IB BIOLOGY
TOPIC 9 PLANT SCIENCE
I. WHAT IS A PLANT?
A. Kingdom Plantae: multicellular, eukaryotic organisms that have a large central vacuole,
chloroplasts for photosynthesis, a cell wall made of cellulose, starch as the food storage
molecule and the life cycle is based on alternation of generation
1. Alternation of Generation
Figure 1. Alternation of Generation
http://www.mhhe.com/biosci/genbio/maderbiology7/graphics/mader07b/online_vrl/images/054
8l.jpg
a. the sporophyte form is diploid
b. in the sporangium, haploid spores are produced by meiosis
c. spores develop into haploid gametophytes
d. gametophytes produce haploid gametes by mitosis
e. hapoid gametes fuse to form diploid zygotes
f. diploid zygotes grow into diploid sporophytes;;
B. Plant Classification
1. Note that the term Division is often used instead of Phylum when discussing Kingdom
Plantae.
Figure 2. The Major Groups of Plants.
http://faculty.southwest.tn.edu/rburkett/classification_of_organisms.htm
Figure 3. Detailed classification of plants. (This figure can be found at both of the following sites;
I do not know which (if either) is the original.)
http://www.blog.montessoriforeveryone.com/the-optimization-of-classification.html
https://sites.google.com/site/scienceandmath4th5th6t/
Table 1. The plant kingdom.
http://www.biologyjunction.com/plant_taxonomy_bi.htm
*Note that the IB terminology uses the term Filicinophyta rather than Pterophyta and the
conifers are referred to as Coniferophyta rather than Pinophyta. As well the flowering plants are
known as Angiospermophyta rather than Anthophyta.
** The Topic 9 in the IB Syllabus focuses on the dicotyledonous Angiosperms (variously referred
to as Angiospermophyta or Angiospermae or Anthophyta)
C. Monocotyledonous versus Dicotyledonous Plants
Figure 4. Monocot versus Dicot.
http://www.mansfield.ohio-state.edu/~sabedon/campbl30_files/image019.jpg
Table 2. Monocot versus dicot characteristics.
Characteristic
1. # cotyledons in the seed
2. # flower parts
3. Pattern of leaf veins
4. Habit
5. Type of growth
6. Type of root
7. Arrangement of vascular
bundles in stem
8. arrangement of vascular
tissues in root
9. # of apertures in pollen
10. Representative members
Monocots
1
Multiples of 3
Parallel
Herbaceous (a very few are
woody)
Primary only
Fibrous
Scattered
Dicots
2
Multiples of 4 or 5
Net
Herbaceous and woody
Form a ring
Xylem forms “spokes” and the
phloem is found between the
spokes
Usually 3
Sunflowers, apple trees, roses,
beans
Usually 1
Grasses, palms, lilies, orchids,
corn
Primary and secondary
Taproot
Ring
1. Monocot stem.
Figure 5. Cross-section through a monocot stem. Note the particular configuration of the large,
xylem cells.
http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookplantanatii.html
Figure 6. Cross-section of a monocot stem, plan diagram.
http://plantphys.info/plant_biology/stems.shtml
2. Dicot stem.
Figure 7. Cross-section of Alfalfa stem. Note the arrangement of the vascular bundles and their
cell arrangements.
http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookplantanatii.html
Figure 8. Cross-section of a dicot stem and plan diagram.
http://plantphys.info/plant_biology/stems.shtml
Figures 9 and 10. Cross sections of a dicot stem.
*For labels on the following, go to the original website.
http://plantphys.info/plant_biology/stems.shtml
3. Dicot stem (showing secondary growth).
Figure 11. Cross-section through a dicot stem showing secondary growth. Note the merging of
the vascular bundles to form a continuous ring of vascular tissue.
http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookplantanatii.html
4. Monocot root.
Figure 12. Cross-section through a monocot root. Note the location of the xylem and phloem.
http://lylscience.blogspot.ca/2010_09_01_archive.html
5. Dicot root.
Figure 13. Cross-section through a dicot root. Note the location of the xylem and phloem.
http://www.doctortee.com/dsu/tiftickjian/plant-anat/root.html
6. Monocot leaf.
Figure 14. Cross-section through a monocot leaf. Note the characteristic shape of the vascular
bundles.
http://www.nvsd44.bc.ca/Staff/UZ/VondetteN10030/Biology%2011/Ch%2022-23/Ch%202223%20Lab.aspx
a. no vascular cambium
b. because the veins are parallel, the sections used in the slide are at right angles to
the cells
7. Dicot leaf.
Figure 15. Cross-section through a dicot leaf.
http://plantphys.info/nrgworkshop/leafcslabel.html
Figure 16. Cross-section through a dicot leaf. Note the shape of the vascular cells because of the
angle of the slice.
http://www.nvsd44.bc.ca/Staff/UZ/VondetteN10030/Biology%2011/Ch%2022-23/Ch%202223%20Lab.aspx
a. a small section of vascular cambium is present
b. because the veins are net-like, the sections used in the slide will have some cuts at
right angles and some at oblique angles to the cells
II. OVERVIEW OF PLANT STRUCTURE AND FUNCTION
A. Generalized Plant
1. the main organs of the vegetative (not reproducing) plant are the roots (root system),
stems and leaves (stems + leaves = shoot system)
a. note that flowers are modified leaves and the fruit of a plant comes from the
flowers
2. the root system is underground (and thus includes specialized stems such as tubers
and rhizomes that are found underground) and the shoot system is above the ground
Figure 17. Generalized plant structures.
http://leavingbio.net/FLOWERING%20PLANTS_files/image002.jpg
Figure 18. Plan diagrams of cross-sections through the various parts of a plant.
http://www.bio.miami.edu/dana/226/226F09_5print.html
B. Root System (usually below the ground)
1. anchors plant to substrate and helps support it
2. absorbs water and mineral nutrients
a. root hairs increase surface area of the root
3. transports water and mineral nutrients
4. store food
a. roots specialized for food storage include carrots, radishes, parsnips etc.
5. produce hormones that will be transported to other parts of the plant
C. Shoot System (usually above the ground)
1. Stems
a. support plant
b. produce leaves, buds, flowers
c. lift the leaves toward the light to maximize photosynthesis
d. has vascular tissue to transport water and minerals from roots and products of
photosynthesis from leaves
e. can be specialized for storage
i. cactus stems store water
ii. tubers are horizontal stems that store food and/or nutrients
a. potatoes
f. can be specialized for vegetative (asexual) reproduction
i. stolons (runners) are specialized stems that grow along the ground and new
individual plants grow along them at locations called nodes
a. e.g. strawberries
ii. tubers can also grow into individual plants
a. e.g. potatoes
*Note that tubers are specialized rhizomes
g. protection (thorns are modified stems)
2. Leaves
a. photosynthesis (mainly in the mesophyll, both palisade and spongy)
b. increase surface area for the capture of light energy
c. gas exchange
d. water regulation/conservation
i. the underside of the leaf has openings called stomata that can open and
close
ii. transpiration: the loss of water from the leaf
iii. the surface of the leaf has a waxy covering, the cuticle, that helps to
minimize transpiration
e. transport of the products of photosynthesis for use by other parts of the plant
f. sexual reproduction (flowers are modified leaves)
g. protection (spines/prickles are modified leaves)
h. hormone production
i. food storage: bulbs are underground shoots with leaves that are swollen and
store food
Figure 19. A cross-section through a leaf.
http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookplantanat.html
Figure 20. A comparison of a micrograph and a diagram of a leaf in cross-section.
http://webpub.allegheny.edu/dept/bio/bio220/Milt_lectures/leafxc.jpg
D. Modifications of Stems and Leaves (IB mandated)
1. bulbs: underground shoots with leaves that are swollen and
store food
a. e.g. tulips, onions
2. stem tubers: specialized underground stems (rhizomes) that store food
a. e.g. potato
3. storage roots: taproots
a. e.g. carrots, parsnips, turnips
4. tendrils: leaves modified to wind/curl around structures and help the plant to climb
a. e.g. peas, grapes vines
E. Plant Cell Types
Figure 21. Xylem cells.
http://www.bio.miami.edu/dana/226/226F09_5print.html
Figure 22. Xylem cells.
http://webpub.allegheny.edu/dept/bio/bio220/Milt_lectures/xylemcells.jpg
1. Tracheids/tracheary elements/tracheid cells
a. long, narrow cells connected to each other by pits
b. cells have tapered ends
c. found in all vascular plants (Filicinophyta, Gymnospermophyta and
Angiospermophyta)
d. cell walls have lignin for added strength
e. dead when mature
2. Vessel elements
a. shorter than tracheids with perforated cell walls at the ends
b. have thinner walls and larger internal diameters than do tracheids
c. joined at the ends to form long continuous tubes
d. cell walls have lignin for added strength
e. found only in angiosperms (and Gnetophyta)
f. dead when mature
3. Sieve tube elements/sieve cells
Figure 23. Phloem cells.
http://webpub.allegheny.edu/dept/bio/bio220/Milt_lectures/xylemcells.jpg
a. do not have a nucleus, have reduced cytoplasm, lack organelles
b. connected to each other by plasmodesmata/sieve plates
c. remain alive
d. arranged end to end to form sieve tubes
e. the actual cell used for transport of sucrose
4. Companion cells
a. found adjacent to sieve tube elements
b. (may) use active transport to load and unload sucrose into or out of the sieve cells
5. Guard cells
Figures 24 and 25. Guard cells.
http://waynesword.palomar.edu/photsyn1.htm#guardcell
a. found on the surfaces of leaves (stems and fruits)
b. regulate the exchange of water vapour, oxygen and carbon dioxide by opening and
closing the stomata
c. stomata: openings on the surfaces of leaves (stems and fruits)
5. Parenchyma
Figure 26. Cross section of sunflower stem. Is it a monocot or a dicot?
http://www.doctortee.com/cgi/image-lookup.cgi?helianthus-stem-lab
a. least specialized
b. thin, flexible cell walls
c. remain alive
d. can differentiate into other cell types for repair
6. Collenchyma
Figure 27. Celery collenchyma.
http://www.doctortee.com/cgi/image-lookup.cgi?celery-collenchyma
In this freehand section of a celery petiole, a large strand of collenchyma can be seen just
beneath one of the ribs that runs along the outside of the petiole. This is angular collenchyma;
the cell wall thickenings are located mostly at the corners where several cells meet.
a. primary cell walls are relatively thick
b. remain alive
c. help to support herbaceous plants
d. e.g. the stringy fibres in celery
7. Sclerenchyma
Figure 27. Sclerenchyma of pear fruit.
http://www.doctortee.com/cgi/image-lookup.cgi?pyrus-sclereids
The fruit of Pyrus (pear) contains clusters of stone cells (brachysclereids). Note the thick lignified
secondary walls with branching (ramiform) pits.
a. thick secondary cell walls
b. do not remain alive
c. 2 types
i. fibres: long, narrow cells with regular secondary cell wall
a. e.g. hemp fibres for making rope
ii. sclereids: short cells with an irregular shape
a. e.g. stone cells in pears and hard nut/seed shells
Figure 28. A comparison of parenchyma, collenchymas and sclerenchyma.
http://www.britannica.com/EBchecked/media/388/Cell-types-and-tissues
F. Plant Tissue Types
1. Vascular
a. Xylem
i. conducts water and ions through the plant
ii. composed of tracheids and parenchyma (and vessel elements and fibres in
angiosperms)
iii. also supports the plant
b. Phloem
i. involved in the transport of sucrose, other organic compounds, and some ions
ii. composed of sieve tube members, companion cells (and often sclerenchyma)
c. xylem, phloem (and sometimes vascular cambium) are located together in the
stem and called a vascular bundle
2. Meristematic: cells that divide to produce other cells (one cell remains meristem, the
other differentiates)
a. Primary meristems
i. allows growth in length (primary growth)
ii. e.g. apical meristems in roots and shoots, and pericycle in roots
b. Secondary meristems
i. allow growth in diameter of a stem or root (secondary growth)
i. e.g. vascular cambium and cork cambium
3. Dermal
a. usually form layers that are 1 cell thick
b. form the outer covering of the plant
c. function for protection
d. made mainly of parenchyma cells
4. Ground
a. makes up most of the plant; anything not any of the other tissue types is referred
to as ground tissue
b. mostly parenchyma but collenchyma and sclerenchyma as also found in ground
tissue
c. can be for photosynthesis, storage, support
Figure 29. Two views of the relative positions of the different tissue types in the plant stem. Is
this a monocot or a dicot? How can you tell?
http://mrb-science.wikispaces.com/Roots+and+Stems
III. PLANT GROWTH
Figure 30. Primary growth in a plant.
http://content.bfwpub.com/webroot_pubcontent/Content/BCS_3/Sadava_9e/Interactive%20Su
mmaries/3410.html
A. Primary Growth
1. increase in length
2. arises from the apical meristem
a. root growth
Figure 31. Primary growth in the root.
http://content.bfwpub.com/webroot_pubcontent/Content/BCS_3/Sadava_9e/Interactive%20Su
mmaries/3410.html
i. Zone of cell division: most mitosis occurs
ii. Zone of elongation: cells grow in length/size
iii. Zone of maturation: cells differentiate
iv. Root cap: protects the tip of the cell as it pushes through the soil
a. the smashed cells help to lubricate the passage of the root through the soil
Figure 32 . Another plan diagram of dicot and monocot roots.
http://content.bfwpub.com/webroot_pubcontent/Content/BCS_3/Sadava_9e/Interactive%20Su
mmaries/3410.html
Figure 33. A comparison of different roots and their tissue locations.
http://content.bfwpub.com/webroot_pubcontent/Content/BCS_3/Sadava_9e/Interactive%20Su
mmaries/3410.html
B. Secondary Growth
Here are some animations of secondary growth in stems:
http://academic.kellogg.edu/herbrandsonc/bio111/animations/0015.swf
http://bcs.whfreeman.com/thelifewire/content/chp35/35020.html
Figure 34. Primary versus secondary growth.
http://content.bfwpub.com/webroot_pubcontent/Content/BCS_3/Sadava_9e/Interactive%20Su
mmaries/3410.html
Figure 35. Detail of secondary growth in a woody plant.
http://content.bfwpub.com/webroot_pubcontent/Content/BCS_3/Sadava_9e/Interactive%20Su
mmaries/3410.html
1. arises from the 2 types of lateral meristems, vascular cambium and cork cambium
2. vascular cambium produces new xylem and phloem while cork cambium produces
periderm (cork, cork parenchyma and cork cambium)
3. bark = periderm + secondary phloem and wood = xylem
4. vascular cambium near the interior produces xylem while vascular cambium towards
the exterior produces phloem
a. as the secondary growth of vascular tissue occurs, the new xylem forms a ring
around the old xylem (which remains in place) and the old phloem (and periderm)
is broken and replaced by new phloem (and periderm) as the stem diameter
increases
b. more xylem cells are made than phloem cells
c. in the spring and early summer, the xylem cells are larger in diameter than later in
the summer (and the remainder of the growing season)
i. due to relatively abundant water and mineral nutrients in the spring followed by
hotter and drier summers accompanied by reduced mineral content of the soil
ii.this produces the annual rings of tree trunks
iii. the relative thicknesses of the annual rings give clues about the environment
and significant weather events during each year
a. dendrochronology: the study of the layers of a tree trunk
d. phloem cells also show the same variation in size throughout the year
e. the xylem in the centre of the tree trunk (heartwood) eventually becomes plugged
with sap/resins and cannot conduct water any longer
i. even if this wood rots away, the tree can stay alive
f. functional xylem is known as sapwood
Q: Why isn’t the phloem used to deduce the age of a tree?
Q: Why aren’t tree rings produced in tropical plants?
Figures 36. Primary versus secondary growth.
http://scidiv.bellevuecollege.edu/rkr/biology213/lectures/pdfs/secondarygrowth213.pdf
Figure 37. Cross-sections through a woody stem. Note the annual rings due to the relative sizes
of the cells in the early versus late growing seasons.