Lecture 3 - Fertilization and Types of Gynoecia Flashcards
Fertilization in Conifers
Plants with imperefct flowers can be described as monoecious or dioecious.
In monoecious plants, staminate and carpelate flowers occur on the same plant (male + female).
In dioecious plants, we have separate male and female sporophytes.
In contrast to angiosperms, all conifers are monoecious.
The life cycle of Conifer
Female cones (ovulate cones) contain two ovules per scale.
One megaspore mother cell (megasporocyte) undergoes meiosis in each ovule.
Three of the four cells break down leaving only a single surviving cell that will develop into a female multicellular gametophyte.
It encloses archegonia. Upon fertilization, the diploid egg will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant.
Fertilization and seed development is a long process in pine trees: it may take up to two years after pollination.
The seed that is formed contains three generations of tissues: the seed coat that originates from the sporophyte tissue, the gametophyte that will provide nutrients, and the embryo itself.
Staminate cones
Microsporophylls are modified needles. Contained within each is a sporangium containing microspore mother cells (2N). Each one will undergo meiosis to produce 4 haploid spores. Each will develop into a pollen grain.
Ovulate cones
Females cones develop on shoots with modified needles called bracts. Within each bract is an axillary bud that produces the megasporophylls. The megasporophylls of each bud fuse to form the ovuliferous scale.
The bract and scale in most conifers are fused together.
How can something like this evolve?
In the spring…
The scale on the small immature female cones flexes open.
During the same few days, male cones release their pollen. The wind-blown pollen falls on the ovulate cones.
After a few days, the scales flex back downward, the opening is sealed off, and further pollination is prevented.
Each scale contains two ovules which will become seeds.
The pollen is drawn into the micropyle via the pollination drop.
The pollination drop is a sticky sucrose solution that rehydrates the male gametophyte -so they (gymnosperms) technically still require water for reproduction.
The micropyle is flanked by two long arms (integuments), each covered with pollination drops to which the pollen grain will adhere.
Following pollination, the pollen grain will remain dormant (within the pollen chamber) while the ovule matures (this can take 6 months to 1 year).
Eventually, the pollen tube digests its way through the nucellus to the egg.
Pines: A 3 year process
The followng example is for a pine tree
YEAR 1: Sexual buds are produced but remain enclosed within bud scales.
YEAR 2: SPRING
Maturation of male cones –> pollen grain
Dispersal by wind to female –> Pollination –> A period of dormanncy (~6 months)
YEAR 3: EARLY SEASON
Fertilization –> Egg + Sperm = Zygote
Seed Maturation: Ovule integuments form seed coat. Embryo has many cotyledons.
YEAR 3: LATE SEASON
Seed release
Cone scales usually open to release seeds in fall.
Some cones stay sealed until appropriate condition for germination occurs.
In angiosperms, upon pollination…
Pollen grain imbibe water and germinates within a few hours of landing on a conspecific stigma. Pollen tube protrudes out from aperture in exine and grows toward ovule(s) within the ovary. This breaking of dormancy mobilizes food reserves for the male gametophyte.
The stigma, style, sepals, and petals all wither, shrink and fall off.
The ovules and their contents swell and develop into seeds.
The ovary swells and becomes the fruit enclosing the seeds.
While the carpel might look simple in structure, internally it is very complex.
Gynoecium types: Monocarpous
made from one carpel
Gynoecium types: Apocarpous
made from more than one separate carpels
Gynoecium types: Syncarpous
Formed from more than one fused carpels.
Within each type, the # of ovules (thus seeds) also varies. Many plants have a single ovule per carpel.
At the opposite extreme: orchids have thousands of ovules per carpel.
Ovule Attachment to Ovary
Ovules are attached to the ovary via the funiculus. Vascular tissue runs through the funiculus.
Placentation refers to the arrangement of ovules in ovary.
Types of Placenta: Axile
(syncarpous gynoecium, ovules in center, divided by septa, multiple locules) In flowers with multiple carpels, ovules are contained within separate chambers called locules.
Types of Placenta: Parietal
Syncarpous, ovules attached to ovary wall. Single locule
Types of Placenta: Free Central
Syncarpous, one locule, ovules on short axis at base
