The Gynoecium - γυνή, gyne = woman and οἶκος, oikos = house - the house of women
- The gynoecium is composed of the carpels of a flower. The carpel is the basic unit of the gynoecium, equivalent to an ovule-bearing leaf.
- “Pistils” are made up of one or more carpels
- Pistils (whether composed of one or more carpels) typically have 3 parts, the stigma, style, and ovary. The style may be absent (e.g. tulips, poppies, in which the stigma sits directly atop the ovary).
- Stigma is special area for pollen reception/germination
- Ovary is chamber where ovules are borne.
- Style is structure connecting stigma and ovary, through which the pollen tubes grow.
The Carpel - ovule bearing leaf - the key to angiospermy
- A carpel is a hollow megasporophyll with ovules attached inside. The pollen (male gametophyte) germinates on the outside, grows into the carpel, and fertilizes the ovules.
- The megasporophylls of cycads and extinct seed ferns are leaf-like, with ovules attached on the margins. Pollen arrives directly at the ovule.
- Angiosperms have the ovules positioned on the inside of a leaf-derived ovary.
Ascidiate vs plicate carpels
Two theories as to how a carpel is produced have been proposed:
1. Plicate carpel - Folding of the carpels with ovules inside – if one looks at many eudicots, this seems to be a good analogy, e.g. pea-pod can be opened like a book
2. Ascidiate carpel - invagination of carpel to form a pitcher-like hollow structure with ovules inside.
How did they originate in the first place? By ascidiation, not by folding…that came later. The primitive carpel is a hollow sac (ascus = sac) with the stigmatic surface at the closed mouth
- Later the plicate carpel evolved, which resembles a folded leaf, by elongation of the terminal (mouth) portion of the primitive ascidiate carpel.
- The plicate carpel is well illustrated by a pea pod, which can be opened up to reveal the seeds inside.
Examples of ascidiate carpels occur in the majority of basal angiosperms:
Amborella (also has free carpels)
Waterlilies (e.g. Brasenia – also an example of a water lily with free carpels)
Some magnoliids (e.g. Persea, Lauraceae)
Ascidiate carpel of avocado (Persea) - sac below with overgrowth on abaxial side, mouth is asymmetric and elongated. Evolutionary directionality: the plicate carpel probably evolved from progressive elongation of the asymmetric mouth of the sac-like primitive carpel
How to turn a leaf into a sac?
Key concept – peltation - difference between a ladle and a goblet
Benefit of enclosing ovules: restrict the access of males to ovule, more control over access and fertilization, protection of ovules.
Fusion of carpels
- A simple pistil is made of one carpel. Flowers with one or more simple pistils are termed apocarpous.
- A compound pistil is made up of two or more fused carpels. It is called a syncarpous pistil (gynoecium and flower are referred to as syncarpous as well).
- The fusion of carpels into a compound pistil is connation (the fusion of similar parts).
- (1) Early stages of evolutionary fusion: carpels cohere, but do not structurally fuse. Each carpel has its own epidermis and vascular bundles.
- (2) Later stages of evolutionary fusion show the lateral carpel walls fused. Also, the ventral and lateral vascular bundles of adjacent carpels are close together in pairs, but not fused.
- (3) In the final stages of evolutionary fusion, the pairs of ventral and lateral vascular bundles of adjacent carpels are intimately fused
Clues to the number of carpels in a compound pistil:
- Number of styles, stigmas, or stigma lobes
- Number of locules
- Number of lobes or bulges on the ovary
- Number of placentae (if placentation is axile)
Poppy (Papaver) = Syncarpous ovary of 12 united carpels – look at capsule of a poppy, the number of stigmas reveals the number of carpels. In plants with a united style the number of stigma lobes is equal to the number of carpels.
Reduction or loss of gynoecium
- In many plants the number of pistils per flower is reduced to one. All compound pistils are solitary - if there are two or more pistils in a flower, then they are simple pistils.
- Loss of gynoecium. Some flowers have no pistils (they are imperfect). They may be male in dioecious, monoecious, or polygamous plants, or the flower may have neither gynoecium nor androecium, in which case the flower is sterile or neutral. It serves to attract pollinators to other, fertile flowers. Example: lacecap hydrangeas.