Botany 2104. Plant Form and Function
Reproduction in Plants
two complementary gametes (egg, sperm) fuse to form a new, genetically distinct individual
reproduction without fusion of two gametes
naturally occurring in most plant species
cells proliferate by mitosis: clone
totipotency of plant cells: A mature, specialized cell has the ability to de-specialize and divide to produce many more cells. The new cells can specialize into any plant cell type.
uses vegetative part(s) of the plant
roots, stems, leaves
a type of asexual reproduction
the resulting plants are genetically identical to the starting plant and to each other (clones)
adventitious = plant organs that develop on different organs. For example, when a stem cutting forms roots, the roots are adventitious roots.
Some vegetative reproduction occurs
naturally, usually via modified vegetative organs.
rhizome = horizontal, underground stem
Adventitious roots form at nodes.
examples: iris, crabgrass
stolons (runners) = horizontal, above ground stems.
Adventitious roots form at nodes.
examples: strawberry, spider plant
root sprouts (suckers) = adventitious buds form on
examples: aspen, raspberry, blackberry, apple, cherry, redwood
tubers = underground stem with an enlarged (fleshy) tip.
example: potato = the eyes are axillary buds; after the shoot system grows from the buds, adventitious roots form
plantlets = some plants form small plants along leaf margins, leaf tips, leaf base, or fleshy leaves of bulbs
Jumping cholla (Opuntia bigelovii)
The detached joints will readily generate new plants by rooting and branching. During the cooler months the terminal joints are detached by a slight touch by a passing animal or even strong winds. The joints that attach to animals may be transported considerable distances before being dislodged.
Some vegetative reproduction is human-assisted.
leaf cuttings = African violet (Saintpaulia), Peperomia, Begonia
stem/shoot cuttings = Geranium, Coleus (photo)
the cutting is placed in water or damp potting soil or vermiculite; adventitious roots form at the base of the cutting
Leaf and stem cuttings are sometimes treated with rooting powder to encourage adventitious root formation. The rooting powder contains a form of auxin, a plant hormone.
Union of two related plants
The shoot system forms from the scion; the root system forms from the root stock.
Match up the vascular cambia of the scion and root stock so that the two plants grow together as secondary growth of the vascular tissues proceeds.
Often done with fruit trees
maintain the fruit quality
disease- and pest-resistant root system
dwarfing of the tree
example: navel orange scions + bitter orange root stock
Why you need to cut off suckers from grafted trees: The suckers are growing from the root stock, which generally has lower quality fruit than the scion.
Examples: whip graft, cleft graft
Micropropagation (Tissue Culture)
Plants are grown under aseptic conditions in special media.
Pieces of a plant (explants) are surface sterilized with dilute bleach and placed in culture vessels.
The vessels contain a pH-adjusted growth medium that includes mineral nutrients, sugar (explants are grown in dim light to control plant development, but the light isn’t bright enough for sufficient photosynthesis), vitamins, plant hormones (to control plant development), and agar (to solidify the medium).
The explants form callus (a clump of parenchyma cells) followed by shoot and root development.
Under some hormone mixtures, the plants are transferred to a new medium for each developmental step:
callus --> shoot --> root
Under some conditions, callus will form entire plantlets. After plants have formed in culture, they are transplanted to sterile soil and hardened off.
Advantages to Micropropagation
propagate many plants from limited supply of starting material
several commercially desirable plants can only be propagated easily, if at all, by tissue culture
examples: orchids; carnivorous plants
faster than conventional vegetative propagation
the plants are disease- and pest-free, making it easier to ship them (interstate or international)
the production of seed without fertilization
results in cloning via seed
the advantages of seed production in the absence of sexual reproduction
occurs in citrus, orchids, some grasses, and dandelions
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1 October 2013