Plant Science
Plants are photosynthetic organisms containing different light
absorbing pigments such as chlorophyll . This kingdom includes
four main groups as shown below:
Bryophytes : mosses
Plants are photosynthetic organisms containing different light
absorbing pigments such as chlorophyll . This kingdom includes
four main groups as shown below:
These are primitive plants with heights not exceeding 10 millimeters.
They have structures similar to roots called rhizoids, stems
and leaves. The rhizoids are mainly used to anchor the plant
to the substratum since absorption of nutrients can take place
by all parts of the plants. This is because they are very small
with a big surface area to volume ratio for exchange of substances,
and they are surrounded by a humid environment which facilitates
the absorption of water and minerals. The plant lacks a transport
system (no vascular tissue), and the nutrients absorbed from
the surroundings can be transported efficiently from cell to
cell without the need for xylem tubes.
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Sexual reproduction
in these plants involves the fusion of eggs and sperms.
Sperms have flagella and so they swim towards the egg.
These plants need water for the swimming of the sperms
and they need to live in moist conditions in order to
absorb water and nutrients from their surroundings. Bryophytes
can also reproduce asexually by spores growing in sporangia,
which burst open when mature releasing the spores. These
spores fall on the soil and grow again into the mature
plant. |
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| Filicinophytes
(ferns)
These plants are more advanced than bryophytes since they
have vascular tissue made of xylem and phloem. This allows
the plant to grow to much bigger sizes than bryophytes
and in some forests they can grow to a height of 10 meters.
The absorbed water and minerals are carried upwards in
xylem tubes to all the parts of the plant. They can live
in dryer habitats than those of the bryophytes, but still
they need water for their reproduction since they produce
sperms that must swim to the egg in the female structure.
These plants can also reproduce by spores which grow in
clusters of sporangia that appear as spots on the surface
of the leaf. |
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Coniferophytes
These plants are more advanced than the filicinophytes
and more adapted to terrestrial life due to the following structures:
- More advanced vascular tissue
- Leaves are adapted to conserve water and
so in most species the leaves are needle shaped, with thick
waxy cuticle and few numbers of stomata .
- The roots grow to deep layers in the soil
in order to absorb more water
- The male gametes are pollen grains and
not sperms. Pollen is carried by wind and insects and so
there is no dependence on water for reproduction .
- The male cone produces huge amounts of
pollen which is shaken by the wind and falls on the female
cone. The female cone is woody and thus it protects the
growing embryo .
- The presence of seeds in the life cycle
of this group is considered a very successful adaptation
to terrestrial life. The seed can withstand dryness on land
and can live dormant for thousands of years. This conserves
the species and leads to its wider spread.
- The seeds are carried by wind and they
have a wing like structure that allows them to be carried
a long distance away from the plant. Seed dispersal is important
in the conservation of the species since it provides more
chances of survival as plants spread in varied environments
where they might adapt.
- Due to all the above adaptations these
plants are spread in many types of habitats and they form
forests in various biomes. The trees are evergreen and so
they grow to huge heights and sizes
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Coniferous trees: female (left)
and male (right) cones grow on the same tree. However the male
cones are short lived they grow mature, shed their pollen which
is shaken by the wind and then they dry and fall off. The female
cones however, live for a longer time and they contain the eggs
which are fertilized by the pollen, they grow into seeds which
fall off the woody cone. These seeds are winged and they are
carried by the wind to other areas where they germinate, grow
into seedlings and then into a mature tree. These pictures were
taken from the same tree at different times of the year. Some
of the cones are open and have shed their seeds, while others
are still closed with seeds inside them. When these cones dry
they open releasing their mature seeds. |
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| Angiospermophytes
These are the flowering plants and in addition to all
the structures possessed by the coniferophytes this group
has developed the following structures: Flowers for sexual
reproduction. The flower has led to more successful and
efficient methods of pollination due to the attraction
of insects. Insects are attracted to large, colorful,
scented and nectar containing flowers. Pollen stick to
the body of insects and by visiting many flowers the insect
transfers pollen to other flowers. The fruit: this structure
contains the seed and it helps in seed dispersal. |
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Different
methods of seed dispersal have evolved in different plants
and this includes dispersal by wind, animals, insects
and water. Juicy fruits are eaten by animals, which excrete
the undigested seeds at a distance away from the mother
plant.
Angiosperms are the most adapted to desert life
and so they are the main representatives of
desert flora. They are called xerophytes and their adaptations
are described in the next section. |
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The
body of an angiosperm is divided into different parts
each with certain functions and structure. The roots are
for absorption and anchoring the plant to the soil. The
stem carries the upper parts of the plant including the
leaves, flowers and fruits.
The bud contains a young developing flower or a cluster
of growing leaves. |
The leaves are the main photosynthetic
organs of flowering plants, since they have chlorophyll and
many other adaptations to make them specialized for this process.
Transverse sections in the different parts of a dicotyledonous
(dicot) plant are shown on the next page. |
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| Cross
section in the leaf of a dicotyledonous plant
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| Cross
section in a dicot stem |
cross
section in a dicot root |
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Transport
in angiospermophytes
Absorption of water
Plants absorb water and minerals from the soil. Water
enters the plant by osmosis and it always moves from dilute
to concentrated solutions. In other terms it moves from hypotonic
to hypertonic solutions. This can also be described as movement
from areas of higher water potential to areas of lower water
potential. Water enters the roots through root hair cells, which
are adapted for absorption of water and solutes.
Plants need to grow in hypotonic soils so that water enters
the root hair cells by osmosis. If the situation is reversed
and plants are grown in hypertonic soils, water will move from
the roots to the soil by osmosis . This results in shrinking
of the vacuole, the cytoplasm and the cell membrane. The plants
wilt and if this persists for some time the plant would die.
In hypotonic solutions water enters plant cells and makes them
turgid, this contributes to support and turgidity of the plant.
In hypertonic solutions, water leaves the plant cell and so
it shrinks and loses support, this is called plasmolysis. A
plasmolysed cell is characterized by shrinking of the cell membrane
and its detachment from the cell wall. Herbaceous plants such
as grasses and weeds depend mostly on cell turgidity for support.
The leaves of plants also depend mostly on turgidity as a means
of support that makes the leaves spread wide and not droop and
fold. This helps them absorb maximum amounts of light and carbon
dioxide from the atmosphere.
Transport of water inside the plant
As water enters a root hair cell, it makes the cell hypotonic
in comparison to an adjacent cortex cell, and so water moves
from it to the cortex cell by osmosis. This makes the cortex
cell hypotonic compared to another cortex cell adjacent to it
and so water moves from it to the next cell. The cells of the
cortex are connected together by channels called plasmodesmata.
These channels connect the cytoplasm of all the cortex cells
into one continuous fluid called the symplast. The fluid outside
the cells also forms one continuous fluid called the apoplast.
Hence water can move in the root from cell to cell through the
plasmodesmata (the symplastic path) or through spaces between
the cells and this is the apoplastic path. This movement continues
until water reaches the xylem tubes, then it starts ascending
in these tubes due to the following properties:
- Capillary action : xylem tubes are very
thin, they are microscopic and so a microscopic amount of
water entering the tube would move a long distance in it.
- Adhesion forces between water and the walls
of xylem, and cohesion forces between the water molecules,
help water to climb in one continuous column without interruptions
and without slipping, since it sticks to the walls of xylem
as it is moving up. Imagine climbing a wall; you need to
be in one piece (cohesion forces), and you need to hold
on to something on the wall (adhesion forces).
- Transpiration is the loss of water from
the stomata of the leaves. This makes epidermal cells in
the leaf hypertonic and so they start pulling water from
adjacent cells by osmosis . This process continues until
it reaches xylem tubes in the leaves. Xylem tubes in the
leaves are continuous with xylem tubes in the roots and
stem, and so water is pulled by transpiration like pulling
water from a straw by suction. Water is also pushed into
the roots by osmosis in the root hair cells. Hence water
is under two forces in the xylem tubes; a pulling force
exerted by transpiration in the leaves, and a pushing force
exerted by osmosis and entry of water into the root hair
cells (root pressure). This allows water to travel in the
xylem tubes from the roots to the leaves like a stream,
called the transpiration stream
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| Diagram showing the direction of water
from the soil into the root hair cell, and its transport
into the cortex, xylem tubes and then to the mesophyll layer
of the leaf. Water evaporates into the air spaces and then
out of the stomata to the atmosphere. The release of water
vapor from the stomata into the atmosphere is called transpiration.
Transpiration is one of the forces that pull water up in
the xylem in a continuous column called the transpiration
stream. As more water is lost by transpiration it is replaced
by absorption from the soil. |
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Absorption
of mineralions
Mineral ions are absorbed by root hair cells by active
transport , which is a process that needs energy. This is because
mineral ions move against their concentration gradient from
lower solute concentration in the soil to higher solute concentration
in the root hair cells. The root hair cell is equipped with
a high number of mitochondria , which are needed for aerobic
respiration that produces energy for active transport. The cell
membrane of these root hair cells has protein channels for the
process of active transport . One of the requirements in soil
management is plowing which helps in aerating the soil and making
oxygen available for absorption by the roots for aerobic respiration
. Water logged soils result in poor plant growth due to the
lack of oxygen needed for aerobic respiration in the root hair
cells.
The roots are highly adapted to efficient absorption of water
and minerals from the soil. These adaptations include highly
branching roots, and cortex cells that have structures specialized
for passage of water.
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Factors
affecting rate of transpiration :
The
rate of transpiration in a mesophytic terrestrial
plant is affected by many environmental factors
as shown below.
• Light
: the stronger the light intensity the
faster is the rate of transpiration . Increased
light intensity increases the rate of photosynthesis
and so carbon consumption increases. This lowers
carbon dioxide concentration within the leaf and
results in increased stomatal opening.
• Temperature
. High temperature causes faster evaporation
of water from the leaves due to increased movement
of molecules, and so the rate of transpiration increases.
Desert plants are adapted to close their stomata
in the day time in order to minimize transpiration
and the loss of water from the plant. They open
their stomata at night when the temperature is lower,
and so they take their carbon dioxide at night and
store it in certain compounds for the day time.
• Wind
increases the rate of transpiration . Wind
blows away the water vapor that comes out of the
leaves by transpiration, thus making more space
for more water to come out through the stomata into
the surroundings.
• Humidity:
the higher the humidity of the atmosphere
the lower the rate of transpiration . This is because
at high humidity the surroundings are saturated
with water and so the atmosphere cannot take any
more water vapor molecules, and so less water evaporates
from the leaves. |
Water availability: shortage of water in the
soil results in closing of the stomata and so it lowers
the rate of transpiration. This conserves water in the
plant and controls the rate of transpiration. In desert
plants the rate of transpiration is greatly reduced by
a multitude of adaptations and measures as will be discussed
in the next section. The rate of transpiration can be
measured by a potometer as shown in the diagram on the
right.
Support in terrestrial plants
Support in plants is manifested by two main properties:
Cell turgor, which results from the entry of water into
the cells by osmosis . The cells become swollen but they
are prevented from bursting due to the presence of cellulose
in the cell wall . Cellulose is strong and elastic, and
so it resists water pressure and prevents bursting
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| Adaptations
in plants |
| Concerning
water availability plants can be divided into three main groups:
Xerophytes live in an environment characterized
by water scarcity and shortage. Desert plants are the most highly
adapted plants to such environments. Water is needed in plants
for different functions, including support, photosynthesis ,
transport , cooling, enzyme action and many other functions.
Hence desert plants have to possess structures and processes
that help them in conserving and obtaining water efficiently.
Some of these adaptations are listed below: |
| Well
established root system that grows in all directions
in order to obtain water from the soil efficiently.
The root of a small plant can sometimes be ten times
longer than the plant itself. These long roots also
help to anchor the plant in the sandy deserts and protect
them from being rooted out by the wind. During desert
studies, in field work, my students were always surprised
and impressed at the length of the roots and how deep
they have to dig to reach the root apex. |
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They have a short life cycle,
and so they grow from seeds to mature plants, produce flowers,
fruits and seeds again in few days that coincide with the
few days of rain.
Desert plants show regular
distribution and they are well spaced away from each other
to avoid competition and to be able to get enough water for
their living.
The leaves have adapted
in different ways to avoid transpiration and loss of water.
Many desert plants have succulent leaves, which are filled
with water and this helps them store water, which they can
get during the short rainy seasons. Other adaptations include
thick cuticle, few number of stomata , reduced leaves, modification
of leaves to spikes, stomata sunken in pits, stomata surrounded
by hairs for minimizing water loss by transpiration .
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| Left: Succulent leaves;
these plants (which do not need watering) are often grown
in dry countries on road sides for their aesthetic value.
Right: a variety of cacti demonstrating the different types
of adaptations in desert plants. |
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Hydrophytes
are plants that live in watery habitats such
as ponds, rivers and swamps. Elodea is a good example
on hydrophytes and the plants here are adapted by having
lots of air spaces in their tissues to help them float,
their leaves stems and other structures are flexible (pliable)
in order not to break due to wave actions and currents.
The leaves are divided to small parts to provide a big
surface area for absorption of substances. The root system
is simple and in many cases it functions as an anchoring
device, since absorption can be carried out by all parts
of the plant. |
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Mesophytes
are plants that live in moderate amounts of water
and so they do not have special adaptations for water
shortage or excess water availability. They have abundant
amounts of rain or irrigation water and so loss of water
by transpiration is balanced by enough water entering
the roots from the soil. |
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Food storage
in plants |
Plants make glucose
in the process of photosynthesis . Glucose is used for respiration
to produce energy which powers different processes in the plant
such as active transport . Glucose is also converted to other
substances such as proteins , enzymes , nucleotides for building
and growth. Excess sugars made in photosynthesis are converted
to storage materials such as starch, lipids and proteins. Some
examples on storage materials in plants are shown below:
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Starch |
Corn seeds,
potato tubers, rice and wheat grains |
Proteins |
Seeds of leguminous
plants such as peas, beans and lentils. |
Lipids |
Olive fruits
and seeds, sunflower seeds |
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Sexual
Reproduction in Angiosperms |
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Flowers
are the sexual structures of angiosperms (flowering
plants). A flower consists of the following parts:
Sepals are
small green leaves surrounding petals and they function in protecting
the flower when it is still in its bud stage.
Petals are
colorful and so they function in attracting insects for the
process of pollination.
The carpel
is the female organ and it is divided into three main parts;
the ovary where meiosis takes place to produce the egg, the
style which carries the stigma on its tip. The stigma functions
in receiving pollen and it secretes a sugar solution that causes
the pollen to stick and to start germinating to produce the
germ tube. The germ tube grows down the style to carry the male
nucleus to the female nucleus.
The stamen is
the male organ and it is made of a long thin structure called
the filament carrying on its tip the anther. The anther is where
meiosis occurs to produce the pollen. It is made of four chambers
containing pollen grains.
As the pollen matures, the anther
bursts open releasing all the pollen; this is carried to the
stigma of the same or other flowers. This process is called
pollination and it can be of two types:
Self pollination
is when the pollen of one flower falls on the stigma of the
same flower or another flower of the same plant.
Cross pollination
is the transfer of pollen from the stigma of one flower to the
anther of a flower of another plant of the same species. Cross
pollination can be by wind or animals, mostly insects.
Insect or animal pollination
Flowering plants have evolved
various methods of attracting insects for the process of pollination.
The insect visits one flower to get some nectar and by doing
this the pollen from the anther sticks to its body, then it
flies to another flower of the same species for the same reason,
and by this it deposits some pollen on the stigma of that flower.
For insect pollination, plants have evolved the following adaptations:
Some flowers have a landing
or walking platform for insects made from an extension of a
petal or some fused petals such as in the case of orchids. The
sunflower attracts insects by being large and brightly colored;
it also has nectar as a source of food for insects. Sweetly
scented flowers are common occurrences in all animal and insect
pollinated flowers. The number of pollen grains produced is
much smaller than in the case of the wind pollinated flowers,
since in most cases the insect as an agent is adapted to visit
flowers of the same species, and so the pollen is specifically
carried to its correct destination.
As for wind pollination
, the flowers are small and dull in color, the filament
hangs out of the flower cup for easy shaking by the wind, the
stigma is feathery and it also hangs out of the flower cup in
order to catch the pollen. The pollen is very large in number
since a high percentage of it is lost in the wind and does not
reach the flower. Examples on this include wheat, rice and corn
flowers. Pollination is then followed by pollen germination
which carries the male nucleus to the egg into the ovule .
Fertilization
During fertilization the pollen
nucleus fuses with the female nucleus to produce the zygote.
The zygote grows into the seed and the ovary into the fruit.
The fruit has different adaptations for seed dispersal. The
seed germinates when provided with all the required factors
including a suitable temperature, water and oxygen. The germinating
seed grows into a seedling and the latter into the mature plant
which starts producing flowers and the cycle continues. |
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The zygote grows
inside the ovule and together they form the seed. The ovary
grows and concentrates some nutrients to become the fruit. Fruits
can be dry such as in roses, dandelion and peas or juicy such
as in apples, cherries and plums.
Seed and fruit dispersal results
in the spreading of fruits and seeds to other environments and
ecosystems. Seed dispersal is important for the following reasons:
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- The seeds will not overcrowd in an area
and this minimizes competition for resources.
- It spreads the seeds in other environments
which might offer better chances of survival for these plants.
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Wind dispersal by wings
such as in sycamore tree |
Wind dispersal by parachute
as in dandelion |
By animals: the hooks
in burdock fruits stick to the fur of animals |
Self dispersal: in
lupins the dry pod splits open and the seeds shoot out
at a good distance |
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Diagrams showing
different methods of seed dispersal
Seed Germination |
Seeds are dormant or inactive plant structures that help in
the survival and conservation of plant species. Seeds are resistant
to various biotic and biotic factors and they can stay dormant
for many years, until all the factors around them are suitable.
When they are provided with the right conditions their dormancy
breaks and they start germinating and growing into the plant
again. Scientists have found seeds aged about 5000 years in
some parts of the world. When these seeds were provided with
their requirements they grew again into a mature plant. This
adaptation helps in the conservation of the species and it is
one of the factors that lead to the success of terrestrial plants
on land. Land is characterized by different degrees of water
shortage, and terrestrial plants have evolved the seed which
can stay dormant for a long time until water and other needed
factors are available. Hence the seed is one of the most successful
adaptations to terrestrial life. Seed plants include gymnosperms
(non flowering plants such as coniferophytes), and angiosperms,
the flowering plants.
When the seed is provides with suitable factors and materials
it starts germinating. In germination dormancy is broken and
the seed is activated. The following sequence of events occur
in the activation and the consequent germination of the seed:
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| Metabolic events in germination
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Water enters the
seed through a hole in the seed coat called the micropyle.
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Water moves into
the tissues and cells by imbibition and osmosis .
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The seed swells
and the seed coat bursts.
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Water activates
gibberelline, the hormone needed for breaking the
dormancy of the seed.
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Gibberelline activates
amylase which hydrolyses starch to maltose, and maltose
is then hydrolyzed by maltase into glucose.
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Glucose is mobilized
(transported) to the embryo .
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The embryo absorbs
glucose and uses it for respiration (oxygen is needed
in this process).
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Cell division, growth
and elongation occur in the embryo . The radicle starts
growing downwards into a root and the plumule starts
growing upwards into a shoot.
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The nutrients needed
for growth are all supplied by the food stored in
the cotyledons.
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As the nutrients
in the cotyledons are consumed and exhausted, the
first leaves start to appear and the plant starts
to photosynthesize and make its own food.
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When photosynthesis
starts, the seedling absorbs water and minerals from
the soil, carbon dioxide from the atmosphere and sunlight.
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The three main
factors needed for the germination of all plant seeds are water,
oxygen and a suitable temperature.
Water :
this is needed for the following processes:
- Activation of hormones and enzymes .
- Swelling of the seeds and bursting of seed
coat.
- Hydrolysis of storage compounds such as
starch into their simple monomers such as glucose.
- Transport (mobilization) of the simple
materials to the embryo to be used for respiration and growth.
- Metabolic reactions and enzyme actions
occur in solution, and so water is needed for many reactions.
Oxygen :
all seeds need oxygen for germination. Oxygen is needed for
aerobic respiration , which is involved in germination. Without
a supply of oxygen, seeds fail to germinate because of the
lack of energy, in the form of ATP . Respiration produces
energy that powers all the reactions involved in this process
 C
6 H 12 O 6 + 6 O2 6 CO 2 + 6 H 2 O + energy
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Suitable temperature
: all the reactions occurring in germination are
controlled by enzymes . Hence an optimum temperature for enzyme
activity leads to a faster rate of germination. Low temperatures
such as freezing inactivates enzymes and stops the process
of germination. Very high temperatures denature enzymes.
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| Diagrams showing
the stages in germination and growth into a seedling in bean,
a dicotyledonous plant. As the embryo grows, the seed shrinks
due to consumption of stored food materials by the embryo. Later
the seedling starts photosynthesizing and making its own food.
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Higher
Level Biology
