Gymnosperms

GymnospermFacts - The late tracheophytes are the Gymnosperms and Angiosperms. Gymnosperms and Angiosperms are the Seed Plants. - Gymnosperms bear cones and have naked seeds. The seeds are termed “naked” because they are not enclosed by fruit as in the angiosperms. Giant Sequoias - Seeds are exposed on modified leaves that form cones. - Gymnosperms are heterosporous. They have pollen and seeds and almost all gymnosperms are woody. - Gymnosperms depend on wind for pollination. - Gymnosperm Examples: pines, firs, redwoods, junipers and sequoia.

A Cladogram of the Gymnosperms. Division Coniferophyta Division Gnetophyta Division Ginkgophyta Division Cycadophyta

Gymnosperm Evolution Gymnosperms were the first seed plants to appear-millions of years before the first angiosperms. The conifers replaced many ferns as Earth's climate became more arid because conifers are better adapted for a dry environment. These modifications for a dry environment include needle-shaped leaves, which have a thick, protective cuticle and a relatively small surface area. Seed and pollen found in gymnosperms are key reproductive innovations for life on land (water is not necessary for sperm to reach egg like in the early tracheophytes).

Division Cycadophyta – The Cycads Cycads were very important in the Mesozoic Era. The were important for herbivorous dinosaurs. Today, they are not so diverse. They are palm-like. Sago Palm (Cycas revoluta) Zamia sp. with ovulate (seed) cones.

Division Ginkophyta – The Ginkgo Only 1 living species, the ginkgo tree. It tolerates pollution and is an urban street tree. Ginkgo is dioecious. Pollen trees are preferred. The fruit of seed trees smell really bad. So what would plant in your yard, a male or female tree? Ginkgo Tree (Ginkgo biloba) Also known as the madenhair tree.

Division Gnetophyta – Gnetum, Welwitchsia, Ephedra These gymnosperms have vessel elements like the flowering plants. Welwitschia mirabilis

Division Coniferophyta (Pinophyta) – The Conifers “Cone-bearers” - Many conifers have woody cones. Pine is an example of this. - Some lack woody cones and have a berry-like tissue around seeds. Eight families are found in this division. The pine family (Pinaceae) includes the pines, fir and spruce. Fascicles of needles born or short shoots from a pine (Pinus sp.).

Gymnosperm Reproduction Each individual tree generally has both cone types (male and female). Therefore these trees are monoecious. Small pollen cones produce microspores that develop into male gametophytes or pollen grains. Larger ovulate cones usually develop on separate branches of the tree and produce megaspores that develop into female gametophytes. Strobilus (plural, strobili) – a number of modified leaves (sporophylls) or ovule bearing or pollen bearing scales grouped together on an axis; papery; pollen is produced here; part of pollen cone. Pollen grain is the immature male gametophyte. Ovulate cone (female) – when young it is softer and smaller than the male strobilus. Pollination replaces the need for free water, which is necessary in early tracheophyte reproduction.

Mature Ovulate (female) cones of Shasta red fir Mature Ovulate (female) cones of Shasta red fir. Cones in this genus are borne upright. Note the long bracts projecting beyond the cone scales.

Douglas Fir (Pseudostuga menziesii) with soft needles and cone.

Pollen (Male) Strobili A cluster of strobili near the end of a branch at the time of pollen release. A longitudinal section of one strobilus, showing microsporangia attached to the underside of each scale. Each microsporangium is filled with pollen grains; the pollen grains consist of only 2 cells; each with its own nucleus.

Ovulate (Female) Cones at Time of Pollination 2- Month-Old cones at the tip of a young branch. Below are large cones just older than 1 year. Longitudinal section through a 2-month-old cone, showing ovules on top of each scale. The mature open cone is 2 years old when seeds are mature. A closer view of one ovule (dashed area). Note the large megasporocyte inside the nucellus and the micropyle and micropylar chamber. Meiosis has not yet occurred.

Life Cycle of Pine A Representative Gymnosperm. A young ovulate (seed) cone. A scale from an ovulate cone with 2 ovules on its upper surface. Longitudinal section of an ovulate cone at the time of pollination. A pollen strobilus One scale from a pollen strobilus A section of a microsporangium with mature pollen grains inside. (g-i) A developing male gametophyte. (c,j,k) A developing female gametophyte. (l-n) A developing seed. (o) Germination of the seed.

Ecological and Economic Importance of Gymnosperms The gymnosperm landscape covers the 1/3 of the landmass of North America. The presence of gymnosperms can cause acidic or nutrient poor soil by presence of decaying pine needles. The harvesting of ponderosa pine, eastern white pine and Douglas fir is important to the forestry economy.

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