Plant Reproduction: Gymnosperms and Angiosperms
Reproduction in Seed Plants
- Cones (gymnosperms) and flowers (angiosperms) are specialized structures for sexual reproduction.
- These structures are vital for species continuation, analogous to the importance of roots, stems, and leaves for individual plant survival.
- Seed plants' life cycles involve alternating diploid sporophyte and haploid gametophyte generations.
- Gametophytes produce male and female gametes; their fusion forms a zygote, which develops into the sporophyte generation.
- In seed plants, the sporophyte generation is dominant, while the gametophyte generation is reduced and often hidden.
- The evolution of cones, flowers, and seeds enabled seed plants to reproduce without dependence on standing water.
- This adaptation allows seed plants to thrive in drier terrestrial environments, unlike mosses and ferns.
Life Cycle of Gymnosperms
- Gymnosperms like pine trees are diploid sporophytes grown from zygotes within seeds.
- Mature pine trees produce male and female cones.
- Male cones contain microsporangia that produce male gametophytes called pollen grains.
- Female cones contain megasporangia that produce female gametophytes which then produce ovules, where egg cells form.
- Wind carries pollen grains from male to female cones.
- Pollen grains landing near an ovule may be caught by a sticky secretion.
- The pollen grain splits open and grows a pollen tube containing two haploid sperm, should it land near an ovule.
- The pollen tube grows into the ovule, delivering the sperm.
- One sperm fertilizes the egg to form a zygote; the other sperm disintegrates.
- The zygote develops into an embryo within a seed, which also contains a food supply.
Life Cycle of Angiosperms
- Angiosperms (flowering plants) are the dominant plant life form on Earth.
- Their evolutionary success is attributed to their life cycle, which is independent from standing water for reproduction.
- Flowers are evidence of angiosperm success, ensuring plant survival and propagation.
Structure of a Flower
- A typical flower produces both male and female gametes, although variations exist.
- Some plants have separate male and female flowers on the same plant (e.g., corn).
- Others have male and female gametes on separate plants (e.g., willows).
- Flowers are modified miniature stems with four types of specialized leaves: sepals, petals, stamens, and carpels.
- These leaves are arranged in circles and modified for reproduction.
Sepals and Calyx
- The outermost circle consists of sepals, which are often green and leaf-like.
- Sepals enclose and protect the flower bud during development.
- All sepals together form the calyx.
Petals and Corolla
- The second circle consists of petals, which are often brightly colored.
- All petals together form the corolla.
- Brightly colored petals attract insects and other pollinators.
- Sepals and petals are considered sterile leaves because they do not produce gametophytes.
Stamens
- Fertile leaves inside the petals contain structures that produce male and female gametophytes.
- Stamens form the first circle of fertile leaves.
- Each stamen has a filament supporting an anther.
- Inside the anther are microsporangia that produce male gametophytes (microspores).
- Most angiosperm flowers have multiple stamens.
Carpels and Pistil
- Carpels form the centermost circle of flower parts, derived from rolled fertile leaves.
- Megasporangia, producing female gametophytes, are located inside these leaves.
- A flower may contain one or more carpels, either separate or fused.
- One or more carpels form the pistil, consisting of the ovary, style, and stigma.
- The ovary is the base, the style is a stalk, and the stigma is at the top of the style.
- The stigma receives pollen, often being sticky or having projections to catch it.
- For example, the style of a corn plant (corn silk) can be more than 30 centimeters long.
Female Gametophyte
- Ovules, containing megasporangia, are located inside each ovary.
- A diploid (2N) megaspore mother cell grows inside each ovule.
- The megaspore mother cell undergoes meiosis, producing four haploid (N) cells, three of which die.
- The remaining haploid cell divides mitotically to produce eight nuclei.
- These eight nuclei and their surrounding membrane form the embryo sac, the entire female gametophyte.
- The angiosperm female gametophyte is smaller and simpler than those of mosses and ferns.
- Inside the embryo sac, two nuclei become polar nuclei in the center, and three nuclei clump at each end.
- One of the three nuclei closest to the ovule opening enlarges to become the egg nucleus, flanked by two other nuclei.
- The three nuclei at the opposite end of the embryo sac die.
- The female gametophyte then contains a female gamete (egg nucleus) ready for fertilization.
Male Gametophyte
- The male gametophyte is even smaller than the female gametophyte.
- Microsporangia (pollen chambers) inside the anthers produce diploid (2N) microspore mother cells.
- Each microspore mother cell divides by meiosis to produce four haploid (N) microspores.
- Each microspore becomes a single pollen grain.
- The pollen grain wall thickens to protect the contents from dryness and damage.
- The pollen grain nucleus undergoes one mitotic division, producing two haploid nuclei: the tube nucleus and the generative nucleus.
- The tube nucleus disintegrates, and the generative nucleus divides to form two sperm cells.
- The pollen grain (male gametophyte) usually stops growing until deposited on a stigma.
- Eventually, the anther dries, its pollen chambers split, and mature pollen grains are released.
- Lily flowers demonstrate this process, with anthers ripening, splitting, and exposing mature pollen.
Pollination
- Pollination is the transfer of pollen from anther to stigma.
- Self-pollination occurs when pollen falls from the anther to the stigma of the same flower.
- Most plants cross-pollinate, transferring pollen from one flower to another on a different plant.
- Cross-pollination ensures seed formation with pollen from another plant.
- Sexual reproduction and cross-pollination increase genetic variation in offspring, enhancing survival and reproduction.
Fertilization
- Once a pollen grain lands on a suitable stigma, it grows a pollen tube.
- The generative nucleus divides and forms two sperm nuclei.
- The pollen tube contains a tube nucleus and two sperm nuclei.
- Following a chemical trail, the pollen tube grows down the style, reaches the ovary, and enters the ovule through a small hole.
- The sperm nuclei enter the female gametophyte (embryo sac), initiating double fertilization.
- Double fertilization occurs only in angiosperms.
- One sperm nucleus fuses with the egg nucleus to form the zygote.
- The other sperm nucleus fuses with the two polar nuclei, forming the triploid (3N) endosperm.
- The endosperm provides food for the embryo.
- The endosperm is a nutrient-rich food source for animals, including humans (e.g., corn, wheat, rice).
- Fertilization triggers rapid changes in the ovule, ovary, and other flower structures.
- The ovule parts toughen to form a seed coat, protecting the embryo and its food supply.
- The ovary wall thickens and joins with other parts of the flower stem to become the fruit that holds the seeds.
- A fertilized flower produces hormones that direct energy into developing fruits and seeds.
- Unfertilized flowers do not produce these hormones, causing them to wither and fall away.
- Seed development was a major factor in the success of angiosperms on land.
- Seeds nourish and protect delicate embryos.
- Angiosperm seeds have either one or two seed leaves called cotyledons.
- Cotyledons store food used when the seed germinates.
- Monocots (e.g., corn) have one cotyledon.
- Dicots (e.g., beans) have two cotyledons.
- The epicotyl is the stem length above the cotyledon(s) and develops into the plant's stem, with the apical meristem at its tip.
- The hypocotyl is the stem length below the cotyledon(s).
- The radicle at the base of the hypocotyl contains the root's apical meristem and becomes the primary root of the plant.
- In many plants, the endosperm is almost completely used up by the time the seed is mature, with food stored in large cotyledons (e.g., beans).
- In other plants, much endosperm remains in the mature seed (e.g., corn, coconuts).
- Coconut "milk" is liquid endosperm, and coconut "meat" is solid endosperm.
- In seeds with endosperm, the cotyledons resemble typical leaves.
Seed Coats and Seed Dispersal
- Seed coats can be thin and fragile or thick and woody.
- Thick seed coats protect seeds from dryness, saltwater, and other adverse conditions.
- Tough seed coats protect seeds from animal teeth and digestive chemicals when fruits are eaten.
- These seeds often germinate after being dispersed by animals along with digestive wastes, which provide natural fertilizer.
- Passing through an animal's digestive system disperses seeds away from the parent plant, reducing competition for resources.
- Animals distribute seeds to other areas that may provide suitable environments for survival.