactions. The light reactions take place in the presence of light. The dark reactions do not
require direct light, however dark reactions in most plants occur during the day. The light
reactions convert light into energy (ATP and NADHP) and the dark reactions use the energy and
carbon dioxide to produce sugar. All green plants around the world perform this simple process
(Van Dersal, 1977).
Chemosynthesis
Photosynthesis is just the start of the process. This next step is called chemosynthesis,
which means building and is done in the dark. The simple sugars are combined to form starch,
cellulose, or lignin. These are all known as carbohydrates because they have the same proportion
of hydrogen and oxygen as water does. Using a series of chemicals that are retrieved from the
roots in a dissolved form, the plant builds proteins. From there the proteins are built into
protoplasm, which is the living substance of any plant (Van Dersal, 1977).
Plant Necessities
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The environment has everything to do with a plants success or failure. There are three
elements of the environment: soil, light, and water. One needs to keep in mind that it is the
collaboration of all three elements, because the absence of one factor can cause a plant to die
(Van Dersal, 1977).
Soil can be comprised of five different elements: mineral or rock material, organic
matter, water, air, and the living organisms that inhabit in or around it. The pores or spaces in
between the particles are filled with air and water. If the soil is wet, the pores may be filled
entirely with water. If the soil is dry, then the pores are filled with air, but when the soil is
apparently dry, each soil particle may have a tiny thin film of water around it. It is in these
spaces that the roots of a plant grow, absorb water, and dissolve chemicals (Van Dersal, 1977).
Sunlight is important for plants in many ways including photosynthesis, food storage,
and much more. Some plants will not grow well unless they are grown in full sunlight. Other can
survive with less, and will grow quite well in the shade or in partial sunlight.
Seasons
In the spring, many plants grow new shoots and leaves. Throughout the summer, different
plants flower and reproduce and some make seeds from which new plants will grow. During
autumn, many plants begin to die down or rest as the leaves on some trees change color and fall
to the ground. In the winter, many plants stop growing and rest during the cold and darkness of
the short days and long nights (Morgan, 1993).
Parts of a Plant
Leaves
The leaves of a plant are very important and are often called the “manufactures” of its
basic food. Each leaf is composed of microscopic units or cells, but the cells on the upper layer
of the leaf have a heavy covering of waterproof material called cutin. The waterproof layer is
very important to the leaf because all of the manufacturing operations inside must take place in
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water. If the leaf was not waterproof, too much water would evaporate, and the chemical process
could not take place. The waterproof layer of the cells are called the epidermis. The epidermis
protects the chemical workings occuring inside, and is also translucent so that the light can reach
the cells below (Van Dersal, 1977).
Underneath the epidermis layer, there is a group of cells known as the palisade layer.
These cells stand up at right angles to the leaf’s surface. Inside of each palisade cell there are
small, green, egg-shaped bodies that are called chloroplasts and they are green because they
contain chlorophyll (Van Dersal, 1977).
The cells on the outside are like a skin. They are tough and protect the rest of the leaf,
and are often waxy. This helps keep water inside the leaf and these skin cells have no
chlorophyll. The sun shines through them to the food making cells (Kelley 1985). Plants make
their food through the leaves from the sun shining down through the skin cells. The cells
underneath have chlorophyll in them. When the sunshine reaches the chlorophyll, sugar is made
and the plant is able to turn the sugar into starch (Kelley 1985). To date, we do not know exactly
how a plant does this, but this is where the food for animals comes from (Kelley 1985).
On the underside of the leaf are little pores (holes). Some plants have them on the topside
too. These pores let carbon dioxide in and oxygen out (Kelley 1985).
The veins look like a bundle of pipes. These pipes are made of long cells that are open at
the ends. The lower part of the vein carries food to the rest of the plant. The upper part brings
water to the leaf (Kelley 1985). The veins also make the leaf stronger.
A lot of water is lost through the pores in the leaves. This does not seem to be a problem as long
as the roots keep sending up more water. But suppose the ground gets dry, and the leaf closes its
pores to stop water loss, this would slow down the food making process. These pores also close
at night, because during nighttime there is no sunlight. The pores close to prevent wasting water
(Kelley 1985).
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The young leaves are on the inside and have veins. The outside is called the bud scale and
it protects the young leaves. When the growing time starts, the scale falls off and in a few weeks
the leaves will be full size. While the leaves are coming out, the stem is growing (Kelley 1985).
Stem
Water and dissolved substances must be carried to the leaves, and food materials must
reach the roots through the stem. There are two cells in the stem. One carries water and dissolved
chemicals from the roots up to the leaves. These tissues are called the xylem. The other cells are
tissues that carry food down are called the phloem (Van Dersal, 1977).
Roots
Roots have several important functions. They anchor the plant in the soil, they absorb
water and nutrients materials from the soil, and in many plants they store food during the winter
(Van Dersal, 1977).
Roots also grow from the end, where the tip of the root has a root cap and it is made of
dead cells. The cap protects the root when it is pushing its way into the ground, and the base of
the root will grow thicker but not longer. Plants that live only one year have very thin roots
(Kelley 1985).
The big roots that you see on top of the ground do not get the water. Roots under the
ground, take water in through the outside cells, but most of the water and minerals are taken in
by root hairs. The cells in roots need some oxygen and obtain it through the air in the ground.
Most plants will drown if the roots are soaked with water, but there are a few that will grow in
water with no trouble. Some plants store food in their roots (Kelley 1985).
Misconceptions
In Stein’s paper called, “Science drawings as a tool for analyzing conceptual
understanding, ” she states that, “According to the National Science Education Standards,
students in grades K-4 should understand that: plants have basic needs that include air, water,
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nutrients, and light; each plant has different structures that serve different functions in growth,
survival, and reproduction; and that plants have life cycles that include being born, developing
into adults, reproducing, and eventually dying (Stein).”
To conduct her study, two samples were used: two samples were used for this study: 24
fifth graders and 61 high school biology students. All students were from the same school
district; a large, suburban district that is well regarded with respect to the science curriculum
provided to students. The drawing activity took place during the second semester of the school
year. Students were instructed not to put their names on the drawings and that the drawings
would not be used for classroom evaluation. Instruction on plants had not occurred prior to the
drawing activity (Stein).
There are many misconceptions that students have about plants that resulted from this
study. Some children think that trees, grass, vegetables, and weeds are not plants. Another
misconception is that plants are not alive. Plants receive their energy from the soil in which they
are planted and through the roots.
Students often believe that plants obtain their energy directly from the sun and from
nowhere else. Also, plants have multiple sources of food, heterotrophic as well as autotrophic,
and that carbon dioxide, water, and minerals are food as well. Another misconception about food
is that plants feed by absorbing food through their roots and that sunlight is food. Along with
that plants absorb water through their leaves. A misconception about photosynthesis is that
sunlight is “consumed” during the process.
There are several misconceptions about seeds. Some students believe that grass, trees,
and flowers do not have seeds. Also, that seeds will not grow if taken from a fruit and then
replanted. Lastly, some students think that seeds are small objects and if they were to plant a
small object that it would then grow.
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Experiments
Experiment 1
In both the roots and the stem, there are veins that help transport water and
minerals to the different parts of the plant (Simmons, 1976). David Burnie explains the
transport process, “…As the food is produced, it has to be transported away from the leaves
to the places where it is needed. At the same time, water and minerals, which the roots
have absorbed from the soil, have to be carried in the opposite direction to the farthest
stems and branches (1989).”
To observe the function of veins in a plant, we put one stalk of celery in a cup with
plain water and another stalk in a cup with water and red food coloring (See Image 1 in
Appendix). After observing for two days, the experiment helped make it evident that the
veins in the celery stalk carried the red water through the stalk and to the leaves. We were
able to observe this because we saw the veins of the celery and its leaves change from
green, to a tint of red (See Image 2 in Appendix). We were also able to affirm the change in
color after comparing the celery in a plain cup of water (control) and the celery in red
water (variable).
Experiment 2
The primary root will almost immediately begin to exhibit a strong positive
gravitropism, growing downward in response to the force of gravity. This will occur
regardless of the position of the seed. The root may curl around the seed to accomplish its
downward growth, or in some plants, it may pull the seed around, thus making it easier for
the shoot to begin its upward growth (Tant, 1992).
To observe gravitropism, we put one sweet potato in a cup of water with toothpicks
placed in the middle of the sweet potato to keep it suspended in the cup (See Image 3).
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After three days, the roots began to grow (See Image 4). This experiment helped us observe
the gravitropism that caused the root of the sweet potato to grow downward in the cup. We
also observed that not only flowers and green plants have roots, but that all plants have
roots for survival.
Experiment 3
The process of the growth of a plant, known as “germination,” starts with a seed.
When the conditions are suitable, germination begins. As the seed absorbs water, the cells
in the embryo begin to divide. This eventually causes the seedcase to open. The beginning
root of the seed sprouts and then grows downward. Soon after, the root begins to grow
which will grow the stem and leaves of the plant (Burnie, 1989).
To observe the beginning steps of germination, we dampened a paper towel inside a
cup and placed lima bean seeds between the cup and the paper towel. The next day we saw
that a root broke out of the seedcase and began to grow as each day progressed. Through
this experiment we were able to observe the root sprout out of the lima bean seedcase.
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Appendix
Image 1
Image 2
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Image 3
Image 4
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Image 5
Image 6
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References
Burnie, David. (1989). Plants: Eyewitness Books. New York, NY: Alfred Al Knopf, Inc.
Carle, Eric. (1987). The Tiny Seed. New York, NY. Simon & Schuster Books for Young
Readers.
Fowler, Allan. (2001). From Seed to Plant. Danbury, CT: Children’s Press.
Gibbons, G. (1991). From Seed to Plant. New York: Holiday House.
HOB Mission. (May 2005). Science Misconceptions. Retrieved from
http://www.huntel.net/rsweetland/
Kelley, P. (1985). Biology of Plants and Animals. Castro Valley, CA: Quercus Corporation.
Moncure, Hane B. (1990). How Seeds Travel: Popguns and Parachutes. Elgin, IL: The Child’s
World.
Science Plants. (1988). Salt Lake City, Utah: Childhood Education International.
Simmons, John. (1976). The Life of Plants. Morristown, NJ: Silver Burdett.
Stein, Mary. “Science drawings as a tool for analyzing conceptual understanding.” Oakland
University.
Tant, C. (1992). Seeds, Etc. Angleton, TX: Biotech
The Visual Dictionary of Plants. (1992). New York: Dorling Kindersley.
Tokarz, D. A. (1987). Project Earth. Plants. Sauk Centre, MN: Vactional Biographies.
Van Dersal, W. R. (1977). Why Does Your Garden Grow? New York: Quadrangle.