1. Plant Reproduction and Development
Chapter 38
Plant Reproduction and Development

2. Alternation of Generations
Angiosperms and other plants exhibit alternation of generations: haploid (n) and diploid (2n) generations take turns producing each other
Sporophyte: diploid plant that produces haploid spores by meiosis
Gametophyte: haploid plant that produces gametes
Reproduction and Development

3. Alternation of Generations
Fertilization results in diploid zygotes, which divide by mitosis and form new sporophytes
Sporophyte dominant in angiosperms
Evolutionary history has reduced gametophytes in angiosperms to only a few cells, not an entire plant
Reproduction and Development

4. Reproduction and Development

5. Reproduction and Development
Flowers
Angiosperm sporophytes produce unique reproductive structures called flowers
Flowers consist of four types of highly modified leaves
Sepals
Petals
Stamen
Pistil (or carpel)
Their site of attachment to the stem is the receptacle
Reproduction and Development

6. Reproduction and Development
Flower Structure
Reproduction and Development

7. Reproduction and Development
Flower Anatomy
Sepals and petals are nonreproductive organs
Sepals – protect the other three, the floral bud
Petals – attract pollinators and act as “landing pads”
Reproduction and Development

8. Reproduction and Development
Flower Anatomy
Stamen and carpels are male and female reproductive organs, respectively
Stamen – consists of filament (long, thin) and anther (pollen)
Carpel – consists of stigma (sticky opening), style (long tube connecting stigma to ovary), ovary (houses ovules; becomes fruit), and ovules (develops female gametes; become seeds)
Reproduction and Development

9. Reproduction and Development
Flower Anatomy
Complete flowers – have all four floral organs
Ex: Trillium
Incomplete flowers – missing one or more of the four floral organs
Reproduction and Development

10. Reproduction and Development
Flower Anatomy
Bisexual flower (perfect flower) is equipped with both stamens and carpals
All complete and many incomplete flowers are bisexual
A unisexual flower is missing either stamens (carpellate flower) or carpels (staminate flower)
Reproduction and Development

11. Reproduction and Development
Unisexual Flowers
Monoecious plants: staminate and carpellate flowers at separate locations on the same individual plant
Ex: corn ears derived from clusters of carpellate flowers; tassels consist of staminate flowers
Reproduction and Development

12. Reproduction and Development
Unisexual Flowers
Dioecious plants: staminate and carpellate flowers on separate plants
Ex: Date palms and Sagittaria (below) have carpellate individuals that produce dates and staminate individuals that produce pollen
Reproduction and Development

13. Reproduction and Development
Gamete Formation
Development of angiosperm gametophytes involves meiosis and mitosis
Reproduction and Development

14. Reproduction and Development
Gamete Formation
The male gametophytes are sperm-producing structures called pollen grains, which form within the pollen sacs of anthers
The female gametophytes are egg-producing structures called embryo sacs, which form within the ovules in ovaries
Reproduction and Development

15. Reproduction and Development
Male Gamete Formation
The male gametophyte begins development within the sporangia (pollen sacs) of the anther
Within the sporangia are microsporocytes, each of which will from four haploid microspores through meiosis
Each microspore can eventually give rise to a haploid male gametophyte
Reproduction and Development

16. Reproduction and Development
Male Gamete Formation
A microspore divides once by mitosis and produces a generative cell and a tube cell
Generative cell will eventually form sperm
Tube cell, enclosing the generative cell, produces the pollen tube; delivers sperm to egg
Reproduction and Development

17. Reproduction and Development
Male Gamete Formation
This two-celled structure (generative and tube cells) is encased in a thick, ornate, distinctive, and resistant wall: a pollen grain; an immature male gametophyte
Reproduction and Development

18. Female Gamete Formation
Ovules, each containing a single sporangium, form within the chambers of the ovary
One cell in the sporangium of each ovule, the megasporocyte, grows and then goes through meiosis, producing four haploid megaspores
In many angiosperms, only one megaspore survives
Reproduction and Development

19. Female Gamete Formation
This megaspore divides by mitosis three times, resulting in one cell with eight haploid nuclei
Membranes partition this mass into a multicellular female gametophyte – the egg sac
Reproduction and Development

20. Female Gamete Formation
At one end of the egg sac, two synergid cells flank the egg cell
Synergids attract and guide the pollen tube formation
At the other end of the egg sac are three antipodal cells – no idea what they do
Reproduction and Development

21. Female Gamete Formation
The other two nuclei, the polar nuclei, share the cytoplasm of the large central cell of the embryo sac
The ovule now consists of the embryo sac and the surrounding integuments (from the sporophyte)
Reproduction and Development

22. Angiosperm Pollination
The successful transfer of pollen from anther to stigma
NOT fertilization: fusion of gametes
Pollination leads to fertilization
Cross-pollination vs. self-pollination
Most angiosperms are pollinated by insects, birds, and mammals (vectors) that reward the species with food in the form of nectar
Some are pollinated by wind (corn, wheat) and have small, plain, non-fragrant flowers
Reproduction and Development

23. Angiosperm Pollination
Fragrance, pattern, and colors are designed to attract the vector so it will pick up pollen and bring it to the next flower
Some vectors get “tricked”
Orchid flowers resemble female wasps; males attempt copulation; the more orchids the wasps “mate” with, the more pollination occurs
Good example of coevolution
Reproduction and Development

24. Reproduction and Development

25. Reproduction and Development

26. Reproduction and Development

27. Reproduction and Development
Animal Pollinators
The Scottish broom flower has a tripping mechanism that arches the stamens over the bee and dusts it with pollen, some of which will rub off onto the stigma of the next flower the bee visits
Reproduction and Development

28. Reproduction and Development
Double Pollination
After pollen grain lands on stigma, the generative cell divides by mitosis into two haploid sperm cells
1 sperm fertilizes egg; forms the zygote (2n)
1 sperm fertilizes polar nuclei; forms endosperm (3n)
Reproduction and Development

29. Reproduction and Development
Double Pollination
Double fertilization ensures that the endosperm will develop only in ovules where the egg has been fertilized.
This prevents angiosperms from squandering nutrients in eggs that lack an embryo
Reproduction and Development

30. Reproduction and Development
Seeds
After double fertilization, the embryo develops to a point and then enters a dormancy period
During this time, the embryo is housed in a tough, protective coating – seed coat
It will remain as the seed until germination, usually brought about by the absorption of water
Seeds allow parent plants to disperse offspring and wait until environmental conditions are favorable for growth
Reproduction and Development

31. Reproduction and Development
Seeds
In bean seeds (dicot), the embryo consists of an long structure, the embryonic axis, attached to cotyledons
Below the point at which the cotyledons are attached, the embryonic axis is called the hypocotyl; above it is the epicotyl
Tip of the epicotyl is the plumule:shoot tip with a pair of mini leaves
End of the hypocotyl is the radicle, or embryonic root
Reproduction and Development

32. Reproduction and Development
Seeds
Monocots have a single cotyledon called a scutellum
Embryo of a grass seed is enclosed by two sheaths, a coleorhiza, which covers the young root, and a coleoptile, which cover the young shoot
Reproduction and Development

33. Reproduction and Development
Fruits
Develop due to hormonal changes after fertilization
Usually develop only after fertilization
Designed to protect the seeds and aid in seed dispersal by wind or animals
Reproduction and Development

34. Reproduction and Development
Fruits
Fruits are simply any structure related to or resulting from the ovary of a flower (Yes! That includes many of the common “vegetables”)
Reproduction and Development

35. Reproduction and Development
Seed Dispersal
Fruits aid in seed dispersal based on how the fruits develop
Lightweight fruits allow wind dispersal
Dandelions and Maples
Reproduction and Development

36. Reproduction and Development
Seed Dispersal
Floating fruits allow water dispersal
Coconuts
Reproduction and Development

37. Reproduction and Development
Seed Dispersal
Clingy fruits allow animal dispersal
Fruits “grab” the animal (cockleburs, “jumping” cholla)
Reproduction and Development

38. Reproduction and Development
Seed Dispersal
Tasty fruits allow animal dispersal
Fruits entice the animal to eat it (mistletoe and birds)
Animals eat the fruit and deposit the seeds (in a nice pile of fertilizer) in new places
Why are unripe fruits bitter?
Reproduction and Development

39. Reproduction and Development
Seed Dispersal
Explosive seed pods allow dispersal by the plant itself
Impatients – get their name from their behavior
Reproduction and Development