Plant reproductive morphology

Christian Konrad Sprengel (1793) studied the reproduction of flowering plants and for the first time it was understood that the pollination process involved both biotic and abiotic interactions.

Each male gametophyte typically consists of two to four cells enclosed within the protective wall of a pollen grain.

A perfect flower, like that of Ranunculus glaberrimus shown in the figure, has a calyx of outer sepals and a corolla of inner petals and both male and female sex organs.

Next inwards there are numerous stamens, which produce pollen grains, each containing a microscopic male gametophyte.

If separate staminate and carpellate flowers are always found on the same plant, the species is described as monoecious.

If separate staminate and carpellate flowers are always found on different plants, the species is described as dioecious.

[7] Members of the birch family (Betulaceae) are examples of monoecious plants with unisexual flowers.

Amborella represents the first known group of flowering plants to separate from their common ancestor.

[11] A species such as Fraxinus excelsior, the common ash of Europe, demonstrates one possible kind of variation.

[8] The Asteraceae (sunflower family), with close to 22,000 species worldwide, have highly modified inflorescences made up of flowers (florets) collected together into tightly packed heads.

These include plants that reproduce vegetatively by runners or bulbils, or which produce seeds without embryo fertilization (apomixis).

The selective advantage of outcrossing appears to be the masking of deleterious recessive mutations.

[citation needed] Dioecy, the condition of having unisexual flowers on different plants, necessarily results in outcrossing, and probably evolved for this purpose.

However, "dioecy has proven difficult to explain simply as an outbreeding mechanism in plants that lack self-incompatibility".

[7] Resource-allocation constraints may be important in the evolution of dioecy, for example, with wind-pollination, separate male flowers arranged in a catkin that vibrates in the wind may provide better pollen dispersal.

[7] In climbing plants, rapid upward growth may be essential, and resource allocation to fruit production may be incompatible with rapid growth, thus giving an advantage to delayed production of female flowers.

Close-up of a Schlumbergera flower, showing part of the gynoecium (specifically the stigma and part of the style ) and the stamens that surround it
Dioicous gametophytes of the liverwort Marchantia polymorpha . In this species, gametes are produced on different plants on umbrella-shaped gametophores with different morphologies. The radiating arms of female gametophores (left) protect archegonia that produce eggs. Male gametophores (right) are topped with antheridia that produce sperm.
Ranunculus glaberrimus flower
Alnus serrulata has unisexual flowers and is monoecious . A pair of maturing male-flower catkins are on the right.
Ilex aquifolium has unisexual flowers and is dioecious : (above and top right) a 'shoot' with flowers from a male plant, showing robust stamens with pollen , and a female-flower stigma , reduced and sterile; and (below and bottom right) a shoot with flowers from a female plant, showing a robust stigma and male-flower stamens ( staminodes ), reduced, sterile, with no pollen.
The basic cases of sexuality of flowering plants.