Biology · Plant Diversity & Evolution

Seedless vascular plants and their diversity

A club moss has conducting tissues; a true moss lacks the characteristic tracheophyte vascular system. Their common names sound close, but their ancestry and visible generations differ. A cone-like cluster on a club moss can bear spores without containing any seeds.

The chapters on life's history and phylogeny explain shared ancestry and branching trees. Meiosis and inheritance supplies the distinction between one and two chromosome sets. Here, n and 2n are introduced again inside plant cycles. No botanical identification experience is assumed; the examples concern biological relationships and reproductive structures rather than cultivation.

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By the end, you can

Separate lycophyte and fern ancestry from the shared seedless condition and follow vascular transport and spore types without assuming seeds.

Seedless vascular plants span separate branches

Lycophytes include club mosses, spike mosses and quillworts. The fern lineage includes familiar frond-bearing ferns, horsetails and whisk ferns. Molecular relationships place ferns closer to seed plants than to lycophytes. The shared absence of seeds is therefore useful for a comparison, but 'seedless vascular plants' excludes a descendant branch of their common ancestry and is not a complete clade.

The contracted tree makes that exclusion visible. One vascular branch leads to lycophytes; the other separates into ferns and seed plants. Removing seed plants from the displayed vascular descendants gives the comparison group, but does not move the fern branch next to the lycophyte branch as a new exclusive sister pair.

Many familiar ferns have large fronds, yet not every fern has that appearance. Horsetails have jointed shoots, and whisk ferns have reduced body structures. Modern small lycophytes also do not represent every extinct member of their lineage. Body shape can help characterise organisms, while ancestry explains why a reduced structure need not signal an early ancestral position.

A useful comparison can exclude a descendant branch

A comparison boundary does not create a clade. Horsetails belong within the fern lineage in this usage.

Read the diagram in words

Lycophytes separate before the displayed fern–seed-plant split. The seedless vascular comparison contains lycophytes and ferns, while excluding seed plants descended from their common vascular ancestry.

A useful comparison can exclude a descendant branch
Description	Schematic horizontal coordinate	Schematic vertical coordinate
Vascular-plant ancestry	0	1.5
Lycophytes	3	0
Shared ancestry of ferns and seed plants	1.5	2.25
Ferns, including horsetails	3	1.5
Seed plants	3	3

A useful comparison can exclude a descendant branch
From	To	Process	Specified route through
Vascular-plant ancestry	Lycophytes	Ancestry branch	—
Vascular-plant ancestry	Shared ancestry of ferns and seed plants	Ancestry branch	—
Shared ancestry of ferns and seed plants	Ferns, including horsetails	Ancestry branch	—
Shared ancestry of ferns and seed plants	Seed plants	Ancestry branch	—

Seedless vascular comparison
Included in the comparison	Excluded from the comparison
Lycophytes; Ferns, including horsetails	Seed plants

Included in the comparison

Excluded from the comparison

No time scale

Contracted relationships of the displayed living groups, with no branch-length scale. The comparison boundary marks membership in the seedless group; it is not a new ancestral junction.

Conducting tissues connect separated organs

Xylem and phloem connect regions of a vascular plant over longer distances. In the simplified body model, xylem conducts water and mineral solutes from roots toward an evaporating shoot. Lignified walls of conducting cells also contribute mechanical support. The tissue includes different cell types; it is too broad to call every cell in all xylem dead.

Phloem distributes organic compounds, including sugars, from sources to sinks. A photosynthesising leaf can be a source when it exports sugar. A growing shoot or storage region can be a sink when it receives it. 'Source' and 'sink' describe the current roles in the transfer, not permanent upper and lower labels on the body.

The two assigned phloem routes therefore lead from one leaf to a growing shoot and to a root. They are alternative destination paths, not one liquid flowing both ways at the same point in the same tube. This chapter uses the routes to explain connected organs; the next chapter examines the physical mechanisms, changing sources and sinks, and whole-plant water relations.

One source can supply sinks in different directions

Sources and sinks can change. The example does not define phloem as always downward or xylem tissue as containing only dead cells.

Read the diagram in words

Water and mineral solutes move from the root to the leaf and growing shoot along the assigned xylem routes. Sugars move from the source leaf toward the growing shoot and root along separate phloem routes.

One source can supply sinks in different directions
Description	Ploidy
Root: water supply and sugar sink	—
Leaf: sugar source	—
Growing shoot: sugar sink	—

One source can supply sinks in different directions
From	To	Process	Conducting system
Root: water supply and sugar sink	Leaf: sugar source	Xylem: water and mineral solutes	Xylem
Root: water supply and sugar sink	Growing shoot: sugar sink	Xylem: water and mineral solutes	Xylem
Leaf: sugar source	Growing shoot: sugar sink	Phloem: sugars to a sink	Phloem
Leaf: sugar source	Root: water supply and sugar sink	Phloem: sugars to a sink	Phloem

Xylem

Phloem

Source exports sugars

Sink receives sugars

Assigned source and sinks in this interval

Schematic body, not a rate or distance scale

Qualitative transport in an assigned actively photosynthesising sporophyte. Xylem direction follows root water supply and shoot evaporation in this case. Two phloem arrows represent separate source-to-sink routes, not opposite simultaneous flow in one tube. Shape and line style distinguish systems independently of colour.

Two spore types are not yet a seed

In a typical familiar homosporous fern, one spore type develops into small gametophytes. Homosporous means one spore type, rather than genetically identical spores. Gametophyte sex and timing can vary; one spore type does not guarantee that every individual simultaneously forms both kinds of gamete.

Heterosporous plants produce microspores and megaspores. These develop into male and female gametophytes, respectively. Selaginella, a spike-moss lineage, provides a seedless example. Heterospory also occurs in water-fern lineages. It is not enough to identify a seed, because a seed combines an embryo with ovule-derived enclosing tissue and a supply of nourishment.

The parallel routes show spore production and gametophyte development without adding an embryo to either spore. Fertilization still links sperm and egg to a new sporophyte. In the displayed seedless sexual examples, motile sperm need liquid water to reach the egg. Vascular tissues support an extensive sporophyte body, but do not by themselves replace that fertilization route.

Spore type identifies a route before fertilization

The spore types are reproductive categories, not guarantees of genetic identity. No fertilized embryo is assigned to either spore.

Read the diagram in words

One spore type can give rise to gametophytes whose sex and timing vary. In the heterosporous route, microspores develop into male gametophytes and megaspores into female gametophytes. Both routes use meiosis before spore development.

Spore type identifies a route before fertilization
Description	Ploidy
Spore mother cells	2n
One spore type	n
Gametophytes: sex and timing can vary	n
Microspore mother cells	2n
Microspores	n
Male gametophytes	n
Megaspore mother cells	2n
Megaspores	n
Female gametophytes	n

Spore type identifies a route before fertilization
From	To	Process	Specified route through
Spore mother cells	One spore type	Meiosis	—
One spore type	Gametophytes: sex and timing can vary	Mitotic development	—
Microspore mother cells	Microspores	Meiosis	—
Microspores	Male gametophytes	Mitotic development	—
Megaspore mother cells	Megaspores	Meiosis	—
Megaspores	Female gametophytes	Mitotic development	—

Spore-to-gametophyte route
Description	Selected comparison
Spore mother cells	Homosporous comparison
One spore type	Homosporous comparison
Gametophytes: sex and timing can vary	Homosporous comparison
Microspore mother cells	Heterosporous comparison
Microspores	Heterosporous comparison
Male gametophytes	Heterosporous comparison
Megaspore mother cells	Heterosporous comparison
Megaspores	Heterosporous comparison
Female gametophytes	Heterosporous comparison

Homosporous comparison

Heterosporous comparison

Spore-to-gametophyte route

Two spore types do not alone make a seed

Selected seedless sexual routes. The homosporous lane permits gametophyte sex and timing to vary; it does not assign every gametophyte both gametes at once. The heterosporous lane is a Selaginella-type teaching comparison, not a seed or pollen model.

Worked example

Test what two spore types establish

An assigned plant produces small microspores that develop into male gametophytes and larger megaspores that develop into female gametophytes. The observation establishes heterospory in the case. It does not establish pollen, a seed, an ovary or a flower. Those structures require additional observations and definitions.

Now place two plants on the supplied ancestry tree: a horsetail on the fern branch and a club moss on the lycophyte branch. Both can belong to the seedless vascular comparison group. The horsetail still shares a more recent displayed common ancestor with a pine than with the club moss. A reproductive similarity and a sister relationship answer different questions.

Check your understanding

Why does the seedless vascular comparison group not form a complete clade in this tree?
Show reasoning
Its common vascular ancestry also has seed-plant descendants, which the group excludes. Ferns are closer to seed plants than to lycophytes in the supplied branching pattern.
A source leaf exports sugars to a growing shoot above it and a root below it. Is that inconsistent with phloem transport?
Show reasoning
No. Phloem transport connects sources to sinks; it is not defined as always downward. The model assigns separate routes to two sinks rather than opposing bulk flow within one tube.
Does a megaspore already contain a fertilized embryo because it is larger than a microspore?
Show reasoning
No. In the model it develops into a female gametophyte that produces an egg. The embryo follows fertilization. Spore size and reproductive role do not make the spore a seed.

What to carry forward

Vascular tissues, spore types and ancestry are separate dimensions; heterospory and a cone-like structure alone do not establish a seed.

References

Resources supporting this lesson’s explanations and examples.

A community-derived classification for extant lycophytes and ferns (2016)Pteridophyte Phylogeny Group IOriginal community classification proposal · PDFuvm.eduRead source
One thousand plant transcriptomes and the phylogenomics of green plants (2019)One Thousand Plant Transcriptomes InitiativeOriginal phylogenomic researchnature.comRead source
Biology 2e — Plant diversity and reproduction (2018)Mary Ann Clark, Matthew Douglas and Jung Choi · OpenStaxOpen textbook · selected sectionsEarly Plant Life Bryophytes Seedless Vascular Plants Evolution of Seed Plants Gymnosperms Angiosperms Pollination and Fertilizationopenstax.org
Salviniaceae — Ferns and Lycophytes (2015)Patrick J. Brownsey and Leon R. Perrie · Flora of New ZealandPrimary regional flora · indexed descriptionnzflora.infoRead source