The Evolution of Land Plants
• The first land plants likely evolved in
estuaries from green algae (“sea weeds”).
• They had no true vasculature (xylem), no
seeds, and no true leaves or roots.
• The earliest plant fossils are Ordovician in
age and resemble modern liverworts.
• More modern groups of plants were well
established by the Devonian, but differed
considerably from today’s flowering and
fruiting plants.
Major Plant Groups
(There is considerable current disagreement about
plant taxonomy, but these are the basics)
• Bryophytes: Mosses and liverworts. Low
growing non-vascular seedless sporebearing plants. Many have symbiotic
relationships with internal fungal cells.
• Ferns, horsetails, lycopods, etc. A
polyphyletic set of seedless vascular
plants that arose in Silurian and Devonian
times.
Mosses and
Liverworts
“Bryophytes”
Ferns,
Horsetails,
Lycopods
Major Plant Groups Continued
• Gymnosperms: Large spore-bearing and
seed-bearing vascular plants and trees.
Includes modern conifers, ginkgos, and
cycads. Primitive forms first seen in
Devonian.
• Angiosperms: Flowering and fruiting plants
that arose in the Cretaceous. These
constitute the majority of modern plant
species, and include grasses, trees, etc.
Gymnosperms
Angiosperms
Plant Vasculature
• A major development in plant evolution was
xylem and (later) phloem tubes that conduct
fluids and nutrients in the plant.
• Xylem: tubes of dead conducting cells that carry
water from the ground to the leaves. Older xylem
layers (secondary xylem) is reinforced with
lignin, and helps support much larger plants –
this is wood.
• Phloem: specialized tubes of living cells that
transport carbohydrates in all directions around
inside the plant. Plant regions not producing
nutrients by photosynthesis are fed by phloem.
Early Devonian
Fossil Plants
• By the Devonian,
plants like
Cooksonia and
Rhynia have xylem.
• Some reached 1m.
• Still localized to
very wet areas
(sperm problem).
• Among these were
the first small
lycopod plants.
Silurian Plant Fossils
• Aglaophyton is a
common Late Silurian
Plant fossil, and is
typical of the period.
• It has a ‘conducting
strand’ of cells instead
of xylem.
• In place of leaves, it
has green stems.
• In place of seeds,
spores on a
sporangium.
• Asexual reproduction
by rhizomes.
Devonian
Trees
• Secondary xylem
(wood), true roots,
and the first leaves
allowed for the first
‘trees’ to appear in
the later Devonian.
• Also, one of the first
trees, Archaeopteris
produced the first
seeds (thus, the first
gymnosperm).
Devonian Floras
• As you can see, there was considerable plant diversity
by the late Devonian.
• Ecosystems were dominated by lycopods, ferns of
several heights, and the first seed trees. Thus, a
‘modern-looking’ forest with canopy and understory
plants was present. The first coal beds are Devonian.