|Weber State University -- Department of Botany|
BOTANY LS1203 - PLANT BIOLOGY
1. Asexual Reproduction
a. Uses vegetative parts of the plant.
b. Is a type of asexual reproduction.
c. All cells produced will be genetically identical: clones
d. Plant cells, particularly parenchyma cells, are totipotent. This means that a mature, specialized cell has the ability to de-specialize and divide to produce many more cells. The new cells made can specialize into any plant cell type.
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, often from modification of vegetative organs.
rhizomes = horizontal, underground stems. Adventitious roots form at nodes. Iris, crabgrass.
runners = horizontal, above ground stems. Adventitious roots form at nodes. Strawberries, spider plant.
root sprouts, suckers = adventitious buds form on roots. Aspen, misc. shrubs (raspberry)
tubers = underground stem with an enlarged (fleshy) tip. Potato (the eyes are buds).
plantlets = some plants form small plants along leaf margins (Kalenchöe) or other plant part (pregnant onion).
layering = a long stem bends down into the soil and adventitious roots grow. Climbing roses, raspberries, blackberries.
Some vegetative reproduction is human-assisted.
leaf cuttings = African violet, Peperomia, Begonia
stem cuttings = Geranium, Coleus
Leaf and stem cuttings are sometimes treated with rooting powder to encourage adventitious root formation.
Grafting = union of scion and root stock of two related plants; often done with fruit trees to maintain the fruit variety while acquiring a disease- or pest-resistant root system.
Tissue culture (micropropagation) = plants are grown under sterile conditions in special media. Pieces of a plant (explants) are sterilized with bleach and placed in culture vessels. The vessels contain a pH-adjusted growth medium that includes mineral nutrients, sugar, vitamins, plant hormones (to control plant development), and agar (to solidify the medium). The explants form callus 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. Once plants have formed in culture, they are transplanted to sterile soil and hardened off.
Other Asexual Reproduction Mechanisms
2. Sexual Reproduction: Flower Structure, Pollination, Double Fertilization
In sexual reproduction, two cells (the gametes) fuse to form a new individual
(zygote). Sexual reproduction gives the flowering plants two opportunities
to spread genetic diversity: via pollen and via seeds.
A. Flower Structure
Four floral whorls: sepals (calyx), petals (corolla), stamens (filament + anther), carpels/pistil (stigma + style + ovary; ovule). These floral parts attach at the receptacle.
A complete flower has all four whorls. An incomplete flower is missing at least one.
A perfect flower has both stamens and pistils. An imperfect flower is missing one or the other. If a plant has both male flowers and female flowers, the plant is moneocious (corn, squash). If male flowers and female flowers occur on separate plants of the same species, that plant is dioecious (willow, hops, spinach).
A regular flower has radial symmetry (rose); an irregular flower has bilateral symmetry (iris).
A superior ovary is located above the attachment point of the other three whorls. An inferior ovary is located below the attachment point of the other whorls.
A solitary flower is the only flower on the stalk (peduncle). An inflorescence has multiple flowers on a single penduncle. Inflorescence types include head (sunflower), umbel (dill), spike (lilac), and raceme (mustard).
Pollen grains form inside the anthers. Inside each anther, hundreds of pollen grains are produced, each with two sperm.
One to dozens of ovules are found in each ovary. Inside each ovule is an egg and a central cell. The layers of cells that cover the ovule are called the integuments.
Pollination is the transfer of pollen from the anther to the stigma. Some flowers self-pollinate (and therefore can self-fertilize). Plants that self-fertilize have very low genetic diversity because there is essentially uniparental inheritance (in-breeding). Example: peas, which bring us the expression "alike as two peas in a pod." If sexual reproduction is to result in genetic mixing (out-breeding), then cross-pollination (and the resultant cross-fertilization) needs to occur. Several mechanisms promote out-breeding:
different maturation times for anthers and stigmas
anatomy of the anthers and stigmas
Pollen vectors (pollinators): agents that transfer pollen from one flower to another. Flower structure, color, scent, rewards, etc. match the vector.
Flowers are generally nondescript, lacking petals, scent, and nectar. Copious amounts of light, dry pollen are produced. Anthers and stigmas are well exposed.
In general, these flowers will have colorful petals (color is vector specific) and will produce heavy and sticky pollen. Nectar might be made in nectaries. A scent might be present that is vector specific.
See yellow and blue. Can also see UV. Often the color pattern of the petals serves as a pollen guide. Nectar. Sweet or other pleasant scent.
See red and orange. The corolla often forms a long tube, with nectar at the base. Not much scent.
White or light colored to be seen at night. Strongly scented.
Generally strong odors, like rotting meat to attract carrion flies.
More white or light colored petals. Strong musky or fruity scent. The flowers are generally robust.
Red and yellow flowers. Not much, if any, scent. Robust flowers. The sugar in the nectar is not as concentrated as it is for insect-pollinated flowers. The flower construction will generally place the pollen on the bird's head. (Pollen sticks to feathers great; beaks, not so much.) Classic hummingbird flower: red, with a long tubular corolla.
Mimicry and Pollination
Some flowers mimic female insects, producing a scent that matches a specific insect pheromone and forming structures that look (sometimes very vaguely) like a female. When the male insects try to mate with these flowers, they end up transferring pollen. As a group, orchids are probably the most notorious mimics.
Some flowers mimic other flowers. The original flower might produce a reward for the pollinator, but the mimic does not.
Rock cress is sometimes infected by a fungus that causes the cress to make false
flowers. When insects visit the false flowers, they carry off fungal
spores instead of pollen.
VIDEO!! The Private Life of Plants: The Birds and the Bees. Starring every PBS fan's favorite guide to the natural world, David Attenborough. The video will show many pollinators in action (including some male wasps having quite a tussle with some orchids), explain how different flower features match specific pollinators, and mention some of the mechanisms to avoid self-fertilization. Take notes!!
C. Double Fertilization
Pollen grain on suitable stigma
Growth of pollen tube through the style to the ovule
At the micropyle, get release of both sperm
One sperm fuses with the egg --> zygote --> embryo
Other sperm fuses with central cell --> endosperm
3. Fruits, Seeds, and
zygote -----> embryo, with one or two cotyledons
endosperm -----> food supply
The endosperm might be used up during seed development, resulting in fleshy cotyledons (as occurs in most dicots), or it might be present in the mature seed (e.g., grains). In either case, a food supply is available to support the early growth of the seedling.
integuments -----> seed coat (testa)
ovary -----> fruit (other floral structures might be included)
ovary wall = pericarp, which consists of three layers: exocarp, mesocarp, and endocarp. All three layers are easily seen in the fruit of a peach. In other fruits, it is harder to distinguish the layers.
The fruit provides protection for the developing seeds and a means of dispersal once the seeds are mature.
I. Simple fruit - from a single pistil
A. Dry fruits
1. Dehiscent fruits - fruit has seams that split open at maturity
a. follicle - splits along one seam. (milkweed)
b. legume - splits along two seams (pea, bean)
c. silique - seeds are attached to the center divider between two fused carpels (mustard)
d. capsule - derived from an ovary consisting of many fused carpels; opens by a variety of means (cotton)
2. Indehiscent fruits - fruit doesn’t open at maturity
a. grain or caryopsis - seed coat fused with ovary wall (wheat)
b. achene - seed coat is not fused with ovary wall (sunflower)
c. nut - ovary is woody (acorn)
d. samara - ovary wall forms a winglike structure (maple)
B. Fleshy fruits
1. Superior ovary
a. berry - thin exocarp, fleshy mesocarp and endocarp, generally have many seeds (tomato)
b. hesperidium - berry with a leathery exocarp (orange)
c. drupe - thin exocarp, fleshy mesocarp, stony endocarp (peach)
2. Inferior ovary
a. pepo - berry with a hard rind (squash)
b. pome - fleshy receptacle; ovary wall is papery layer around a core of seeds (apple)
II. Aggregate fruit - fruit is composed of many pistils in one flower (strawberry)
III. Multiple fruit - fruit is composed of many pistils from a cluster of flowers (fig)
IV. Accessory fruit - some structure, in addition to the ovary, forms the fruit (apple)
Be familiar with the following terms: plumule, radicle, cotyledon, scutellum,
coleoptile, coleorhiza, endosperm, testa, hilum, micropyle.
The seeds of annual crop plants will germinate once the seed has imbibed and the temperature is moderate. These seeds were not dormant prior to germination; they were simply dry. The dry seeds were easy to store and transport between growing seasons.
Most plants that have not been domesticated produce dormant seeds. Even under conditions when we would expect the seeds to germinate, they won't until some additional requirement has been met.
Many seeds of desert plants contain chemicals that inhibit germination. If enough water is present to leach out the chemicals, then there is enough water to support establishment of the seedling.
Some seeds, like those of many woody temperate plants, require stratification. The imbibed seed must experience weeks to months of cold temperatures before it will germinate at a moderate temperature. This mechanism prevents the seed from germinating in the fall; it won't germinate until winter is over.
Some small seeds require an exposure to light once they have imbibed. If these seeds were to germinate too deep in soil, they would run out of food reserves before becoming established seedlings.
A number of seeds require scarification. Their seed coats or dry pericarps must be abraded before imbibition can take place. The abrasion could be from tumbling in sand, by passage through an animal's digestive tract, by the action of soil microorganisms, or by fire.
Dormant seeds contribute to seed banks, the viable, ungerminated seeds
in soil. Other seeds in the seed bank might not germinate because of a
lack of minerals or the presence of inhibitory chemicals from established
plants. If conditions change, some of the seeds in the seed bank will germinate.
For example, a fire might remove the plants that produced the inhibitory
C. Seed Dispersal
Some dehiscent fruits will scatter the seeds as they open.
light weight seeds can just blow like dust (orchid)
some fruits have wings that will carry the enclosed seed (maple)
feathery plumes can also carry seeds: the calyx-derived pappus of dandelion; the plume directly attached to seeds of milkweed
whole shoot system detaches from the root and is blown around (tumbleweed)
rain can wash fruits or seeds down a slope
as long as a fruit or seed can float, it can be water dispersed ==> some seeds can float along irrigation ditches until they imbibe so much water that they sink
some fruits are designed for prolonged floatation ==> the mesocarp of the coconut is a lightweight, fibrous material
Some fruits stick to fur, socks, etc. via hooks (the source of the idea for Velcro), spines, barbs, and sticky secretions.
Some fruits entice birds and mammals to eat them by being fleshy and sweet; the seeds then pass through the digestive tract, are regurgitated, or are simply spit out. In some cases, the seeds require an interaction with a digestive or gizzard in order to germinate.
How does a plant get an animal's attention when the seeds are ready to be dispersed? The fleshy fruit will go through specific changes as it ripens when the seeds are mature: the color changes from green to something a bit more obvious against a background of leaves; the fruit softens; the sugar content goes up as acids and starches decrease; various pleasant flavors and odors are made.
Some fruits are climacteric. As they ripen, they produce large amounts of ethylene and have a sharp increase in respiration. These fruits can be picked before they make their own ethylene and stored under conditions that inhibit or greatly slow ethylene synthesis. When we are ready to eat them, they can be exposed to ethylene to trigger ripening. Fruits that are not climacteric cannot be handled this way. These fruits don't store easily and are only available fresh when they are in season.
Ethylene is a plant hormone which happens to be a gas. As a gas, it can diffuse from one plant to another. It is involved in a number of plant responses, including responses to stress. Are you familiar with the saying about one bad apple spoiling the whole barrel? A bad apple is one that has been damaged; it is under stress and releasing ethylene. If that apple is packed in a barrel with other apples for storage, the ethylene will be trapped in the barrel and increase in concentration. On exposure to the ethylene, the other apples will overripen and spoil in storage.
VIDEO!!: Branching Out from The Private
Life of Plants series. The video shows seed dispersal mechanisms.
What is meant by vegetative reproduction? How vegetative reproduction related to totipotency? If a root is described as adventitious, what does this mean?
Be able to explain the role of each of the following in different occurrences of vegetative reproduction; be able to give an example of each. rhizomes, runners, root sprouts/suckers, tubers, plantlets, layering, cuttings, tissue culture.
What is apomixis?
How does sexual reproduction differ from vegetative reproduction? Sexual
reproduction gives the flowering plants two opportunities to spread genetic
diversity: what are they?
Be able to identify the parts of a flower: sepals (calyx), petals (corolla), stamens (filament + anther), carpels/pistil (stigma + style + ovary; ovule), receptacle, peduncle.
Be able to distinguish between the following pairs of words:
complete flower == incomplete flower
perfect flower == imperfect flower
monoecious plant == dioecious plant
regular flower == irregular flower
superior ovary == inferior ovary
solitary flower == inflorescence
Be able to describe and give an example of each of the following types of inflorescences: head, umbel, spike, and raceme.
What is pollination? What are pollination vectors? Be able to associate specific flower characteristics with specific vectors. BE SURE TO GO OVER THE VIEWING GUIDE FOR THE BIRDS AND THE BEES VIDEO!!
What are some of the mechanisms (other than pollinator attractants) used by the flowering plants to promote out-breeding?
seed coat (testa)
pericarp: exocarp, mesocarp, and endocarp.
What are the functions of a fruit with regard to seeds?
Be able to describe various ways in which seeds can be dispersed.
What is seed dormancy? What are some ways in which it is broken? Why is it important for seeds to be able to be dormant? Why was dormancy bred out of annual crops when they were domesticated?
Types of fruits: be able to describe and distinguish between the following types of fruits; be able to give at least one example of each type.
Simple fruits: follicle, legume, silique, capsule, grain or caryopsis, achene, nut, samara, berry, hesperidium, drupe, pepo, pome
Compound fruits: aggregate fruit, multiple fruit
Flower Structure, Seeds and Seed Germination, From Flower to Fruit (by Ross Koning at Eastern Connecticut State University)
Grafting (from the University of Missouri Extension)
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Revised 14 February 2007. Links checked 27 July 2011.