Why Ferns Need a Film of Water but Pine Trees Do Not

Mosses stay small because they have no tubes to carry water. Ferns, horsetails and pine trees solve that problem with xylem and phloem, so they can grow tall — and gymnosperms go further by making seeds, with fertilisation that no longer needs water.

This lesson covers the features and classes of pteridophytes, the fern life cycle with homospory and heterospory, the gymnosperm life cycle, and the uses of both groups.

Pteridophytes are seedless vascular plants with true roots, stems and leaves, a dominant diploid sporophyte and a small, free-living gametophyte; they are classified into Psilopsida, Lycopsida, Sphenopsida and Pteropsida.

General features:

- The plant body is a sporophyte with root, stem and leaves, and with xylem and phloem
- Leaves may be small microphylls, as in Selaginella, or large macrophylls, as in ferns
- Spores form in sporangia, often on leaf-like sporophylls; in some, sporophylls cluster into cones called strobili
- The gametophyte, called a prothallus, is small and green and needs damp, shady places
- Fertilisation needs water, since the male gametes swim

The four classes:

- Psilopsida — the simplest forms, without true roots and with tiny scale-like outgrowths; Psilotum
- Lycopsida — small leaves, often in strobili; Selaginella, Lycopodium
- Sphenopsida — jointed, ribbed stems with whorls of small leaves; Equisetum (horsetail)
- Pteropsida — large, divided leaves called fronds; Dryopteris, Pteris, Adiantum

An everyday example. The maidenhair fern grown in pots on shady balconies is Adiantum, a member of Pteropsida.

The substance. Pteridophytes still depend on water to reproduce — vascular tissue freed the sporophyte from wet ground, but not the gametophyte.

A fern alternates between a large diploid sporophyte that makes spores by meiosis and a tiny haploid prothallus that makes gametes; most pteridophytes are homosporous, producing one kind of spore, while a few are heterosporous, producing small microspores and large megaspores.

Fern life cycle:

- On the underside of fronds, clusters of sporangia called sori produce haploid spores by meiosis
- A spore germinates into a heart-shaped green prothallus, the gametophyte
- The prothallus bears antheridia and archegonia
- Male gametes swim through a film of water to the egg in the archegonium
- The diploid zygote grows into a new sporophyte, and the prothallus withers away

Homospory versus heterospory:

- Homospory — all spores are alike, and each grows into a gametophyte that usually bears both sex organs; most ferns and Lycopodium
- Heterospory — microspores grow into male gametophytes and megaspores into female gametophytes; Selaginella and Salvinia

An everyday example. The rows of brown dots under a fern frond on a damp wall are sori, each packed with spores.

The substance. Heterospory is a step towards the seed habit — when the megaspore stays on the parent plant and the embryo develops inside the female gametophyte, the arrangement begins to resemble a seed.

Gymnosperms are seed plants whose ovules are not enclosed in an ovary, so the seeds lie naked on scales; the sporophyte is dominant, the tiny gametophytes stay inside the cones, and a pollen tube carries male gametes to the egg without any need for water.

General features:

- Medium-sized to very tall trees and shrubs; the redwood Sequoia is one of the tallest tree species
- Generally taproots; Pinus roots have fungal mycorrhiza, and Cycas has coralloid roots with nitrogen-fixing cyanobacteria
- Needle-like leaves in conifers, with a thick cuticle and sunken stomata that reduce water loss; Cycas has large, pinnate leaves
- Heterosporous: microsporangia on male cones make pollen grains, and megasporangia, the ovules, sit on female cones

Life cycle, using Pinus:

- The male cone releases pollen grains, which are carried by wind
- A pollen grain reaches an ovule on a female cone and grows a pollen tube
- Inside the ovule, a megaspore mother cell divides by meiosis, and one megaspore develops into a multicellular female gametophyte with archegonia
- The pollen tube delivers male gametes to an archegonium, where fertilisation forms a diploid zygote
- The zygote develops into an embryo, and the ovule becomes a seed lying uncovered on the cone scale

An everyday example. Pine cones picked up on a walk in Shimla or Darjeeling are female cones whose woody scales once held naked seeds.

The substance. The female gametophyte of a gymnosperm never leaves the ovule — a pollen tube, not a swimming gamete, brings the partners together, which is why gymnosperms could spread into dry lands.

Pteridophytes are used as ornamentals, soil binders and biofertilisers, while gymnosperms supply timber, paper pulp, resins, oils, medicines and edible seeds.

Pteridophytes:

- Ornamental plants — many ferns are grown in gardens and homes
- Soil binders — ferns and horsetails help hold soil on slopes
- Biofertiliser — the water fern Azolla hosts the nitrogen-fixing cyanobacterium Anabaena and enriches rice fields

Gymnosperms:

- Timber — pine, deodar and fir are used for construction and furniture
- Paper pulp — soft conifer wood is pulped to make paper
- Resins and oils — turpentine comes from pine resin, and cedarwood oil from deodar
- Food — chilgoza pine nuts come from Pinus gerardiana
- Medicine — ephedrine, used to treat asthma, is obtained from Ephedra
- Ornamentals — Thuja, Araucaria and Cycas are planted in gardens

An everyday example. Farmers in many rice-growing parts of India add Azolla to flooded paddy fields because it adds nitrogen and reduces the need for chemical fertiliser.

The substance. Azolla is useful only because of its partner — the nitrogen is fixed by Anabaena living inside its leaves, not by the fern itself.