Download _What is a plant?_ _Defining Characteristics_

_What is a plant?_
Although this may seem like an easy answer, a plant may be
defined differently depending on group the people. An
elementary school teacher may describe a plant as a “living green
thing”, a high school teacher may narrow this definition, and a
plant scientist may have a technical understanding of a plant. All
these definitions serve a purpose and the definition needs to be
modified depending on the audience.
Plants are “green things”, which means they carry out a
process called photosynthesis.
There are many green,
photosynthetic organisms, but some of these, such as bacteria,
are not plants. Algae are a group of organisms that may be
considered plants by some teachers, but would be called protists
by scientists. This distinction is probably not important to school
children; the important message is that these organisms use the
sun to make their own food. In contrast, a group such as fungi,
although plant-like, do not make their own food and should not
be considered plants. For the purposes of this handout, we will
concentrate on plants that live on land and are familiar to
students of all ages and levels.
_Defining Characteristics_
ROOTS: Absorption of water & minerals; anchorage of plant to the
soil; usually found underground
STEMS: Support of leaves, flowers and fruits; transportation of
water –minerals from roots to leaves; found above
and below ground.
LEAVES: Site of photosynthesis or food-making; usually found
above ground.
FLOWERS: Flowers attract animals and provide nectar while
completing the needed process of pollination.
FRUITS: Fruits protect and disperse the seeds inside.
SEEDS: Seeds are young plants that continue the lifecycle.
_What parts does a plant have?_
The plant body can be divided into three parts: roots, stems, and
leaves. Most students have a good understanding of what these
look like, but the distinction between these can sometimes be
fuzzy.
Consider that some plants have stems that look like roots (e.g.
ginger), or stems that look like leaves (e.g. cacti). It is important
that students understand that plants need certain materials to
survive; these may be obtained by roots in some plants but
through the stems or leaves in another. The determination of
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Basics of Botany
whether a plant part is a root or stem, such as the potato, seems
to be less important. The flexibility, adaptability of plants and the
function of their parts should be the focus.
_What are roots?_
Roots serve as the most fundamental organ to health of most
plants. The root or radicle is the first plant part to appear from a
seed. Roots act to anchor the plant and absorb water and
nutrients for photosynthesis; these are called true roots. True
roots are usually divided into two forms: fibrous and tap. Some
plants have a mass of string-like, fibrous roots (e.g. grasses);
others have a large, main taproot with smaller side roots (e.g.
oaks, carrots). Sometimes roots “sprout” from the side of a stem,
like in a vine; these are called adventitious roots. A plant with an
underground stem will have adventitious roots.
_Functions of roots_
¾ Absorption: Absorption of water and nutrients are one of the
primary functions of roots. Roots absorb water through a
passive, physical process. Water being released from leaves
is linked to other water molecules in the plant like a chain.
As the water is pulled out of the leaf into the atmosphere,
the chain pulls new water molecules up through the stem
and into the roots.
¾ Anchorage: Roots also act to anchor the plant in the soil or on
rock. Thus, roots act as the major supportive organ for all
other organs.
¾ Conduit: Roots (like stems and leaves) serve as the initial
pipeline for water that is needed in the plant. Sugars
(energy) are also transported down to the actively growing
portions of the root.
¾ Storage: Most roots store some quantity of sugars, water, or
toxic chemicals (like leaves and stems), but some roots are
quite large and fleshy storage organs (e.g. carrots).
¾ Respiration: Some swamp or flooded plants produce roots or
“knees” that grow up and out of the water, allowing the
roots to exchange gases in this oxygen-free environment
(e.g. bald cypress, mangrove)
¾ Support: Specialized adventitious roots called prop, stilt, and
buttress roots can be found on tropical trees that
experience frequently flooding (e.g. mangrove).
¾ Photosynthesis: In rare cases, plants have green, aerial roots
that photosynthesize (e.g. orchids).
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_What are stems?_
Stems are the most basic organs on plants. Simple examples of a
stem are twigs, branches, and the trunk(s) of a tree. Stems can range
from hard & woody to soft & green (herbaceous), but almost all stems
serve a supportive function on a plant.
Most people know what a stem looks like, but some plants make
the distinction difficult. Some plants have small stems, or
underground stems (rhizomes), or leaf-like stems, or stem-like leaves
or sometimes lack stems altogether. Although we make neat and
clear categories called stems, leaves, and roots, this distinction can be
blurred. Grasses are a good example in which the difference between
leaf and stem is not easily observable.
_Functions of stems_
¾ Structure: Most stems serve the role of support in a plant. For
example in a tree, the trunk supports the branches, which support
the twigs, which support the leaves, flowers, and fruits. In nonwoody plants (“herbs”), there is no trunk. Instead, these plants
have soft, green stems that support the leaves, flowers, and fruits.
¾ Transportation: Stems serve to transport the materials needed in
photosynthesis. Inside a stem there are tissues called xylem and
phloem. Both are tubes that transport materials through the plant:
xylem water and sugars, respectively (See xylem & phloem below)
¾ Storage: Many stems store food, water, and waste products. An
example of a storage stem is the white potato (Solanum tuberosum),
which could be called a tuber. Food and water storage in common
in arid conditions. Plants cannot easily get rid of harmful
chemicals from the air and soil; therefore, they store these
chemicals in special cells.
¾ Photosynthesis: Almost all plants have stems that can
photosynthesize. Some plants have stems that are always green,
such as cacti and palo verde. Other plants, such as oaks, just use
their young twigs and leaves for photosynthesis.
¾ Protection: In many plants, branches have been modified over
time to serve as protection against large animals; these are called
thorns. Many trees have thorns (e.g. hawthorn, crabapple, honey
locust). Note: Cacti and rose bushes do not have thorns, because
these are not modified branches (see leaves & hairs below)
¾ Clonal growth: Many plants have stems that can creep on top
(stolons) or under the ground (rhizome) producing new
“plantlets”, which are clones of the parent. Although common in
herbaceous plants (e.g. strawberries, crabgrass), some trees also
exhibit clonal growth. These trees are said to have “suckers”
growing from their base (e.g. crabapple, aspens).
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_What are leaves?_
Many leaves are composed of three parts: the blade, the petiole, and
stipules. The blade is the flat, wide, photosynthetic part of the leaf.
The petiole is the thin stalk that holds the blade and attaches the leaf
to the stem. Stipules look like tiny leaves that are found at the base of
the petiole; they are not always present. An oak leaf has a clear
example of blade and petiole, but grass leaves have blades only. Both
are example of simple leaves, because the blade is entire and not
split/sectioned into leaflets. A leaf that is segmented into leaflets is
called a compound leaf.
Discerning a leaf from a leaflet, and thus a simple from a
compound leaf, can be challenging. The simplest way is to look at the
intersection between the “stem” and the base of the “leaf”. If there is
a bud at this location, then you are observing the attachment of a leaf
to a stem. If the bud is missing, then you are looking at a leaflet
attached to a larger, compound leaf.
All leaves have stomates, which are pores in the leaf, with guard
cells that regulate the exchange of gases and water vapor with the
environment. These stomates can be opened and closed to regulate
the physiology of the plant. Oaks, like many trees, have stomates only
on the bottom of their leaves; grass leaves have stomates on both
sides.
Almost all people know what a leaf looks like, but contemporary
leaves can range from microscopic (water weed: Wolffia) up to 83 feet
long (Raphia palm).
Raphia
Wolffia
A typed
letter “O”
_Functions of leaves_
¾ Photosynthesis: The leaves of almost all plants photosynthesize.
Sunlight is used to convert carbon dioxide and water to sugars and
oxygen
¾ Conduit: Just like stems/roots, there are portions of leaves that
transport water and sugars to and from, respectively, those areas
of photosynthesis.
¾ Storage: Many leaves store water, sugars, and toxic materials.
Some desert plants are succulent in which they store water in their
leaves. Food storage is common in underground leaves, such as
the fleshy, edible leaves of the onion bulb. Many plants can store
toxic chemicals in leaves, and then drop the leaf when “full”.
¾ Support: Many vines have modified leaves called tendrils, which
are thin, stem-like leaves that wrap around other plants, fences,
wire, etc. for support.
¾ Protection: In many plants, leaves have been modified to a sharp
point to serve as protection: spines. Many arid-adapted plants,
such as cacti, have spines.
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_What are flowers?_
Flowers are beautiful structures that are composed of many parts.
The flower forms in a bud, and emerges on a short stalk or stem.
The end of this stem has four different parts: sepals, petals,
stamens, and pistils. Each part has a specific function (discussed
below) that will ultimately lead to fertilization of new seeds.
A group of flowers is called an inflorescence. To understand
the difference between a flower and an inflorescence, compare
the following examples. The largest flower in the world is the
stinking corpse lily (Rafflesia arnoldii); flowers can reach 3ft across
and weigh 15lbs. The largest inflorescence is from the talipot
palm (Corypha umbraculifera), which can reach 30ft tall with
millions of flowers
_Parts of a flower_
¾ Sepals: The sepals are usually green, thick and very leaf-like,
but in some cases (e.g. tulips, lilies), they petal-like.
Sometimes a mature flower lacks sepals all together. If
present, sepals are the lowest most part on the flower. Sepals
serve as a series of scales that protect the unopened flower
anther
stigma
filament
style
petal
¾ Petals: The petals are usually colored, thin, and leaf-like in
appearance. The petals are above the sepals on a flower.
Plants that are wind-pollinated usually do not have large,
showy petals. The main function of the petals is to attract
pollinators (insects, birds, or mammals).
¾ Stamens: Above the petals are the male or pollen-producing
parts. The stamens can range in size and shape, but most are
composed of two parts: the anther and the filament. Some
animals collect pollen for food (e.g. bees), but others feed on
different parts of the flower (e.g. nectar). The main function of
the anther is to produce pollen (the “male” part of the plant).
The filament’s function depends on its shape and size.
¾ Pistils: The innermost part of the flower is the pistil, or the
female portion of the flower. A pistil is the composed of three
parts: ovary, style, and stigma. Inside the ovary are ovules or
pre-seeds, which are unfertilized seeds. The stigma is the
landing-pad for the pollen, and the site of pollination. The
style is a thin stalk that raises the stigma up so it can catch the
pollen. After fertilization, the ovary becomes a fruit. As this
fruit grows and matures, many other flower parts like petals
and stamens wither and fall off
ovary
sepal
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_What are fruits?_
Fruits are, by definition, the mature, seed-containing ovaries of
flowers. We sometimes use the terms “fruit” and “vegetable”,
which are frequently confused. The term “fruit” is a botanical
term (ovary after fertilization), but “vegetable” is not. For
simplicity, any plant structure that contains seeds is a botanical
fruit; those edible plant parts that are roots, stems, or leaves could
be considered vegetables. Some plant parts may be a plant’s
fruit, but a grocery store vegetable (e.g. tomato)
Fruits come in a wide variety of shapes, sizes, and colors, but
not all fruits are edible to humans or other animals. The smallest
fruit belongs to the smallest plant, the waterweed (Wolffia),
which has fruits the size of a salt grain. The largest fruit on record
is something in the gourd family; specifically a pumpkin that
weighed 1,300 lbs. The largest fruit from a tree is the jackfruit
(Artocarpus heterophyllus), which can weigh up to 75 lbs.
_Functions of fruits_
¾ Protection: Fruits are a plants way of protecting their young:
the seeds.
¾ Dispersal: Fruits disperse their seeds through a variety of
methods. Fruits are commonly thought of as large, juicy
structures, such as grapefruits, apples, or pumpkins, but they
can be dry and hardened structures as well.
_What are seeds?_
Seeds are baby plants, or the fertilized ovules of a plant. Seeds
come in a range of size and shapes. The smallest seed belongs to
orchids, which can been dust-sized and weigh as little as
0.0000035 oz. The largest seed comes from the coco-de-mer or
“double coconut” palm (Lodoicea maldivica), which can be 12 in
long, and weigh up 40 lbs. In every flowering plant, seeds are
born in fruits
_Functions of seeds_
¾ Protection: The seed seems to have evolved as a way to
protect the embryo from the environment; a seed allows the
plant young to survive dry or harsh condition for short periods
¾ Dispersal: Besides the fruit, some seeds have special features
that aid in their dispersal by animal, wind or water.
¾ Warning: Some seeds are poisonous and exhibit a coloration
pattern that warns animals; these seeds do not need an animal
disperser, and use other dispersal methods
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in soil
SEED:
DISPERSAL:
fruit releases new seeds
into the world
seed takes in water
& sends out a root
GERMINATION:
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shoots and roots grow
plant still using food
reserves
SEEDLING:
fruits begin to develop and ripen
FRUITING:
buds start
to open
more leaves, stems, and
roots. Plant makes its
own food through
photosynthesis
SAPLING:
POLLINATION:
flowers open &
attracting insects
MATURE PLANT:
roots, stems and
leaves continue
to grow
flower buds
form
FLOWERING:
Basics of Botany
Basics of Botany
_Do all plants have fruits?_
No, but a majority of plants possess fruits. Flowering plants, the angiosperms, are
plants that reproduce through a flower (specifically the pistil), which forms into a
fruit. If a plant doesn’t have flowers, it won’t have fruits. Conversely, in nature, if
a plant has a flower and is pollinated, then it produces a fruit. Remember that
the fruit is the structure that contains and disperses the seeds, and comes from
the pistil of a flower. There are fruit-like structures in the plant world, such as a
cone. The gymnosperms house their seeds inside of a cone, but this would not be
considered a fruit by a botanist. The main difference lies in the process of
pollination. For angiosperms, the seeds are protected inside the pistil. For
gymnosperms the seeds are in cones, but exposed to the environment; this is
why they are called “naked-seeded” plants. Some elementary school teachers
may call a cone a “fruit”, however, this is the main difference between flowering
and cone-bearing plants.
_Do all plants have seeds?_
No, but a majority of plants possess seeds. Seed-bearing plants, (spermatophytes), are plants that reproduce with their embryos in a tough, resistant
coating called the seed. Besides seeds, some plants (and non-plants) reproduce
with spores. For examples, mosses and ferns have spores as a method of sexual
reproduction. Spores and seeds are different in many aspects, not only size.
Sexual reproduction in spore-bearing plants occurs when sperm swim to eggs
outside of the plant. As you can imagine, this requires droplets of water so that
the sperm can swim to the female. Sexual reproduction in seed-bearing plants
occurs internally, and does not require water droplets. The evolution of the seed
was one of the advantages that the spermatophytes had over spore-bearing
plants in dry environments. Nevertheless, plants have many adaptations, and
spore-bearing plants are successful in their own niches. Overall, flowering (seedbearing) plants have the most species in the world (>250,000 spp.). The second
and third-most successful groups are the ferns and mosses, respectively
(~11,500 and ~10,000 spp.). Spore-bearing plants tend to be prolific at
propagating themselves through non-sexual or vegetative means.
_How long do plants live?_
Some plants live for thousands of years; others may live as little as a few months.
Botanists tend to look at the lifecycle of a plant as it relates to the age of the
plant and use the terms annuals, biennials, and perennials
Annuals and Biennials: Some plants only live for a year or two; these are
called annuals and biennials, respectively. Annuals start from seed grow,
mature, flower, fruit and set seed in one growing season (e.g. sunflowers,
peppers). Biennials use two seasons to complete this life cycle (e.g. carrots,
lettuce, asparagus). They will produce leaves in the first year, and flower the
second. Annuals and biennials can be monocots or dicots, and in rare cases,
ferns.
Perennial plants return year after year to grow and reproduce. Any woody
plants would be perennial; some trees can live up to 5,000 years old (Pinus
longaeva). Some non-woody perennials may look dead above the soil surface
during winter but emerge in spring from underground stems or bulbs. These
plants live in open sun prairies or are adapted to lower light conditions under
trees.
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_What examples of plant form exist?_
Most students can recognize that plants come in many different
heights, widths, and overall forms. Below is a listing of some of
general plant forms that exist in the world.
Trees: Plants that are large or tall and possess wood are
considered trees by botanists. Trees may be shade-tolerant or
prefer full sun. These are usually flowering dicots or cone-bearing
evergreens. True trees (vs. tree-like plants) are the height record
breakers, such as the redwoods of California (Sequoia sempervirens),
which can reach almost 400 ft tall.
Tree-like plants: Many monocots and some ferns reach tree size
and stature, such as palms, bananas, bamboo and tree ferns.
Botanists avoid calling these plants “trees”, because they do not
contain wood. Woody trees increase in girth over time, but these
plants remain the same width from top to bottom. Therefore,
they use roots, leaves and other features to support their height.
Shrubs
or
Bushes: Similar to trees, shrubs are plants that are
small and woody. Unlike most trees, shrubs produce many
smaller stems than one large trunk. Shrubs may grow in full sun
as a successional species, or as a shade-tolerant plant in a forest.
Shrubs are usually dicots.
Vines and Lianas: Unlike the other plants mentioned, vines and
lianas obtain light by growing up on top of other trees or
structures. Vines are usually herbaceous, but lianas are woody.
Young vines are adapted to low-light, but mature plants are
usually in full-light of the tree canopy. This form exists in dicots,
monocots, gymnosperms and even ferns!
Herbaceous plants or Herbs: Plants that are soft and green are
called herbaceous. Herb is short for herbaceous, but we usually
think of fragrant plants used for cooking. These plants are usually
small or smaller than trees. Ferns, grasses, tulips, and most
vegetable crops are examples of herbaceous plants.
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_What is plant diversity?_
Traditionally organisms have been grouped as plants or animals.
Today, some scientists classify organisms into a five-kingdom
system: algae, animals, bacteria, fungi, and plants. The following
is a list of plant groups; these plants are grouped based upon their
evolutionary history.
Mosses (“bryophytes”): Plants in this group are the smallest and
most ancient land plants to inhabit the earth. Even though they
have been around for a long time, mosses are extremely diverse
in the rainforests as epiphytes: plants that grow on other plants.
These plants lack xylem, phloem, flowers, fruits, and seeds.
Ferns (“pteridophytes”): This group includes a wide range of
distantly-related plant groups. These plants are larger than
mosses; possess xylem and phloem, but still lack flowers, fruits,
and seeds. Ferns are an extremely successful group. The fern
allies are an extremely ancient group, which formed large
swampy forests before dinosaurs ruled the earth. Our abundance
of oil and coal comes from the poorly decayed remains of ancient
fern allies.
Cone-bearing plants (“gymnosperms”): Plants in this group
produce cones and seeds for reproduction. Examples of this
group include pines, spruces, cycads, and Ginkgo. This group
evolved from the fern allies over 300mya. These plants have
xylem, phloem, and seeds, but lack flowers or fruits. Not all conebearing plants are evergreen, a few are deciduous.
Flowering plants (angiosperms): The flowering plants are
currently the most successful and prolific group of plants in the
world today. Angiosperms evolved from the cone-bearing plants,
acquiring traits throughout history that make them unique; the
best known are flowers and fruits. Scientists believe that these
features gave flowering plants an edge over the dominant conebearing plants during the time of the dinosaurs.
There are two major groupings of flowering plants: dicots and
monocots. Dicots have flower petals in multiples of four or five.
Woody trees and shrubs are dicots. Examples are oaks, maples,
roses, beans, tomatoes, apples, oranges, cacti, daisies, and olives.
Monocots have flower petals in multiples of three. Over 80%
of all food comes from three monocots: corn, rice, and wheat.
Examples are grasses, bananas, lilies, irises, palms, bamboo, and
orchids.
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_What is xylem?_
Xylem (zī ▪ ləm) is very important to the structure and function of
plants. Xylem is composed of cells that are dead at maturity;
these cells are basically hollow tubes that have thick walls. Xylem
is always found inside of the cambium. A synonym for xylem is
“wood”. In a tree, xylem can be divided into two different types:
heartwood and sapwood. Sapwood is the portion of xylem that
moves water from the roots to the leaves; sapwood is found just
inside the cambium. The other type of tree xylem is heartwood,
which is located in the center of the trunk. Heartwood is old
sapwood that has become clogged and now serves to support the
tree.
_Functions of xylem_
¾ Water-mineral transport: The sapwood moves water from the
roots to areas of photosynthesis.
¾ Strength: The heartwood provides support for the branches of
the tree.
¾ Storage: Many useless or harmful chemicals are stored in the
heartwood of the trunk.
_What is phloem?_
Phloem (flō ▪ əm) is usually closely associated with xylem and
forms a thin layer outside the cambium, but inside the bark. The
expanding tree is constantly crushing the old cells in this area and
new cells are created in their place.
_Functions of phloem_
¾ Food transport: The primary function of phloem is to
transport food (sugars) created by photosynthesis (i.e. leaves)
to actively growing parts of the plant (i.e. meristems).
¾ Water-mineral transport: Phloem transports water to actively
growing areas, where the xylem is not mature.
parenchyma
collenchyma
sclerenchyma
_What other cell types exist in plants?_
Besides xylem and phloem, plants have other cells types that aide
in storage, bulk mass, and support.
¾ PARENCHYMA (pə▪rĕng’▪kə▪mə): these cells are the bulk of soft
green plants that store water and other materials. It the most
common tissue in herbaceous plants.
¾ COLLENCHYMA (kə▪lĕng’▪kə▪mə): this cell type is thick but
elastic, which allows flexible support for plants. The “teethflossing” properties of celery are due to collenchyma
¾ SCLERENCHYMA (sklə▪rĕng’▪kə▪mə): this cell type is thick and
tough, which allows rigid support to plants. For example, the
strength of bamboo is due to sclerenchyma
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_What protects the plant from the environment?_
The epidermis of a leaf or a stem is outermost covering around a
plant, just like an animal’s skin. This layer usually has a waxy
coating on top called the cuticle. Plant hairs are simply
outgrowths of the epidermis, and come in a variety of shapes,
serving many functions. As the girth of a tree is increased, the
epidermis is replaced with hard substance called bark.
_Function of the epidermis & bark_
¾ Physical protection: The epidermis and later bark are
protective layers that prevent bacteria, fungi, animals, and
sometimes other plants from stealing sugars and water.
¾ Air-water exchange: The epidermis of a leaf is dotted with
pores, called stomates, that allow carbon dioxide to enter and
oxygen and water to escape; this is crucial to photosynthesis.
_Functions of hairs_
¾ Physical protection: Hairs that are dense and matted can
prevent attacks by small insects. The rose bush does not have
spines or thorns, but sharp prickles, which are modified hairs,
protect the stem from large herbivores.
¾ UV protection: Many desert plants are covered with thick,
white hair. These hairs are efficient at reflecting harmful UV
rays from the sun.
¾ Limit dehydration: A dense mat of hair can also protect the
plant from dehydration. These hairs form a pocket of moist air
around the epidermis, which prevents water loss.
¾ Root hairs: These hairs are the primary conduits for absorbing
water from the soil.
¾ Dispersal: Many fruits and seeds have Velcro™-like hairs that
stick to the fur of passing animals, in effect “hitching a ride” to
a new location. In fact, a Swiss naturalist (George de Mestral)
who went on a hike and returned with burs on his pants
invented Velcro® in 1948 and applied for a patent in 1951.
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