14: Plants: Reproduction, Growth, Sustainability Seeds are the embryos of the next generation of...

14: Plants: Reproduction, Growth, Sustainability

Seeds are the embryos of the next generation of plants. By saving and exchanging seeds produced by plants with desirable traits, farmers and gardeners ensure the plants’ genetic continuation.

UNIT 5 Chapter 14: Plants: Reproduction, Growth, and Sustainability

Plants reproduce by both asexual and sexual reproduction. Sexual reproduction is by sporic reproduction (alternation of generations). Haploid gametophyte cells (1n) produce gametes, while diploid (2n) sporophyte cells produce spores. Male and female gametes unite to form the sporophyte that continues the life cycle.

14.1 Plant Reproduction

UNIT 5 Chapter 14: Plants: Reproduction, Growth, and Sustainability Section 14.1

All plants have a life cycle involving alternation of generations. The cycle varies among species. The variation is mostly due to the type of structure that releases the spores.

Seedless plants include vascular ferns and non-vascular mosses. Fertilization requires the sperm to swim from the male gametophyte to the egg in the female gametophyte. Thus, water must be present, and the sperm must have a flagellum.

In non-vascular plants, the gametophyte is the dominant generation. In vascular plants, the sporophyte is the dominant generation.

Sexual Reproduction in Seedless Plants


Peat, or sphagnum, moss commonly grows in boggy areas. Its antibacterial andabsorbent properties led to its historic use by some Aboriginal peoples as dressing for wounds.

Seed plants include gymnosperms and angiosperms. The gametophytes are not free-living. The male gametophyte, called a microspore, develops into a pollen grain and sperm.

The female gametophyte, called a macrospore, develops into an egg cell. In order for pollination to occur, the male gametophyte must be transferred to the female reproductive structure.

Sexual Reproduction in Seed Plants


During pollination, the pollen grain develops a pollen tube to reach the egg. Sperm develop in the tube and travel to the egg. The zygote becomes an embryo with a small amount of food protected by a tough seed coat. The seeds remain in the female structure until maturity when they are released. Seeds grow into sporophytes.

Sexual Reproduction in Gymnosperms


Angiosperm sexual reproduction involves the flower organ. Flowers have:

• sepals – protect the flower bud• petals – attract pollinators

Pollination takes place on the stigma. Female gametophytes develop in the ovules, where eggs are formed.

Sexual Reproduction in AngiospermsUNIT 5 Section 14.1Chapter 14: Plants: Reproduction, Growth, and Sustainability

• stamens – male reproductive structure• pistils – female reproductive structure

• Petal number can distinguish monocots from dicots:o multiple of four or five: dicot (meadow beauty)o multiple of three: monocot (trillium)

Variations Among Flowers


The meadow beauty (A) and sulfur cinquefoil (B) are dicots. The trillium (C) is a monocot.

• Self-pollination: Plants pollinate themselves or another flower on the same plant. This can lead to loss of genetic variation and species vulnerability.

• Cross-pollination: Plants receive pollen from another plant, ensuring genetic diversity.• Animal Pollination: Insects and other small animals move from flower to flower collecting

nectar and moving pollen. Bright, sweet-smelling flowers attract these pollinators.• Wind Pollination: Some plants lack colourful reproductive organs but produce large

quantities of light pollen grains to increase the chances of pollen landing on a receptive reproductive organ.

Pollination MechanismsUNIT 5 Chapter 14: Plants: Reproduction, Growth, and Sustainability Section 14.1

As a pollen tube grows to reach the ovule, the male gametophyte performs mitosis to create two sperm cells. One fuses with the egg, the other fuses with the polar nuclei, forming a triploid (3n) cell that divides into nutrient-rich endosperm tissue.

Follow the illustration for the complete life cycle of a peach.

Life Cycle of Flowering PlantsUNIT 5 Chapter 14: Plants: Reproduction, Growth, and Sustainability Section 14.1

The zygote (2n) is a sporophyte that divides to produce a monocot or dicot embryo with a 3n endosperm. The outside layers of the ovule form a protective cover called the seed coat. As the ovule develops into a seed, changes occur in the ovary wall that lead to the formation of a fruit. The fruit may also be made from other flower parts.

Seed and Fruit Development


Seeds of monocots differ in structure and function from those of dicots.

Germination is the resumption of growth after dormancy. The seed absorbs water, and the seed coat breaks. The stored food supports the growth of the embryo. The radicle emerges first and becomes roots. The hypocotyl then emerges as an early stem. It may have the cotyledons and the embryonic leaves on it. Monocots leave their cotyledon below ground.

Seed Germination


Photosynthesis begins when chloroplast-containing cells emerge.

Asexual reproduction (cloning) can be an advantage when plants are well-adapted to their environment. Farmers and gardeners have studied and perfected techniques of artificial propagation that involve asexual reproduction from a plant’s roots, stems, or leaves (vegetative propagation).

How does this method of reproduction affect genetic diversity? Justify your answer.

Asexual Reproduction


• split the plant into two or more; each piece contains roots and shoots

• simple and inexpensive way to propagate or thin out plants

• examples: bulbs (tulips), plants with more than one stem (peonies, hostas)

Asexual Reproduction: Division (splitting)


• shoot or root part is cut and joined to the vascular cambium of another • allows combination of characteristics of two varieties of plants; helps repair damaged trees; quickens fruit production• examples: fruit trees (apple), nut trees (almond), grapevines

Asexual Reproduction: Grafting


• a part of a leaf or entire leaf is cut and placed in growth medium (water, soil, or vermiculite) so that meristem cells can grow shoots and roots• faster than propagating from seed; can be done out of season• examples: African violets, snake plants, aloe vera

Asexual Reproduction: Leaf Cutting


• A stem or shoot tip is cut and placed in growth medium to grow roots from meristem cells

• Faster than propagating from seed; can be done out of season

• examples: herbs (basil), gymnosperm and angiosperm trees (pine, willow), flowering bushes (roses), grapevines

Asexual Reproduction: Stem Cutting


• A root is cut and placed in a growth medium in the same orientation as the original plant; meristem cells form a new root and shoot system

• can be done when plant is in spring or autumn dormancy

• examples: trilliums, mint, irises

Asexual Reproduction: Root Cutting


• A long, vine-type stem is bent to touch the ground, slightly cut to promote growth of roots, and buried until a plant develops that can be cut from the parent to grow independently

• Large clone produced quickly; water and nutrients from parent will support rooting process• examples: honeysuckle, willow, hydrangea

Asexual Reproduction: Simple Layering


• A strip of outer bark is removed from a woody stem; moist sphagnum moss is packed around the wound and plastic-wrapped until roots develop; the rooted stem can then be cut from the parent plant and planted• Large woody plant clone produced quickly• examples: tropical plants (rubber trees), lilacs, magnolias, fruit and nut trees

Asexual Reproduction: Air Layering


• A cell or small piece of tissue is placed in a sterile nutrient medium that promotes shoot and root growth; a tiny plantlet develops• Can be used to produce millions of plantlets for genetic modification• examples: most ferns, gymnosperms, angiosperms

Asexual Reproduction: Cell Culturing


Summary of Plant ReproductionUNIT 5 Chapter 14: Plants: Reproduction, Growth, and Sustainability Section 14.1

Plant hormones are chemical compounds. They:

14.2 Plant Growth and Development


• play a role in determining cellular differentiation and gene expression in any cell as a plant grows from shoot and root apical meristem

• regulate the differentiation of plant cells, plant growth, and the plant’s response to a given stimulus (gravity, light, touch)

• act as chemical signals between cells and tissues• include auxins, cytokinins, gibberellins, ethylene, and abscisic

acid

Stimulatory hormones:

Stimulatory vs. Inhibitory Hormones


• include auxins, cytokinins, and gibberellins• can stimulate cell division, cell differentiation, early flowering and cell elongation that develops into apical dominance (growth is upward with little lateral growth)• used in industry to increase fruit and cluster size• examples: indoleacetic acid (auxin)

Continued…

(A) Auxin stimulates apical meristem growth and inhibits the growth of side branches.(B) Removing the apicalmeristem decreases theamount of auxin. As a result,side branches grow.

Inhibitory hormones:

Stimulatory vs. Inhibitory Hormones


• include ethylene and abscisic acid• can inhibit growth by weakening cell walls, promoting breakdown of starch, blocking stimulatory hormones and blocking the intake of carbon dioxide• commercial use: In order to protect fruit in transit, it is shipped long distances in an unripened state; ethylene gas is then used to ripen it when it reaches its market.

Bananas and other fruits ripen due to ethylene.

Plant Hormones Summary


Tropic responses are growth responses to external stimulation coming from one direction in the environment. They include:

Phototropism

• a growth response to light produced by an unequal distribution of auxin. More auxin on the side with less light causes those cells to elongate and bend the plant toward the light.Gravitropism

• a “positive” growth response of the roots downward or a “negative” growth response of stems upward

Thigmotropism

Tropic Responses


• a growth response to mechanical (contact) stimuli

What tropism(s) are evident here?

A plant’s ability to grow is affected by:

Each plant species grows best within a narrow pH range. Plants that thrive in acid soil include pine, blueberry, and potatoes. Plants that thrive in alkaline soil include lawn grass, beans, pears, and lettuce.

Macronutrients are nutrients that are needed in amounts greater than 1% of a plant’s dry weight and include nitrogen, potassium, calcium, magnesium, phosphorus, and sulfur. Micronutrients are needed in amounts less than 1% of a plant’s dry weight.

Other Plant Growth Factors


14.3 Succession and Sustainability


In addition to the other human and ecosystem services plants provide, they also play a role in establishing and developing communities. This is called ecological succession. Ecological succession is the change in an ecosystem when one community replaces another; it results from changes in abiotic and biotic factors.

Events that change the structure of a biological community and sometimes destroy all actively growing organisms are called ecological disturbances. They include:

• forest fires• floods

Within months of most disturbances, new vegetation sprouts and animals repopulate until, over time, the area is again thick with growth. Ecologists believe that disturbances, both large and small, are the norm rather than the exception in communities. Understanding how disturbances help and hinder plant and animal species is critical for the preservation of natural communities.

Ecological Disturbances


• volcanic eruptions• retreating glaciers

Primary Succession


Primary succession is the establishment of a community in an area after an ecological disturbance has left exposed rock that does not have any topsoil. Often, liverworts are the first species (pioneer species) to colonize a barren area. With other organisms, such as bacteria, algae, and lichens, they form a pioneer community that can survive in harsh conditions.

What traits do these pioneer species have in common that make them suited to living in barren areas?

Primary Succession


As pioneer species continue to grow in a barren area, early organisms die and begin to form soil. As soil accumulates, nutrient and moisture content builds and pH changes. This allows larger species to grow in the area.

Scientists measured changes in the nitrogen content and the number of plant species during primary succession in Glacier Bay, Alaska.

Primary Succession


The area continues to change as larger plants move in. Many early species are vascular and non-vascular seed species; their seeds have dispersed and germinated quickly. They compete for space and light. Successive surviving species colonize the area. Different types of plants provide habitat for different birds and animals. Changes continually occur until a stable climax community of plants and animals forms.

It will remain stable until a major disturbance occurs.

Secondary Succession


Secondary succession is the recolonization of an area after an ecological disturbance that has left the soil intact. It also includes changes in the composition and number of species over time.

Biodiversity and Ecosystem Resilience


Research has shown that ecosystems (climax communities) with high diversity are better able to withstand disease, competition from invasive species, and extreme weather events.

Maintaining sustainable and diverse natural and human-made ecosystems is critical to the health of the biosphere.

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