BIOSCI 108: Lecture 15 - Gymnosperms (and some Angiosperms)

Seed Plants

The seed plants, which also has vascular tissue, are the gymnosperms and angiosperms. Their sporophyte phase is considered the dominant stage. While their gametophyte stage is essentially microscopic and entirely dependent on the sporophyte. They have seeds, pollen, and ovule.

There are several benefits from completely relying on the sporophyte:

• UV protection

• Water loss prevention

• Resistance to extreme temperatures

• Readily available nutrients and energy

The megasporangial tissue is incharge of making the female gametophytes. The microsporangial tissue is incharge of making the male gametophytes. They develop within the parental sporophyte.

Gymnosperms

Plants that produce ‘naked seeds, essetially cones. There are four divisions to gymnosperms: conifers, ginkgos, cycads, and gnetophytes.

There are female and male cones (sex organs) and a plant can produce both or either one of them.

Gymnosperm: Gametophyte Phase

The female gametophyte is produced within the female cone (2n).

• Meiosis occurs in the megasporangium tissue producing 4 haplood megaspores, only 1 of which will survive. It’ll undergo mitosis to develop into a female megagametophyte ovule. Each one produces archegonias that hold one egg cell each.

The male gametophute is produced within the male cone (2n).

• Meiosis occurs in the microsporangial tissue, producing 4 haploid microspores that undergoes mitosis to develop into pollen (a 3-celled structure that holds two sperm). This pollen is carried by the wind into the female cone.

These cones are modified leaves and the cones themselves are diploid.

Gymnosperm: Sporophyte Phase

Once a pollen grain lands inside a female cone, it has a tube cell that will make a tunnel to allow the sperm cell to access the egg cell. The sperm cell is not flagellated. Once fertilised, the embryo will continue developing into a seed.

The seed is then dispersed, typically through winds, and once it lands on suitable soils, it’ll germinate into a sporophyte.

Gymnosperm: Embryo Formation

Occurs within the female cone, the female cone will undergo changes as the seed develops.

The female cone will start openned, allowing pollen to enter. Once pollen enters, the cone will shut and development will continue (can take years).

The female gametophyte will start differentiating into cotyledons (seed leaves), shoot apical meristem tissue (future stem), root apical meristem (future roots), and a protective root cap.

Once fully developed and conditions are right, the cone will open again to release the seeds which are dispersed by wind.

Aotearoa’s Native Gymnosperms

We have four native conifer families in New Zealand. Cycads, ginkgos, and gnetophytes are all introduced. Some examples of our native conifers are:

• Kauri

• Tanekaha

• Totara

• Rimu

Kauri

Agathis australis, Araucariaceae, a large forest tree found in isolated pockets in the Coromandel Peninsula and north of Auckland. It does well in cultivation, preferring rich moist soil and a sheltered location.

Tanekaha

Phyllocladus trichomanoides, Phllycladacease, a fast growing medium-sized forest tree that tolerates sun and shade. Mature specimens don’t have true leaves but instead flatenned leaf-like branches/stems called phylloclads. This is because their ancient species had reduced leaves.

Totara

Podocarpus totara, Podocarpaceae, unlike other gymnosperms, have edible faux fruit-like cones (pseduo-fruit). Its female cones are reduced but have this red tissue filled with sugars attached to it. Birds eat this pseudo-fruit and disperses the seeds.

Rimu

Dacrydium cupressinum, Podorcarpaceae, also have pseudo fruit like totara. They have fine delicate leaves that aren’t individually effective at photosynthesis, but are collectively effective.

Angiosperm

Plants that make ‘encased seeds’, which means they bear flowers that fruit. They’re the most diverse group of plants. The flowers and plants can be hermaphrodites, producing both gametes, or one or the either.

Angiosperm: Flower Structure

Sepal, modified leaves that protects flowers when they’re buds and later petals.

Petal, modified leaves that help attract and guide pollinators.

Receptable, a thickened part of the stem from which the flower organ grows from.

It’s female parts, the carpel, where the female gametophytes are produced, include:

• Stigma

• Style

• Ovary, where the megasporangia is

It’s male parts, the stamen, where the male gametophytes are produced, include:

• Filament/Stalk

• Anther, where the microsporangia is

Angiosperms: Fruit

Once a flower’s ovary is fertilised, it’ll undergo modifications to change into a fruit that contrains seeds. Many of the flowers parts become new parts of the fruit.

• The previous peduncle becomes the pedicel (stem)

• The ovule turns into seeds

• The ovary turns into the core/endocarp

• The receptacle turns into the fleshy parts/mesocarp

Bryophytes vs. Pteridophytes vs. Gymnosperms

Dominant Generation:

• Bryophytes: Gametophyte

• Pteridophytes: Sporophyte

• Gymnosperms: Sporophyte

Sexual Reproduction:

• Bryophytes: Water

• Pteridophytes: Water

• Gymnosperms: Wind

Dispersal:

• Bryophytes: Spores

• Pteridophytes: Spores

• Gymnosperms: Seeds

Vascular Tissue:

• Bryophytes: No

• Pteridophytes: Yes

• Gymnosperms: Yes

Independent Gametophyte:

• Bryophytes: Yes

• Pteridophytes: Yes (reduced)

• Gymnosperms: No (micro)

Independent Sporophyte:

• Bryophytes: No (reduced)

• Pteridophytes: Yes

• Gymnosperms: Yes