Green algal relatives and the origins of land plants

Living relatives are branches, not earlier stages

Land plants belong within the green-plant lineage. Within that larger lineage, they are streptophytes, together with several algal branches. Comparisons of many genes support Zygnematophyceae, a group that includes filamentous and single-celled algae, as the closest living algal lineage to land plants. 'Closest living relative' means that the lineages share a comparatively recent common ancestor. It does not mean that a living species of that group was the historical organism from which a modern moss developed.

The ancestry diagram contracts many branches so that a few relationships remain readable. Land plants include bryophyte lineages and vascular plants. Within vascular plants, lycophytes are separate from the branch containing ferns and seed plants. Living gymnosperms and flowering plants belong to the seed-plant branch. Every labelled tip represents descendants continuing to evolve; none is a rung that the next tip must climb.

Green colour, aquatic habitat and large body size are individually poor substitutes for an ancestry analysis. Similar functions can occur in distant groups, while close relatives can look different. A contracted tree also cannot settle every internal relationship or date a split. Here, equal branch lengths carry no time or species-number scale.

Water retention and gas entry create a tradeoff

Moving carbon dioxide into photosynthetic tissue and retaining water are different demands. A surface exposed to air can lose water while gases exchange. A cuticle is an extracellular surface barrier that reduces water loss in many land-plant tissues. A completely sealed photosynthetic surface, however, would restrict the carbon dioxide needed for carbon fixation. Surface structures and their regulation influence both processes.

In the leaf example, a pore between guard cells provides a route connecting air to internal gas spaces. Carbon dioxide enters while photosynthesis draws its internal concentration down; water vapour leaves under the stated dry-air condition. Narrowing that pore generally reduces both routes. The diagram compares directions, not measured rates, and it does not make water loss a form of carbon dioxide uptake.

This leaf model has a limited scope. Not every land-plant tissue has stomata, and an air pore is not automatically a guard-cell stoma. Water supply, light, carbon dioxide, humidity and living regulation interact. A structure that helps in one condition can constrain growth in another; adaptation is not a promise that a plant can flourish in every terrestrial habitat.

Reproduction also changes the challenge

Land plants are also called embryophytes. In their sexual cycles, a young multicellular embryo is retained and supported within maternal tissue. Protecting that early stage changes its surroundings and supply of resources. It is different from protecting a released spore, and it does not mean every embryo is enclosed in a seed.

Other features developed and diversified across plant branches: resistant spore walls, conducting tissues, pollen, seeds and flowers. These features did not appear together as one package in a single modern group. Some aspects of stress response predate the land-plant branch, and some modern algal relatives live on damp surfaces exposed to air. The water-to-land contrast is useful, but the ancestral environments and gene histories were more varied than a single leap from ocean to meadow.

To explain an innovation, ask two questions separately: where is the trait distributed on the ancestry tree, and what does it do in a specified condition? An embryo-retention trait helps distinguish a reproductive organisation; a reduced water-loss surface explains a physiological effect. Neither alone tells the complete history of the lineage.

Worked example

Check your understanding

1. Does a living Zygnematophyceae species have to be the ancestor of a living liverwort because the groups are close relatives?

   Show reasoning

   No. The claim concerns shared ancestry of two lineages. Their living species occupy descendant branches, and the tree does not identify the actual ancestral organism.

2. In the stated leaf condition, why can narrowing a gas-exchange pore reduce photosynthetic carbon supply as well as water loss?

   Show reasoning

   The pore provides a diffusion route for both carbon dioxide entering and water vapour leaving. Narrowing the route restricts both exchanges; their rates still depend on the conditions and gradients.

3. Is a retained embryo enough to call an organism a seed plant?

   Show reasoning

   No. Retained embryos occur across land plants, including bryophytes and seedless vascular plants. A seed involves additional structures and reproductive organisation, including an embryo within an ovule-derived covering.

What to carry forward

Trace shared ancestry and a trait's function separately; living plant groups are continuing branches with different combinations of features.