Download Not all plants even live in the ground. Some specialized plants

Plant Basics
If you're not a microbe and you're not an animal, chances are you are a plant.
There are loads of species of plants on Earth. Just as there is a system of
classification for animals, there is also a system of classification for plants.
Because plants adapt so well to any climate, scientists need a way to organize
the hundreds of thousands of species.
What Makes a Plant?
What do they all have in common? The big thing that connects plants is
photosynthesis. Photosynthesis is the process that allows plants to take energy
from the Sun and create sugars. Not all plants go through the process of
photosynthesis. As with all of biology, there are exceptions and you may learn
about plant species that are parasites. Plants also have cell walls. In the cells
tutorials we explained that all cells have a membrane. Only plants have an
additional cell wall made from cellulose.
Let's look at photosynthesis. Plants are able to turn sunlight into energy but not
directly. Plants are actually able to store energy in some chemical bonds that can
be used later. Before we get into details, we'll explain that there are two
processes on Earth: Photosynthesis and Respiration. Photosynthesis stores the
energy and respiration releases that energy. It all starts with the Sun. Check out
the tutorial on photosynthesis.
Learning from Plants
Not only do you see plants everywhere in the real
world, but they are also all over the scientific
world. Scientists use them for studies in genetics.
A guy named Gregor Mendel used pea
pods and their flowers to come up with
some of the first ideas on how traits
are passed from one generation of
organism to another (genetics). We
also use plants for food. Scientists are
constantly developing new plants that are more
resistant to disease and insects. Scientists also
help create plants that grow faster and make more
food.
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.
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.
THE ROOTS BELOW 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 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.
PLANTS VASCULAR SYSTEMS
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
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.
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.
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.
CONFIERS 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.
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.
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.
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.
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.
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.