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Plant Basics
If you're not a microbe and you're not an animal, you are probably a plant. There are about 300
thousand known species of plants. Because plants adapt so well to almost any climate, scientists
needed a way to organize the hundreds of thousands of species. In the same way there is a
system of classification for animals, there is also a system of classification for plants.
Common Plant Traits
What do plants have in common? Photosynthesis is the biochemical process that connects plants.
Photosynthesis is the process that allows plants to take energy from the Sun and create sugar
molecules. As with all of biology, there are exceptions to the rule and you will learn that not all
plants survive through photosynthesis. Some plant species are parasites.
There are two big biochemical processes on Earth: Photosynthesis and Respiration.
Photosynthesis stores the energy and respiration releases that energy. It all starts with the Sun.
Plants are able to turn sunlight into energy but not directly. Plants are able to store energy in the
chemical bonds of sugar molecules.
Plants also have cell walls that are made of cellulose. In the cells tutorials we explained that all
cells have a membrane but plants have an additional cell wall made from cellulose. Cell walls
serve as support structures by protecting individual cells and creating a larger structure for the
entire plant. Cellulose is called a structural carbohydrate. Redwood trees that are hundreds of
feet tall are only possible because of cellulose in the trunks and branches.
Learning from Plants
Not only do you see plants everywhere in the real world, but they are also all over the scientific
world. Some scientists use them to study genetics. Gregor Mendel used pea plants and their
flowers to come up with some of the first ideas on how traits are passed from one generation to
another. Botanists might look at weeds and learn how they are able to become resistant to
herbicides. Other scientists develop new plants that are more resistant to disease and insects.
On a non-scientific level, farmers have been observing plants for thousands of years. They look
for plants that grow quickly and have higher yields. They also know about good fertilizers and
the amount of water needed to have the most productive crops. Once they understand how one
species works, they take that knowledge and apply it to other crops.
A General Plant Structure
We're going to look at plant structure in this section. The plants we discuss will be vascular
plants that have systems of tubes (xylem and phloem) for the transport of nutrients and water.
Remember that there is a wide variety of plants on Earth and even a whole group that doesn't
have vascular systems. Mosses and liverworts may still have photosynthesis, but they do not
have that 'classic' plant structure. Then you will find species such as cacti that don't have leaves.
They conduct photosynthesis in their stems. Anyway, just remember that there are many other
possibilities in the plant kingdom.
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Alike But Different
We just told you about the many exceptions to the basic plant structure, so let's look at some
similarities. An easy similarity is on a cellular level. Plants conduct photosynthesis. This process
of converting the Sun's energy into molecular energy happens in chloroplasts with the help of
chlorophyll molecules and a variety of enzymes. Vascular plants share a similar set of structures
called roots, stems, and leaves. Many plants have specialized versions, but the basics are there.
One specialization might be the petals of a flower. Those flower petals are specialized leaves that
surround the reproductive structures of the plant.
Roots Below the Ground
We'll start at the bottom with the roots. These structures are designed to pull water and minerals
from whatever material the plant sits on. For water plants, the roots may be in the water. For
traditional trees, the roots go deep into the soil. There are even plants called epiphytes that live
in trees and their root system clings to branches. Humans often capitalize on the roots of plants
for food. Carrots are just one big orange root.
Root systems also provide support for plants in the form of an anchor in the soil. If the wind
blows hard, those roots keep the plant from falling over. Some plant species have roots above
ground that provide support for the entire plant. Roots are further broken down into the primary
root and lateral roots that each has apical meristem at their tips. Root hairs are also a common
structure on roots. They make the roots look fuzzy and help in the absorption of water and
nutrients.
Shoots Above the Ground
Sure we said that there are some roots above the surface, but the majority of the plant you see is
made up of stems and leaves. Think about a tree. The stems are the trunks and branches. Leaves
are self-explanatory. Stems are all about transporting food and water and acting as support
structures. Leaves are all about photosynthesis, creating food molecules and absorbing carbon
dioxide for the plant. These parts are connected by the vascular system of xylem and phloem that
spreads through the entire plant.
The tip (terminal bud) of the main stem has a specialized structure that is the source of new
growth for plants. You will find the apical meristem that develops into young leaves
(primodium). There are other points of growth at each node where leaves and branches develop
on the stems. Those branching points are home to axillary buds that can also develop into new
branches.
Vascular Systems of Plants
Xylem and phloem make up the big transportation system of vascular plants. As you get bigger, it
is more difficult to transport nutrients, water, and sugars around your body. You have a
circulatory system if you want to keep growing. As plants evolved to be larger, they also
developed their own kind of circulatory systems. The main parts you will hear a lot about are
called xylem and phloem.
It all starts with a top and a bottom. Logically, it makes sense. Trees and other vascular plants
have a top and a bottom. The top has a trunk, branches, leaves, or needles. The bottom is a
system of roots. Each needs the other to survive. The roots hold the plant steady and grab
moisture and nutrients from the soil. The top is in the light, conducting photosynthesis and
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helping the plant reproduce. You have to connect the two parts. That's where xylem and phloem
come in.
Zippy Xylem
The xylem of a plant is the system of tubes and transport cells that circulates water and dissolved
minerals. As a plant, you have roots to help you absorb water. If your leaves need water and they
are 100 feet above the ground, it is time to put the xylem into action! Xylem is made of vessels
that are connected end to end for the maximum speed to move water around. They also have a
secondary function of support. When someone cuts an old tree down, they reveal a set of rings.
Those rings are the remains of old xylem tissue, one ring for every year the tree was alive.
Phloem Fun
The fun never stops in the plant's circulatory system. Most plants have green leaves, where the
photosynthesis happens. When those sugars are made, they need to be given to every cell in the
plant for energy. Enter phloem. The phloem cells are laid out end-to-end throughout the entire
plant, transporting the sugars and other molecules created by the plant. Phloem is always alive.
Xylem tissue dies after one year and then develops anew (rings in the tree trunk). What is the
best way to think about phloem? Think about sap coming out of a tree. That dripping sap usually
comes from the phloem.
Different Parts = Different Advantages
Obviously, not all plants look the same. They have different flowers, stems, and even root
structures. Extreme examples have given some plants big advantages. These advantages have let
them settle in new environments and become more successful.
Specialized Leaves
What kinds of leaves are there? What kinds aren't there? There are thick ones for storing water
as in succulents. There are long twisting vine-like leaves that can wrap around and dig in for
support as in grapes. There are also thorns. Nothing says, "Don't eat me" like a bunch of sharp
thorns on your branches.
Focus On: Succulents
Even though succulent species are flowering plants, we thought it might be nice to add an
example of plants that are able to survive with little water. Instead of leaves, succulents have
fleshy structures that efficiently store water. Many succulents have also developed thorns for
protection. Many animals in arid environments feed on succulents because they provide a source
of water in their tissues. This image displays a spiral pattern that several succulents have
developed. Their growth is not a random growth like a tree or weed. They grow in specific
geometric patterns.
Flowers
Flowers have developed such a wide variety. That variety is often dependent on what kind of
creature helps out with the pollination. If I am a big insect, I will be looking at plants with big
flowers. If I am a tiny little bug, I might live my whole life inside a flower. There are also a variety
of colors that attract different insects and animals.
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Stems
Stems are a good place to store water. It's very efficient to develop a big protected area. Think
about a barrel in hot areas where water is scarce. Enter a cactus. All stem and trunk. No leaves.
Having no leaves means very little evaporation on hot days. Other extremes are plants with no
stem. They could grow one, being vascular plants, but they have found it to be an advantage to
stay near the ground. Vines are another extreme.
The bark of a tree or plant can also perform a specific function. Corks in wine bottles are actually
from the bark of a tree (cork tree). Some bark has been designed to peel away as the tree grows.
Other types of bark are very thick to protect the plant from animals and insects.
Epiphytes - Specialized Roots
Not all plants even live in the ground. Some specialized plants called epiphytes actually live on
the side of other trees or on rocks. They are able to collect water themselves but do not use roots
to gather it up. Their roots have been specialized to dig in or grab on to the larger object. They
don't always hurt the trees; they just hang out on the outside. Epiphytes can include some
seedless species, bromeliads, and orchids. There are also epiphyte species that can grow very
large and even break tree limbs. They can suck nutrients away from the tree and weaken it over
time. Several ficus species are killer parasitic epiphytes.
Mosses and Liverworts
These are the little ones. The most important feature of mosses and liverworts is that they have
no vascular system. A vascular system in plants is a series of tubes that can transport water and
nutrients over a distance. That vascular system of xylem and phloem allows redwood and
sequoia trees to grow to over one hundred feet tall.
Limited in Size
Without a vascular system, mosses, and liverworts cannot grow very large. If you have seen
mosses, you know that they are actually carpets of individual plants. They are rarely taller than
one inch high. Another important characteristic of these little guys is that they require water to
reproduce. It's another characteristic of their low place in plant evolution. While all plants need
water, mosses and bryophytes need droplets of water to enable their haploid reproductive cells
to combine. They are all known as the bryophytes.
Mosses
Let's start with mosses. These are waxy little plants with no leaves and no stem that use each
other to stay upright. Their inability to stay up is why you never see one little moss plant; it's
always a group. That grouping also helps them retain water in the area. A waxy covering across
their bodies helps keep water from evaporating. You will usually find them in moist areas out of
the direct sunlight.
Good Worts
We'll cover liverworts and hornworts together. If you can believe it, the worts are even simpler
than mosses. These are considered to be the simplest of all plants and often grow flat along the
ground in large leaf-like structures. None of the bryophytes have roots. They all have rhizoids
(little hairs), and the worts are no exception. Like mosses, they are found in very moist areas, and
some species even spend their whole lives in the water.
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Ferns and Horsetails First Plants with Pipes
These are the first of the vascular plants you will study. Mosses and worts are non-vascular. The
ferns were the first plant species to develop a circulatory system that lets them grow larger. They
have roots, leaves, stems, and trunks. With their new vascular system, the sky was the limit for
plants.
They Like Water
Ferns are often used in landscaping. There's a good chance you've seen them. They are also able
to live in a variety of climates as long as it is moist. You will find ferns in Canadian rain forests
just as easily as you will find them near the equator. They are similar to mosses in that they need
liquid to reproduce. When water is around, they are able to have baby ferns called zygotes. Ferns
have some neat structural features. Some have large stems, several feet in length. Scientists call
those bad boys the tree ferns. Ferns also have specialized leaves called fronds. They unroll as
they mature and spread out in a fan shape.
Horsetails
Horsetails are related to ferns in that they have a vascular system. They never developed the
ability to reproduce with seeds. They might be a little hard for you to see because many of them
are extinct. Because they are better able to survive in various environments, you can find them
from very northern and southern latitudes to the equator. Unlike ferns, these are tough plants.
While ferns are soft, horsetails are rough plants and even have silica (silicon-based compound)
in their epidermal cells. Ouch!
Gymnosperms - First Plants with Seeds
So you've got a vascular system. What comes next? Seeds. Seeds let you send your offspring out
into the world. Seeds provide a protective coat so that the embryo plant can develop when it
finds a nice piece of soil. But remember this: gymnosperms have not developed the ability to
make flowers. Flowers are an evolutionary advancement after seeds. So if you have a vascular
system, seeds, and no flowers, what are you? A gymnosperm!
Seeds are a protective structure that lets a plant embryo survive for long periods of time before it
germinates. Seeds have food sources pre-packaged for plant embryos to provide for an embryo's
needs in early growth. Seeds let plants spread their embryos over large areas. Some are even so
lightweight that they are carried across the planet by strong winds. Seeds are an advantage if you
want to be a plant that can grow anywhere. Seeds are da bom'!
Cycads in the Tropics
Looking like a fern. Looking like a palm tree. It's actually neither! It's a cycad. These are another
favorite of landscape designers. These are sturdy little plants that can survive in harsh
conditions. You won't find them in cold areas like the conifers. Cycads need warmer weather to
survive. They have cone-like structures for reproduction. Instead of being on branches, their
cones are in the center of the plant and can get really large. They also have big waxy fronds, and
when it's time to reproduce, the female plants have a great fruit that grows in the middle of their
stem.
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Conifers in the Forests
Pine, cedar, redwood, and spruce. Sounds like we're at a hardware store buying lumber. Not so.
We are listing off a bunch of trees that are called conifers. If you've ever gone skiing or to
northern latitudes you have seen loads of conifers. The conifers most people think of are pine
trees. Every year millions of trees are grown for Christmas and they are all conifers. They usually
have needles and cones (thus the name CONifer).
They are also evergreens: even in cold winter months they are able to keep their needles. That
ability is one reason they do so well in northern latitudes. The ever-present needles allow
conifers to take advantage of the Sun whenever it is around. They are also some of the tallest
plants in the world. They are able to get very tall and strong because of heavy-duty xylem that
hardens and makes them sturdy. That sturdiness is why these kinds of trees make good lumber hard and strong wood.
Ginkgoes on Your Street
Not every plant made it to the modern day. Fossil evidence shows what plants used to be alive in
other geological eras. The Ginkgo is one of the ones that made it. Some people call it a
"Maidenhair Tree". It's the last one of its kind. It has needles that have combined to form very
sturdy leaf-like structures. You need to remember they are not like leaves in the traditional
sense. You've probably seen these all over. Landscape designers love to use them because they
look very nice and are very resistant to pollution. They are great for cities. Being able to resist
insects and disease has let this species survive beyond all of its close relatives.
Angiosperms - First Plants with Flowers
We asked it before. What would give you an advantage if you were a plant? You have a vascular
system to transport nutrients. You have seeds for reproduction that allow your babies to spread
out in new areas. What next? Flowers! Flowers are the most recent evolutionary advantage for
plants.
Looking Good for the Birds and Bugs
When we talked about gymnosperms, we spoke of seeds. That was a big advantage. The
angiosperms took it one step further. They not only have seeds, but they also have flowers. What
kind of an advantage is that? Many angiosperm species use wind for pollination the way that
gymnosperms do. What if you didn't need to rely on the wind to spread your pollen around
anymore? What if another creature could do it for you? Maybe an insect? Sounds like a new
advantage.
Those specialized flowers are able to attract organisms to help pollinate and distribute seeds.
Another cool advantage is the fruit/seed packaging. Would you rather eat a pine cone or an
apple? A lot of animals would go for the apple. When they do, they are able to spread the seeds
across wide areas after the animal poops out the seeds.
Some Withy One Cotyledon
There are two kinds of seeds in the angiosperms, monocots and dicots. Monocot is short for
monocotyledon. A cotyledon is the seed leaf. When you are a monocot, your seed only has one
package of food. "Mono" means one or a single cotyledon. Monocots are made up of simple
flowering plants like grasses, corn, palm trees, and lilies. Two of the characteristics of monocots
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are that their flowers have petals in numbers of three and their leaves are made of long strands.
Think of the leaves of grass or a palm frond.
And Some with Two
The other kind of plant in the flowering plant world is called a dicot. Dicot is short for
dicotyledon. "Di" means two or a double cotyledon. These plants have seeds that have two
cotyledons, two seed leaves of food for the embryo. Most of the flowers you see every day are
dicots. They have flowers with petals in numbers of four and five. They also have really complex
leaves with veins all over, not long like monocots. Some examples of dicots are roses, sunflowers,
cacti, apple, and cherry plants.
Humans and Plants
Humans need plants. All animals do. Humanity's relationship with plants has actually made it
possible for us to have a civilization. Before we had cities, humans went around in little packs
and were hunter-gatherers. We ate rats, birds, berries, and whatever food we could find. It
wasn't very efficient. One day someone had the bright idea to plant the plants we like to eat.
When humans did that, they were able to stay in one place full time. Then came the cities and a
huge system of agriculture to support millions of people.
Big Time Farming
As time has passed, we have taken farming to new levels. We have manipulated species to create
big apples and large ears of corn. The plants would never have done it in the wild. It took man to
change the plants. We are also moving toward the genetic alteration of plants. We're trying to
make plants that are resistant to disease and bugs. These stronger plants will allow our crops to
give us more food from the same amount of space.
Medicines
One of the good examples of plants giving medicine to man is an aloe plant. Inside the leaves of
an aloe plant are compounds that soothe burns on our skin. Man also gets something called
digitalis from plants. The truly exciting discoveries are in the future. Scientists are analyzing
plants every day to find out if they have any compounds that can help humans survive and lead a
better life.
And Places to Live
If you live in a house, chances are it was made with wood or there is wood in the house. We have
used wood to build our homes for thousands of years. Huge lumber companies grow entire
forests to supply the amount of wood needed for our world. As buildings got larger, construction
turned to concrete and steel. You will probably still find wood fixtures such as paneling inside
these large steel buildings.
Plant Reproduction - They'll Make More
If you are an organism, you will need to reproduce. Otherwise, there will be no more of your
species and the species will die off. You may have heard of endangered animals. There are also
endangered plants. These endangered species have very few individuals left and
scientists/naturalists are working together to make sure the species don't become extinct.
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We talked a little about reproduction when we discussed meiosis in the cells tutorials.
Reproduction is one of two things.
(1) One cell can split into two, giving you two identical cells. That type is asexual reproduction.
(2) The second type is when two cells, each with half of the DNA needed, combine and create a
living cell. That type is sexual reproduction.
When plants hit a point in evolution, the second is the one that occurs more often.
Making More Mosses
Sporophytes are the reproductive structures you will find in mosses. They are actually a phase of
the moss life cycle that feeds off the green parent plant (the gametophyte). The sporophyte is a
stalk that grows after the haploid sperm of one moss plant is able to mix with the haploid egg of a
female moss plant. The resulting diploid cell grows into the sporophyte stalk. When ready,
spores stored in the sporophyte are released and they grow into new moss plants.
Conifers and Their Cones
While there are male and female mosses, conifers produce two types of cones on the same tree.
One of the cone types gives off pollen (the staminate cone). The other type of cone catches the
pollen if the wind is moving in the right direction. Better yet, the wind blows the pollen to
another conifer of the same species, and a cone (called the ovulate cone) catches the pollen.
Again, the pollen and megaspore (receiving haploid cell) are haploid and combine to form a
diploid cell. That diploid cell grows into a zygote (baby conifer) that eventually lives in a seed.
Flowers and Pollen
The most advanced of the plants have their own way of sexually reproducing. It is a very fancy
and very complex process. Plants that rely on flowers for reproduction are also very dependent
on outside help such as insects and animals. While conifers have the two structures on one tree,
flowering plants went one step further and put the devices that make and receive pollen in the
same structure.
How does that help? A bee might go to one flower and get a little pollen on its back. If it goes to
another flower of the same species, that pollen can land on the stigma. From that point, one
haploid male nucleus combines with a female nucleus and the other haploid male nucleus
combines with a polar nucleus. If successful, an embryo and seed/fruit develop respectively.