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.

Formation of Seeds

• 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.