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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
Chapter 17: Plants, plant evolution & Plant life cycles
- Part 1 
on your way to College take some time and look around you. Even though you
should live in a very urbanized area, I am sure that you have encountered, but not
recognized, hundreds of different “green-colored, immobile creatures” standing left
and right of the street

these motion-less, green creatures belong to a huge group of life forms or, as
Biologists say, to a kingdom of biological organisms called plantae (= green plants)

plants are multi-cellular biological organisms, that do a living by harvesting solar
energy to make ATP and organic carbon molecules by a process called
photosynthesis (see Chapter 7)

the huge kingdom called plantae does not only comprise the so-called green plants,
which we commonly refer to as plants, but also the mono- or multi-cellular green
algae
e.g. the seaweed Alva or the common pond inhabitants,
called Volvox, Chlamydomonas or Ulothrix
THE PLANT CELL

the smallest functional units of plants are, as in animals, the so-called cells
(for more detail regarding cells see Chapter 4)

a plant cell shares many structures and organelles with an animal cell
 a plant cell is also surrounded by a biological membrane and harbors
microtubules, centrioles, mitochondria, Golgi vesicles, a rough
endoplasmic reticulum and a tubular trans-Golgi network

but plant cells evolved some unique structures and intracellular organelles
 it contains a unique organelle called chloroplast; with these pigmentfilled compartments, the plant cell is able to collect solar energy and
to use it for biological synthesis in a process called photosynthesis
(see Chapter 7 for more details)
 it has a huge central vacuole necessary for regulation of the intracellular water
household which is filled with nutrients and metabolites
 the membrane of the plant cell is surrounded by two, so-called cell
walls;
1. the primary cell wall is laid down first
2. then a second cell wall is synthesized between the cell membrane and the
first one
1
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.

plant cells are connected with each other via so-called pits, which itself have several
inserted so-called plasmodesmata (singular: plasmodesmosom);
 plasmodesmata are small channels between plant cells
which enable an unconstraint communication and rapid
circulation of nutrients and other constituents

plants are complex biological, multi-cellular organisms, which are made up by many,
differently shaped cells; each so-called cell type has different functions within the
plant body

5 major plants cell types are known
11.. P
Paarreenncchhyym
maa cceellllss

parenchyma cells are the most abundant cell type of plants
 they have thin cell walls and lack a second cell wall!
Biological Function:

1. food storage
2. photosynthesis
3. aerobic respiratory function
most parenchyma cells can divide and differentiate into other cells; i.e. they can be
used for vegetative regeneration
Image of plant parenchyma cells
2
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
2. C
Coolllleenncchhyym
maa cceellllss

collenchyma cells have an unevenly thickened, primary cell wall
 they are able to stretch and elongate

they are characterized by thickenings of the wall; they are alive at maturity and tend
to occur as part of vascular bundles
Biological Function:
 give growing young stems support and
stabilization
Image of plant collenchyma cells
3. S
Scclleerreenncchhyym
maa cceellllss

sclerenchyma cells have a rigid secondary cell wall hardened with the help of the
biomolecule lignin

these cells cannot elongate and occur in regions which stopped growing

most sclerenchyma cells are dead at maturity, but their remaining cell walls form a
hard support scaffold
Biological Function:
Support and protection
 2 forms of cell types exist
1. fibrous type
 e.g. hemp or pineapple fibers
2. the scleroid (= stone cell) type
 e.g. in nut shells and seed coats
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
Image of plant sclerenchyma cells
4. W
Waatteerr--ccoonndduuccttiinngg cceellllss

or xylem cells conduct water and minerals from the roots to the leaves
 while parenchyma cells do occur within the "plant xylem", two other
cell types are identifiable within this plant tissue
 these two cell types are called tracheids and vessel elements
Biological Function:
 convey water from the roots to the upper
parts of the plant (= leaves, stems)

the tubes are formed by the remaining lignified cell walls of dead cells

the two cell types of xylem (see Figure below)
1. Tracheids
 long, stretched cells with tapered end
 have angled end plates which connect from cell to cell
 are the structurally more primitive of the two cell types
(= “evolutionary old model”)
 they occur in the earliest vascular plants on planet Earth,
e.g. in horsetails, ferns
2. Vessel elements
 are much wider and shorter, lack end plates and
have open ends
 are the evolutionary more “modern version” of water-conducting
vessels
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
 occur only in Angiosperms, the most recently
evolved group of plants on our planet
Image of water-conducting cells in plants
Xylem
Phloem
Tracheids
Vessel
elements
Sieve tube cells
Architecture of the vessels of a plant xylem
5
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
55.. FFoooodd--ccoonndduuccttiinngg cceellllss ((== ssiieevvee ttuubbee cceellllss))

sieve tube cells are live plant cells, that are arranged as end-to-end forming tubes;
they build-up the plant’s phloem tissue, which is responsible for the active transport
of metabolites, sugars and other complex nutrients (see Figure above)

they have a thin primary cell wall and no second cell wall; the cell walls at either end
of the cell form so-called sieve plates, which are especially enlarged plasmodesmata

they are flanked by neighboring and tightly associated companion cells
PLANT TISSUES

as in animals, plant cells form tissues, each with characteristic functions
 in general terms: tissues are functional body units, which are
made up by several major cell types

some plant tissues consist of only one cell type
e.g. parenchyma cells
mesenchymal cells
 form a parenchym
 form a mesenchym
Cross section of a typical plant leaf

most plant tissues are composed of more than one cell type and are called complex
tissues
e.g. the water-conductive vascular tissue called xylem or
the sugar-transporting phloem, which consists of
sieve type cells and supporting sclerenchym cells

a green plant consists of 3 typical tissues which make up the plant body
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
11.. E
Eppiiddeerrm
miiss

the epidermis is a one-cell layer made up from epidermal cells, which covers and
protects the plant’s leaves and young stems

the epidermis has numerous functions:
1. it protects the plant leaves from various chemical, physical and environmental
influences, such as draught, wind, environmental toxins, etc.
2. it is the first line of defense to protect the plant from invading fungi or bacteria
3. it participates in the gas exchange, secretes metabolic compounds and plays a
role in water absorption
 the gas exchange is enabled by leaf openings, called stomata,
which can be found at the underside of a leaf
E
Elleeccttrroonn m
miiccrroossccooppiicc iim
maaggee ooff lleeaaff ssttoom
maattaa
Leaf under side
Leaf stomata
( made up from
2 guard cells)
Epithelial cell
4. the epidermis also releases chemicals, e.g. repellants, which are aimed to
protect the plant leaves from infestation by parasites; or warns other neighboring
plants from existing herbivorous attacks ( “plant communication)

the epidermis cells of the leaves or some fruit bodies produce wax-like chemicals,
which form a thin protective layer called cuticule
 e.g. in the case of the tomato fruit, the cuticle is pigmented with
carotenoids
 often are additional waxes, oils, resins, salt crystals and (hydrophilic)
mucilage excreted
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
Electron microscopic images of epidermal cells
Epidermal cells at the flower
surface of a Daisy (Bellis perennis)
Epidermis cell of the seed coat of
a wood sorrel species
22.. V
Vaassccuullaarr ssyysstteem
m ((xxyylleem
m//pphhllooeem
m))

a tube-like system made of cells which provides support and transports water (=
xylem), as well as nutrients (= phloem) to the different parts of the plant
 it is arranged in so-called vascular bundles which differ between
monocotyles ( dispersed) and dicotyles ( ring-like)
33.. G
Grroouunndd ttiissssuuee ssyysstteem
m

consists of cells (mainly parenchyma cells) which fill the spaces between epidermis
and the vascular system
 it has diverse functions, mainly photosynthesis, storage
and support

the ground tissue system in leaves is called mesophyll; which consists of primarily
chloroplast-filled parenchyma cells

the ground tissue system of roots forms the cortex, which consists mostly of
parenchyma tissue; it has primarily food storage function
 roots also have a so-called endodermis, which is a one cell
layer between the ground tissue system and the vascular
system; it has protection and filter function
8
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
A
Arrcchhiitteeccttuurree aanndd ttiissssuueess w
wiitthhiinn aa ppllaanntt lleeaaff
TTH
HE
EP
PA
AR
RTTS
SO
OFF A
AP
PLLA
AN
NTT

a green plant has a distinct architecture and consists basically of four major parts:
1. Leaves


are the plant’s biological solar panels and the places of photosynthesis
have a wax-like layer on the surface called cuticule and pore-like openings,
called stomata - mostly on the bottom of the leave
2. Stem





supports the leaves and flowers
the stem consists of so-called nodes and internodes; nodes are the places where
the leaves are attached, while internodes mark the spaces between the nodes
within the stem thin, tube-like structures are visible, these tiny, cell-made tubes
are called xylem and phloem; both have nutrition and water transport function
the rigid stem of trees is called trunk ; the rigidity of the trunk and it’s surrounding
bark, is due to the molecule lignin, which hardens the cellulose fibers
the rigid cellulose and lignin structures in trees have support function and keep
the plant upright and above the ground
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
3. Roots

roots anchor the plants upper parts in the soil; they enable the plant to absorb
water and minerals from the soil

in some plant species they also have the function to store food in form of starch
in tubers
e.g. potato, horseradish, carrot, yam, ginger, etc.

the root system of both, monocotyle and dicotyle plants, contains root hairs,
which are the places of active water and nutrient absorption

certain nodule-shaped root regions of some plant species, e.g. legumes (beans,
peas) are the place of so-called symbioses with other organisms, e.g. fungi (=
mycorrhiza) with trees or nitrogen-fixing bacteria (= Rhizobium) in legumes
plants
4. Buds

buds are specialized endings of a stem

2 types of buds are classified:
1. terminal buds
 at the apix (= top) of the stem
 develops into the flowers head
2. auxiliary buds
 located in the angles between a leaf and the stem
 are usually dormant = they do not grow into a
flower because of the influence of inhibitory
hormones from the terminal bud;
 this phenomenon is also called apical dominance
 some develop into shoots which later bear flowers
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