Chapter 18: Growth and Reproduction in Plants Problem solving with flowers and wood Lecture by Danielle DuCharme, Waubonsee Community College

Learning Objectives Understand and be able to explain the following:  Reproductive strategies of plants, and how and why they are different from other organisms.  What roles flowers play in plant reproduction.

Learning Objectives  How pollination, fertilization, and seed dispersal are reliant on other organisms or environmental conditions.  The differences between the two types of growth in plants, and how they help protect and enhance the life of the growing plant.

18.1 Plant evolution has given rise to two methods of reproduction. Plants are extremely varied in their appearance and strategies used to attract pollinators.

Plants Have Different Reproductive Strategies Because plants cannot move, they have different reproductive strategies that allow their species to be successful.

Take-home message 18.1  Most plants have two very different options for reproduction: asexual and sexual reproduction.

18.2 Many plants can reproduce asexually when necessary. For most plants, sexual reproduction is more common.

Plants Can Reproduce without a Partner Nearly all plant species have the ability to reproduce without another if the need arises.

Asexual Reproduction in Plants Occurs in a Variety of Ways Raspberry plants can be propagated by cutting sprouts from their roots.

Asexual Reproduction in Plants Occurs in a Variety of Ways Or, plant a potato and wait for it to sprout a genetically identical individual.

Asexual Reproduction in Plants Occurs in a Variety of Ways Or, “plantlets” develop, drop to the soil, and grow as an independent plant.

Asexual Reproduction is a Necessary Adaptation Because plants cannot move, these modes of reproduction are a necessary evolutionary adaptation.

There are advantages and disadvantages to asexual reproduction.

There Are Advantages and Disadvantages to Asexual Reproduction Main disadvantage: Reduction of genetic variability

Take-home message 18.2  Many plants can reproduce asexually. This involves the growth of new, individual plants directly from the tissue of an established plant through mitosis.  The new individual is genetically identical to the individual from which it was produced.

Take-home message 18.2  Asexual reproduction can be energetically efficient and fast and can preserve successful genetic combinations. But it also has the disadvantages, most notably that it does not lead to genetic variability among an individual’s offspring.

18.3 Plants can reproduce sexually, even without moving. Sexually reproducing populations generate large amounts of genetic diversity.

18.3 Plants can reproduce sexually, even without moving. Because of this genetic diversity, organisms can better adapt to new environments, increasing their survival and reproductive success.

Sexual Reproduction in Plants

Sexual Reproduction Is Important to Agriculture Sexual reproduction and artificial selection allow farmers to breed a better crop.

Non-flowering Plants and Flowering Plants Have Different Life-cycles

Alternation of Generations The life-cycle in plants is called an alternation of generations and is characterized by an extended period in which a plant is in a multicellular haploid form and a period in which it is in a multicellular diploid form.

Take-home message 18.3  Many plants can benefit from producing genetically varied offspring by reproducing sexually, with the flower as the chief structure for sexual reproduction.

18.4 The flower is the chief structure for sexual reproduction.

All Flowers Have the Same Fundamental Structures

Flowers Have Four Distinct Parts Flowers have four distinct parts: 1)Sepals Leaf-like structures 2)Petals Brightly colored, leaf-like structures

Flowers Have Four Distinct Parts 3)Stamens (male reproductive parts) Including the filament and anther 4)Carpel (female reproductive parts) Including the stigma, style, and ovary

Take home message 18.4  Flowers are plant structures specialized for sexual reproduction.  Most flowers have the same fundamental structures: sepals, petals, stamens, and a carpel.

18.5 The male reproductive structure produces pollen grains.

Pollen Grains Can Cause Misery for Those Who Suffer with Allergies

Take-home message 18.5  The male reproductive structure produces pollen grains, each grain a two-cell structure that is water-tight and has a sticky surface.  One of the cells in the pollen grain will form a pollen tube, and the other will divide to produce two sperm cells.

18.6 Female gametes develop in embryo sacs.

Take-home message 18.6  Within the ovary, diploid cells differentiate into ovules, each of which is a group of outer protective cells around a diploid egg-producing cell, which undergoes meiosis to produce haploid megaspores.

Take-home message 18.6  One of these megaspores undergoes mitosis several times to produce the embryo sac, the structure that contains the egg and the place where fertilization will occur.

18.7 Plants need help getting the male gamete to the female gamete for fertilization.  Reproduction can be tied to water, like in mosses and ferns.  Some gametes can be transferred by wind.

This Process Can Involve Enlisting Animals to Carry Male Gametes To reduce the dependence on water, land plants have evolved mechanisms to have animals transport male gametes.

How does a flowering plant get an animal to transport its gametes? The plant attracts animals with a flower:  visual cues (color, shape)  olfactory cues (smell)  tactile cues (soft, bristly, hard, rough, smooth)

This Can Also Involve Two Clever Strategies for Achieving Pollination  Bribery: Plants bribe some animals (i.e., with nectar) to carry pollen from one plant to another.  Trickery: They deceive animals into doing the job.

After Attraction, Attachment of Gametes Is the Goal The goal: to physically attach some male gametes to an animal so that they rub off on the female reproductive parts of another plant.

Pollination  A pollen grain from a plant must journey to the stigma of another plant of the same species.  This is called pollination.

There Is a Wide Variety of Pollinators  Pollinators include: birds, bees, flies, beetles, butterflies, moths, and even some mammals (mostly bats).  There is strong co-evolution between plants and their animal pollinators.

Take-home message 18.7  Plants usually utilize trickery or bribery to get the assistance of animals in carrying the male gametes to the female gametes.  There has been strong co-evolution between plants and their animal pollinators.

18.8 Fertilization occurs after pollination.  Pollination brings the male and female gametes close to each other.  Fertilization requires that the male and female gametes fuse so that their genetic material can be combined.

Pollen Grains Are Built to Stick to the Stigma of a Plant of the Same Species

Fertilization in Flowering Plants Is Considered Double Fertilization

Take-home message 18.8  Pollination is necessary but not sufficient for achieving fertilization.  Following pollination, a pollen tube must grow down the style and into the ovule, where the sperm-producing cell within the pollen tube produces two sperm cells.

Take-home message 18.8  One of these cells fertilizes the egg cell in the embryo sac to form the embryo, and the other fuses with the diploid central cell of the embryo sac to form the endosperm, which will nourish the developing embryo.

18.9 Plants can avoid self- fertilization. Self-fertilization leads to less genetically varied offspring, offspring that are more likely to express one or more lethal or negative genes due to inbreeding.

How can plants avoid this bad outcome?

Take-home message 18.9  Plants can reduce the likelihood of self- fertilization in any of several ways: producing separate male and female flowers staggering the time of maturation of male and female reproductive parts producing separate male and female plants

18.10 Following fertilization, the ovule develops into a seed. In rapid cell division, developing embryos form:  root meristem  shoot meristem  one or two cotyledons

Seed and Fruit Formation  Ovule tissue surrounds all of this in a hard casing, forming a seed.  The ovary wall, surrounding the seeds, develops into a fruit.

Take-home message  Following fertilization, the ovule develops into a seed, containing a root meristem, a shoot meristem, and one or two cotyledons, surrounded by a hard casing.  The seed is protected within a fruit, which can aid in dispersal.

18.11 Fruits exist to help plants disperse their seeds. Just as with pollination, plants can also use the wind, water or animals to transport their seeds.

Wind-Dispersed Fruits and Seeds Characteristics of fruits and seeds aid in dispersal:  Hairy  Winged  Tiny, dust-like  Explosive

Water-Dispersed Fruits and Seeds

Animal-Dispersed Fruits and Seeds Animals take one of two approaches to disperse the seeds:  Carry  Consume

Take-home message  Following pollination and fertilization, plants utilize the assistance of animals, water or wind to disperse their fruits and seeds, depositing them at a new location where the seeds can germinate and new plants can grow.

18.12 How do seeds germinate and grow? The seed does not start to grow until the water, temperature, and oxygen conditions are good for germination or growth.

Some Seeds Have Coats That Require Extra “Processing” before They Can Germinate  Some seeds need an environmental trigger to start germination and growth.  This is a protection mechanism for the seed, and ultimately, the plant.

Seed Germination Is a Multistep Process

Take-home message  A seed, containing a new embryo and a supply of nutrients begins to grow only when the water, temperature, and oxygen conditions for life are just right.

Take-home message  Seeds sometimes must pass through an animal’s digestive system before they can germinate.  Initial growth utilizes fat and starch reserves stored in the endosperm and the embryo.

18.13 Plants grow differently from animals.  In animals, growth is determinate (growth comes to an end).  Plants show indeterminate growth.

Plants Grow Differently from Animals  Animals are made up of living cells.  Plants are made up of both dead and living cells.

Plants Grow Differently from Animals  Plants can lose relatively large structures without harming the organism.  In animals, critical appendages are meant to last a lifetime.

Plants Have Two Methods of Growth Primary growth allows plants to grow taller.

Plants Have Two Methods of Growth  Secondary growth allows plants to grow thicker and stronger.  Both growth processes are similar and involve regions called meristems.

There Are Two Types of Meristem Apical meristems are clusters of meristem cells at the ends of roots and shoots or branches that repeatedly divide to produce primary growth.

There Are Two Types of Meristem Lateral meristems, which are not present in all plant species, give rise to secondary plant growth.

Take-home message  Plants generally grow for their entire life, using two types of growth. Primary growth makes plants taller and plant parts longer and forms new tissues. Secondary growth makes plants thicker and sturdier.

18.14 Primary plant growth occurs at the apical meristems.

Take-home message  Plant growth occurs as a result of cell division in meristems, small collections of totipotent cells.

Take-home message  Primary growth—the lengthening of stems, branches, and roots, and the formation of new tissues such as buds and leaves—results from the division of apical meristem cells.

18.15 Secondary growth produces wood. Wood is amazing, not only because of its human uses, but, also, it allows plants to grow to mammoth heights.

How does secondary growth occur?

Wood Structure Woody plants include the following structures:  Pith  Primary xylem  Secondary xylem  Vascular cambium  Primary phloem  Secondary phloem

Cross-section of a Tree Looking at the color of a tree cross-section can tell you the amount of “heartwood” (clogged, structural support wood) and “sapwood” (recently created—better at conducting water).

Cross-section of a Tree This also can reveal the age of the tree— in the number of growth rings that are observed.  Quicker growth is observed in wider, lighter bands.  Slower growth is observed in darker bands.

Cork Cambium Provides Protection for the Tree  Bark tissue is replaced by cork cambium.  A layer of waxy cork cells is produced that protects the outer surface of the trunk from water loss, fire, and microbe infection.

Actions That Damage the Bark of the Tree’s Circumference Are Called “Girdling”  Relatively shallow areas to the trunk can destroy the secondary phloem and, if the damage goes deeper, the cells that give rise to new phloem.  This can cause the death of the tree.

Take-home message  Secondary growth results from cell divisions in a thin cylinder of tissue between the primary xylem and the primary phloem—the vascular cambium, a lateral meristem.

Take-home message  As this tissue divides, it produces a ring of non-living xylem cells, closer to the center of the trunk, that conduct water and minerals while also providing structural support to the plant.  We call these cells, collectively, wood.