Seed Plants: Gymnosperms

Core Concepts

The seed: a new way to protect the next generation

The seed is the defining breakthrough of this group. A seed consists of three parts: a diploid embryo (the young sporophyte), a supply of stored food (tissue that nourishes the embryo during germination), and a protective seed coat derived from the parent plant's tissues. Compare this with the spores of the seedless plants in the previous topic. A spore is a single cell that must land in a moist spot and fend for itself; a seed is a packaged, dormant embryo that can survive drought, cold, and time, then germinate when conditions improve. Seeds can also be dispersed far from the parent by wind or animals, which reduces competition between parent and offspring.

Pollen: sperm that no longer needs water

In seedless plants, a swimming sperm must reach the egg through a film of water. Gymnosperms solved this problem with pollen: a microscopic male gametophyte enclosed in a tough wall. Wind carries pollen grains to the female structure, where the pollen tube grows to deliver the sperm to the egg. Fertilization therefore no longer depends on standing water, which is a huge advantage on dry land. This is why gymnosperms are sometimes described as "wind-pollinated" — they release enormous clouds of pollen, relying on chance encounters with a female cone.

The cone (strobilus): the reproductive structure

Most gymnosperms produce cones, which are clusters of scale-like leaves specialized for reproduction. Male cones are small and produce pollen in microsporangia. Female cones are larger and bear ovules on the surface of their scales — this exposed position is exactly why the seeds are "naked." In pines, a female cone takes roughly two years from pollination to mature seed. Many conifers have separate male and female cones on the same tree (monoecious), while some species, like ginkgo, have separate male and female trees (dioecious).

Adaptations of conifer leaves

Conifer leaves are typically needles or scales: narrow, thick, and covered with a waxy cuticle, with stomata sunk into pits to reduce water loss. These features let evergreens keep their leaves through winter and photosynthesize on mild days when deciduous trees are bare. Needle shape also sheds snow, and the dark green color helps absorb heat. The tracheids that transport water in conifer wood are efficient enough to raise water hundreds of feet into a redwood's crown, although the same cells do double duty in support.

The four living phyla

• Coniferophyta (conifers): the largest group — pines, spruces, firs, cedars, redwoods, and junipers. Woody trees and shrubs with needle or scale leaves; the source of most timber.
• Cycadophyta (cycads): tropical and subtropical plants with a stout trunk and palm-like leaves. They bear large cones and, unusually, have swimming sperm that travel through a pollen tube.
• Ginkgophyta (ginkgo): one living species, Ginkgo biloba, a deciduous tree with fan-shaped leaves and foul-smelling seeds. Famous as a living fossil and a tough urban street tree.
• Gnetophyta (gnetophytes): three very different genera — Ephedra (shrubs of dry regions), Gnetum (tropical vines), and Welwitschia (a two-leaved desert plant of Namibia). Molecular evidence places them among the gymnosperms, though their relationships are still debated.

Life cycle: alternation of generations

Like all land plants, gymnosperms show alternation of generations: a diploid sporophyte generation alternates with a tiny, dependent haploid gametophyte. The pine tree you see is the sporophyte. Its cones produce spores by meiosis; spores grow into microscopic gametophytes — a pollen grain (male) or the tissue inside an ovule (female). Fertilization produces a zygote, which develops into the embryo inside the seed. The gametophyte generation is so reduced that it lives entirely inside the sporophyte's cones.

How It Works / Step-by-Step Process: the pine life cycle

1. A mature pine tree (sporophyte, 2n) produces male cones (pollen) and female cones (ovules).
2. Wind carries pollen grains to a young female cone; a pollen grain lands near an ovule and grows a pollen tube.
3. Inside the ovule, meiosis produces the female gametophyte with an egg cell.
4. The pollen tube delivers sperm to the egg (months later, in pines); fertilization forms a diploid zygote.
5. The zygote develops into an embryo; the ovule matures into a seed with stored food and a seed coat.
6. The female cone scales open when dry; winged seeds fall and blow away.
7. A seed germinates in suitable soil, and the embryo grows into a new sporophyte tree.

Common Confusions