The Ovule (also seen with Coniferophyta)

The Micropyle provides a specific locus for successful fertilization.

In other gametophytes the Archegonia may be located in a general region but nothing approaches the specificity of the Micropyle.

A Pollination Drop helps to capture Pollen and draw it inside the Micropyle.

Archegonia may secrete chemotactic agents to attract sperm but I am not aware of anything like the pollination drop in seedless plants like ferns.

The Stigmas of flowering plants can secrete a fluid which helps collect Pollen and also stimulates or blocks pollen germination.

The restricted location of Archegonia and especially the Embryo Sac (Anthophyta) helps to reduce multiple fertilizations. This is especially true for Angiosperms (Anthophyta) where only one egg may be present at the Micropyle.

Multiple fertilizations are still common in Gymnosperms like Conifers, however. This sets up a competitive situation in which several embryos compete for resources. Consequently, there will be a suboptimal level of nutrients for the dominant embryo.

Megagametophyte development is completely Endosporic.

This protects it at all phases of its life-span.

The Ovule has a vascular connection to the parent plant. Consequently, the Megagametophyte receives Nutrition from the Sporophyte during most of its existence.

The Integument eventually becomes the Seed Coat which Protects the Seed's contents and can enhance its Longevity. The latter is very significant because it increases the possibility that seeds will encounter favorable germination conditions over time. Otherwise, seeds could germinate regardless of environmental conditions. This could lead to excessive mortality and competition.

Seeds represent a tremendous investment of Energy on the part of the parent organism. Organisms which pump a lot of calories into dysfunctional propagules would not compete with more efficient organisms.

The Seed Coat may specifically control when germination occurs by responding to environmental cues like light, water and temperature.

The Seed Coat may provide a mechanism for seed dispersal. Some seeds have fleshy coats and can attract herbivores who eat the seeds and distribute them via their feces.

Most of the endemic plants of Hawaii arrived on birds who carried seeds in their digestive tracts and plumage. Pomegranate is a good example of this. The fruit is dry and brittle. The seeds have a fleshy outer integument which entices herbivores to dine.

Seeds of wiliwilw (Erythrina sandwichensis) have and
Magnolia have Red Fleshy Integument that attracts disseminating animals.

Cycads which are related to conifers have a Red fleshy Integument which attracts Iguanas. These are extinct in Florida and Puerto Rico but are alive and well in Cuba. Consequently, Cycad seeds are disseminated in Cuba but not in the other two locations.

Some Seeds have oil-rich structures called Eliasomes which are attached to the seed coat. These also attract herbivores especially ants.

Seeds of Podocarpus (related to Conifers) have a Colored Fleshy Aril just below them. The Aril is not part of the seed but herbivores dislodge the seeds and may eat them when they feed on the Aril.

Seeds may have hairs or barbs which attach to animals. Cotton is a good example. The seed coat produces extremely long hairs which adhere to animals.

Milkweed Seeds have long fluffy Trichomes which facilitate Aerial Dissemination.

  Some Seeds produce a mucilaginous coating which expands when hydrated and fosters aquatic dispersal. Hydrophilic mucilage may also prevent desiccation during germination.

In one case the mucilage places the seed in the correct orientation so that it is vertical with the root end facing the soil. How about that for room service.

  Some Seeds like those in the Coniferophyta produce wing-like projections which favor aerial distribution.

Seeds contain energy reserves in the Megagametophyte (Coniferophyta) or Endosperm (Anthophyta).

This promotes the rapid growth and survival of Germlings.

Cotyledons may also contain storage products which may augment or replace the Endosperm.

Animals have learned to eat seeds which are rich in storage
products. This leads to seed dispersal. In some cases, passage through an animal's digestive system is a prerequisite for successful germination. Oak Acorns (right) are a favorite food of squirrels.

Orchid Seeds are so small such that they have the aerodynamic properties of Pollen. This allows them to be disseminated passively in the air.

The Flower

Sepals and Petals protect immature reproductive organs.

They may also attract pollinators. Western Hummingbirds are attracted to red. I got buzzed frequently when I lived in Montana because I had a red back-pack. They are partial to Columbine flowers which have Nectaries in their "Spurs".

Some Perianth parts have pigmentation patterns which guide pollinators directly to reproductive structures. These are not always apparent with visible light. Some insects can perceive ultraviolet patterns that are invisible to humans.

An Extreme Example of Orchid Pollination

The reproductive organs of most orchids are adnate (fused) into a Column. Pollen is contained in masses which are interconnected and called Pollinia. The Stigma is usually located near the Pollinia but they are positioned so that self-fertilization is rare.

Pollinia stick to pollinating insects who transfer them to the next orchid they visit.

Orchid Flowers in the genus Ophrys probably have elaborate disguises which lure insects into mating with them. Their flowers resemble insect females.

Males are attracted to them and try to mate with them. In their frenzy to copulate with the female surrogate, they pick up specialized pollen sacks called Pollinia and transfer them to the next Ophrys they visit. One species mimics a small wasp while another resembles a bumble-bee queen.

Perianth parts often produce Nectar which attracts pollinators, or defending insects who repel marauders.

Plumbago produces large complex Trichomes which trap nectar robbers. This insures a ready supply of nectar for its pollinator.

Epidermal cells may also produce odors which attract Pollinators. Various types of scents may be produced by Osmophores.

Microgametophyte development is also Endosporic. The Pollen Wall is one of the hardest biological substances produced on earth. This is why Pollen is preserved in fossils and archeological sites. This protects the Pollen Grain during its transport to the ovule.

Pollen is rich in Protein and some animals like certain Bats are pollen eaters. This leads to cross-pollination.

Bees also collect pollen. The Pollen can often be seen on the hind legs of bees.

Some Pollen Grains have bladders or wings which help them
float on air currents.

The Pollen Grains of plants which depend on insects for pollination have sticky, waxy coatings.

The barbs on some pollen grains help them adhere to potential pollinators.

Orchids and members of the Milkweed family (Asclepiadaceae) produce specialized  Pollinia which adhere to pollinators and help insure cross pollination.

The Stigmas of most plants have Hairs which help to catch pollen.

The Stigmas of many plants secrete copious amounts to nutrient rich exudes which help to collect and stimulate the germination of compatible Pollen. This can include specific Recognition Factors which promote the germination of compatible pollen and inhibit the germination of alien pollen.

The Style may have specialized "Transmitting Tissue" which nourishes the pollen as it grows towards the Ovule. It can be rich in Pectins which can be degraded by the Pollen Tube. This provides an easy route to the Ovary.

Some Styles are Hollow. The Pollen Tubes may grow along the Secretary Epidermis or the space may be filled with a liquid which promotes the growth of the Pollen Tube.

The Fruit

The Ovary Wall (Pericarp) protects and nourishes the Ovules during their life-span.

Fruits with Fleshy Pericarps attract Herbivores who disseminate their seeds.

The Ovary of Pisonia  brunoniana (papala kepau) produces a sticky exude that presumably attached to animals who distributed the seeds. The exudate was used by Hawaiians to trap birds without damaging their plumage. This worked all too well and led to the extinction of some bird species.

In some cases the Fruits have wing-like projections which help them disseminate as they swirl and fall like helicopters.

Some Fruits contain plumes of hairs which assist their dispersal.

  
Inflorescence of Tragopogon. The individual dry fruits have a Parachute-like plume of hairs.

In some cases Hydrostatic Pressure develops in the Ovary that acutely leads to the forceful ejection of seeds. Certain cucumbers spit out their seeds. The most famous of these is the Squirting Cucumber.

Some Fruits create mechanical force as they mature and forcefully eject their seeds.

It should be clear that there are many positive adaptations for sexual reproduction associated with the Ovule and the rest of the Flower. It also helps to explain the success of Angiosperms compared to other taxa.

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