Plants on Land

The green cushion of moss on a shady wall is made of cells with one set of chromosomes each, the same count as a human egg, yet it photosynthesizes, grows and lives for years. In your body, cells with one set exist only as eggs and sperm. Plants grow one multicellular body from haploid cells and another from diploid cells. Which body are you looking at in a moss, a fern, a pine and a flowering plant, and what changed as plant lineages spread over dry land?

Two bodies in one life cycle

A cell is haploid (n) when it carries one set of chromosomes and diploid (2n) when it carries two. Meiosis is the division that halves the chromosome number, from 2n to n; mitosis copies a cell without changing the number. The mechanics of each belong to meiosis in Genetics and mitosis in Cell Biology.

Alternation of generations is a life cycle with two multicellular stages, one haploid and one diploid, each giving rise to the other. In land plants it runs like this:

1. The diploid body, the sporophyte (“spore plant”), has cells that undergo meiosis. The products are haploid spores, single cells that grow into a new body without fusing.
2. A spore divides by mitosis into the haploid body, the gametophyte (“gamete plant”).
3. The gametophyte makes eggs and sperm by mitosis. Its cells are already haploid.
4. Sperm and egg fuse at fertilization, giving a diploid zygote, the single fertilized cell.
5. The zygote divides by mitosis into a new sporophyte.

Compare your own cycle. In humans, as in other mammals, meiosis makes eggs and sperm directly, and there is no haploid body. Many fungi sit at the other extreme: the zygote is the only diploid cell, and it undergoes meiosis soon after it forms (see microbes, protists and fungi). In land plants both bodies are multicellular.

To read any life-cycle diagram, find meiosis and fertilization first. They are the only two places the chromosome number changes. Every stage from meiosis to fertilization is n, every stage from fertilization to meiosis is 2n, and mitosis never changes the label. The commonest slip is to carry the animal rule across: plant meiosis makes spores, not gametes.

In the 1840s nobody had this scheme, and some botanists, Matthias Schleiden among them, held that a seed plant’s embryo formed from the tip of the pollen tube. Wilhelm Hofmeister tested the question with a microscope. He followed germinating spores, the plants they grew into, their flask-shaped female organs (archegonia), and the youngest embryos in mosses, liverworts, ferns, horsetails, clubmosses and conifers. If spore and seed plants shared no plan, the sequences should differ. Instead, in every group the embryo arose from an egg inside the female organ, and a spore-bearing generation alternated with a gamete-bearing one. The conifer seed fitted too. His 1851 comparison ruled in one alternation of generations across land plants, and his work on flowering plants from 1847 ruled out the pollen-tube origin of the embryo. He knew nothing of chromosome numbers; tying the two generations to n and 2n came decades later.

Moss, fern, pine, flower: which body do you see?

Moss. Take Physcomitrium patens (long called Physcomitrella patens) or the haircap moss Polytrichum. The leafy green shoot is the gametophyte (n). After a sperm swims through a film of rain or dew to an egg, the zygote grows into a sporophyte (2n): a thin stalk topped by a capsule. It stays attached, depends largely on the gametophyte for nutrients, and makes spores by meiosis in the capsule. A cap called the calyptra covers the young capsule, and it comes from the gametophyte, not the capsule. In P. patens, stomata occur on the capsule and not on the leafy shoot.

Fern. In a homosporous fern (one that makes a single kind of spore) such as Ceratopteris richardii, the frond is the sporophyte (2n). Spore cases (sporangia) under the fronds release haploid spores made by meiosis. A spore grows into a prothallus, a small, flat, often heart-shaped gametophyte (n) that lives on its own and photosynthesizes. Its sperm swim in a film of water to the egg. Ferns are vascular, with xylem and phloem, even though they have no seeds.

Pine. The tree is the sporophyte (2n), and its cones make two kinds of spores. In small pollen cones, meiosis makes microspores, each of which becomes a pollen grain: a male gametophyte of a few cells, not a sperm. In the ovules of larger seed cones, meiosis gives four megaspores; one survives and grows into the female gametophyte (n), which forms eggs. Wind carries pollen to the ovule, and a pollen tube delivers sperm to an egg without outside water. (In cycads and Ginkgo the pollen tube instead releases large swimming sperm into fluid inside the ovule.) A pine seed holds three kinds of tissue: the seed coat, from the parent sporophyte (2n); the food store, which is the female gametophyte (n); and the embryo, the new sporophyte (2n).

Flowering plant. In Arabidopsis thaliana, the green plant is the sporophyte. The male gametophyte is the pollen grain, and the female gametophyte is the embryo sac inside the ovule. A megaspore mother cell undergoes meiosis to give four megaspores; three degenerate, and the survivor goes through three rounds of nuclear mitosis, giving eight nuclei. Walls then form, giving seven cells and eight nuclei: an egg, two synergids beside it, three antipodal cells at the far end, and one large central cell with two polar nuclei. In Arabidopsis the two polar nuclei fuse before fertilization, so the central cell becomes diploid. One sperm fertilizes the egg, giving the 2n zygote; the other fuses with the central cell, giving triploid (3n) endosperm, a food tissue. Triploid endosperm holds in Arabidopsis and most flowering plants, not all; double fertilization returns in plant reproduction.

endosperm = central cell (n + n) + one sperm (n) = 3n

Worked problem. The numbers here are invented. Suppose a plant whose embryo sacs develop exactly as in Arabidopsis has 2n = 24 (real Arabidopsis has 2n = 10). Five ovules each have one megaspore mother cell, and all complete meiosis. Assume normal development and no losses beyond the three megaspores of each four that degenerate.

1. Megaspores. 5 mother cells x 4 megaspores = 20 megaspores.
2. Survivors. 5 x 3 = 15 degenerate, so 20 - 15 = 5 functional megaspores, one per embryo sac.
3. Nuclei. Three mitoses double the count three times: 2 x 2 x 2 = 8 nuclei per sac, and 5 x 8 = 40 nuclei.
4. Cells. 7 cells per sac, and 5 x 7 = 35 cells.
5. Chromosomes. Egg: n = 24 / 2 = 12. Zygote: 12 + 12 = 24. Central cell after polar fusion: 12 + 12 = 24. Primary endosperm nucleus: 24 + 12 = 36, which is 3n.

Check: 15 + 5 = 20, and 40 nuclei exceed 35 cells by 5, one extra nucleus per central cell.

Plant

Moss Homosporous fern Pine Flowering plant (Arabidopsis)

Test me: hide the n and 2n labels

Diploid number 2n =

24

Invented numbers for the sketch. Real Arabidopsis thaliana has 2n = 10.

In this sketch: a schematic cycle, not to scale; chromosome numbers are invented examples.

Keep the rule: find meiosis and fertilization first, because they are the only two places the chromosome number changes, and plant meiosis makes spores, not gametes. Moss, fern, pine, flower is a teaching order, not a line of descent.

Problems of land and the innovations that eased them

Land plants arose from green algae (see history of life). Water keeps an alga wet, holds it up and carries its sperm; air does none of these. Read the traits below as history and present effect, not intention: lineages carrying them left descendants on dry ground.

Problem on land	Trait that eased it	Seen in
Drying out	Cuticle, a waxy layer on the outer surface	moss, fern, pine, flowering plant
Gas exchange through a sealed surface	Stomata, pores bordered by guard cells	moss capsule, fern, pine, flowering plant
Transport and upright support	Lignified xylem with phloem	fern, pine, flowering plant
Fertilization without a water film	Pollen and the pollen tube	pine, flowering plant
Protecting and feeding the embryo	Seed	pine, flowering plant
Getting pollen and seeds moved	Flowers and fruit	flowering plants

A cuticle slows water loss; in Physcomitrella patens it limits drying. But a seal that keeps water in also slows carbon dioxide entry, and photosynthesis fixes carbon dioxide drawn from the air (the chemistry is in the Calvin cycle). Stomata are adjustable pores that regulate exchange of gases and water vapour. But wet-grown P. patens capsules typically have plugged stomata, so do not assume moss stomata work like a leaf’s.

Xylem is tissue that carries water and dissolved minerals up from the roots to the shoot; lignin, a tough polymer in its walls, also stiffens the plant for upright growth. Phloem carries sugars from where they are made or stored (sources) to where they are used (sinks), which can be up or down. Mosses lack lignified vascular tissue and stay low and close to moisture. Details are in plant transport.

Pollen carries the male gametophyte to the ovule, and its tube takes the sperm the last distance. A seed wraps an embryo in a food store and a coat, and dormancy lets germination wait. Flowers promote pollination, often by animals, and the ovary wall can develop into a fruit that protects seeds and aids dispersal.

Branches, not a staircase

Moss, fern, pine, flower is a teaching order, not a line of descent. Living mosses did not turn into ferns; each living group is the tip of a branch that has evolved as long as every other. Characters such as stomata are ancestral or derived; whole organisms are not (see reading phylogenetic trees).

The tree itself has shifted. In 2019 the One Thousand Plant Transcriptomes Initiative compared gene sequences from 1,124 species. Its analysis of nuclear gene trees recovered living mosses, liverworts and hornworts as one branch, and all its analyses rejected liverworts alone as the sister group to every other living land plant. A moss is a cousin of vascular plants, not a picture of their ancestor.

Nor is each body set simply by chromosome number. When KNOX2 regulatory genes were deleted in Physcomitrella patens, diploid embryos grew into gametophyte bodies without meiosis. Gene activity decides which body forms, a clue to Frederick Bower’s question of where the sporophyte came from.

Design problem

In 2017 Renault and colleagues deleted the gene PpCYP98, whose relatives in flowering plants act in lignin synthesis, in the moss Physcomitrella patens. Mosses are considered not to make lignin. Design the test of whether the gene is required for a normal cuticle. Say what you would compare and measure, what result would support a role, and what the test cannot tell you.

One way to do it

Compare wild type with several independent deletion lines, so that a stray second mutation in one line cannot explain the result; the paper used three. Measure surface permeability (dip gametophores in dilute toluidine blue and score dye uptake; the paper used 0.05 percent dye for two minutes), cuticle structure by transmission electron microscopy, and whether leafy gametophores form normally.

Then rescue. Grow mutants with caffeic acid, a compound made downstream of the missing enzyme (20 micromolar in the paper). If the pathway matters, mutants should take up more dye, show a disrupted cuticle and grow poorly, and supplementation should restore growth and the barrier at least partly. Renault and colleagues reported partial rescue.

Limits: dye uptake shows a leaky barrier but does not measure water loss, so add a weighed drying test for that claim. And a living moss’s cuticle is not the first land plants’; the result shows what this pathway does in this moss.