Gymnosperms

Seeds without a fruit

The word gymnosperm comes from two Greek words: gymnos, naked, and sperma, seed. In these plants the ovules are not enclosed by an ovary wall. They lie exposed, before fertilisation and after it. So the seeds that grow from them are naked: there is no fruit around them.

Familiar gymnosperms include Cycas, the sago palm of gardens, Pinus, the pine of hill stations, Cedrus, the deodar, and Ginkgo, the maidenhair tree. The giant redwood, Sequoia, is one of the tallest kinds of tree in the world.

The plant body

The main plant is the sporophyte, a diploid tree or shrub.

• Roots. Gymnosperms have tap roots. In Pinus, the roots live with fungi, an association called mycorrhiza. The fungi help the roots take in water and minerals. In Cycas, small special roots called coralloid roots hold cyanobacteria that fix nitrogen.
• Stems. The stem may be unbranched, as in Cycas, or branched, as in Pinus and Cedrus.
• Leaves. Leaves may be simple or compound. The large pinnate leaves of Cycas last for a few years. In conifers such as pines, the leaves are needles.

Needle leaves suit cold, dry and windy places. A needle has a small surface. It has a thick cuticle and sunken stomata. All three reduce the loss of water.

Cones and two kinds of spore

Gymnosperms are heterosporous: they make two kinds of spore, both haploid.

• Microsporophylls carry microsporangia, which make microspores. The microsporophylls are packed spirally on an axis to form a male cone, or male strobilus.
• Megasporophylls carry the ovules. Clusters of them form a female cone.

Each microspore develops into a pollen grain. A pollen grain is the male gametophyte, reduced to only a few cells. It develops inside the microsporangium.

In Pinus, the male and the female cones grow on the same tree. In Cycas, the male cones and the megasporophylls grow on different trees.

The female gametophyte inside the ovule

An ovule is a megasporangium, the nucellus, wrapped in protective layers. One cell of the nucellus becomes the megaspore mother cell. It divides by meiosis into four megaspores.

One megaspore grows into the female gametophyte. It is many-celled, and it carries two or more archegonia, the female sex organs. It stays inside the ovule.

So in gymnosperms, neither gametophyte lives on its own. Both develop and stay inside sporangia, on the sporophyte. This is a big change from mosses and ferns, whose gametophytes are free-living plants.

Pollination and fertilisation

Pollen grains are light. The wind carries them from the male cones to the opening of the ovules. A pollen grain sends out a pollen tube, which carries the male gametes towards the archegonia.

One male gamete fuses with the egg in an archegonium. The zygote grows into an embryo, and the ovule becomes a seed. The seed sits on the scale of the cone, with no fruit around it.

The food stored in a gymnosperm seed is the tissue of the female gametophyte. It formed before fertilisation, so it is haploid.

Chromosomes through a cycad's life

A cycad's leaf has two sets of chromosomes, $2n$. Its pollen grains, its female gametophyte and its eggs have one set, $n$. Change $n$ and watch which stages carry one set and which carry two.

Try it in Python

The program lists the parts of a cycad, with the number of chromosomes in each. Cycas has $2n = 22$.

Practise with new numbers

Common mistakes

• "Gymnosperms have no seeds." They do have seeds. What they lack is an ovary, so the seeds are not inside a fruit.
• "A pine cone is a fruit." A cone is a group of sporophylls. A fruit forms only from an ovary.
• Calling the food store of a pine seed endosperm made by triple fusion. Gymnosperms have no triple fusion. Their food store is the haploid female gametophyte.
• "The gametophytes of gymnosperms grow in the soil." They never live free. They stay inside the sporangia on the sporophyte.
• Mixing up the root partners. Fungi (mycorrhiza) help Pinus. Nitrogen-fixing cyanobacteria live in the coralloid roots of Cycas.

Remember these

• Gymnosperms have naked ovules and naked seeds, with no ovary and no fruit.
• The main plant is a diploid sporophyte with tap roots. Pinus has mycorrhiza, and Cycas has coralloid roots with cyanobacteria.
• Needle leaves, a thick cuticle and sunken stomata reduce water loss.
• They are heterosporous. Male cones make pollen grains; ovules on megasporophylls hold the female gametophyte with its archegonia.
• Pinus bears both kinds of cone on one tree; Cycas bears them on different trees.
• The wind carries pollen; a pollen tube brings the male gametes; the ovule becomes a naked seed.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why are gymnosperms called naked-seeded plants?

2. What helps the roots of a pine take in water and minerals?

3. Name three features of a conifer needle that save water.

4. A single cycad grows alone in a park, far from any other cycad. It never makes seeds. Give a likely reason.

5. Is the food tissue in a pine seed haploid or diploid? Explain.