TRANSITION OF PLANTS TO LAND
Dr. Paul F. Ciesielski
University of Florida
Note: Check out the linked web sites
For most of Earth history (90%), the land has been virtually
void of all plant life!
I. BARREN LANDSCAPES
Our solar system is 4.6 billion years old. Within our vast universe
there are millions of older solar systems, many of them probably with planets
and life. It is entirely possible that more intelligent life forms may
exist elsewhere. If so, what may have they seen as they passed Mars and
If our visitors had passed by Mars early enough, they may have seen
a planet with a rocky landscape with some active volcanoes and a considerable
quantity of surface water. Over the land surface, barren rocky soil totally
devoid of life.
Let's say our visitors came by recently, oh.. ...some 480 million years
ago (90.6% of the time from Earth formation to today). Our visitors would
first be struck by the vast watery expanse of our oceans, similar in area
and volume as today. Different would have been the shape of the oceans
and distribution of continents.
As our visitors moved closer, for a better view of the continents, they
would have observed the same variety of topography as today, vast flat
plains to high snow covered mountain ranges. Yet to these explorers, Earth's
land surfaces would have been more like Mars, where they had just visited.
WHY? Because there was no visible life, no green, no plants, except for
perhaps some very limited cyanobacteria.
So when did plant life first inhabit the continents? That is the subject
we now explore. First, plants had to solve the problems of desiccation
(drying), support, and the effects of gravity. Land plants probably evolved
from marine plants, moved into freshwater and finally onto land. This probable
transition is from marine green algae to simple bryophyte type plants to
Marine Algae ---------> Bryophyta -----------> Tracheophyta (green) "amphibious
vascular plants" plants
No fossil evidence of transition from marine algae to non-vascular plants
A. Non-vascular Land Plant Characteristics:
Lack specialized cells for the movement of nutrients
They are small and limited mostly to low, moist areas.
B. Vascular Land Plant Characteristics
(not all evolved at the same time)
roots: to gather water and nutrients
vascular system: for fluid transport
cuticle (cutin in outer wall layers): prevent drying
support: lignin and cellulose
III. A BRIEF SUMMARY OF MAJOR STEPS IN PLANT EVOLUTION
Food for thought:
The oldest fossils reveal evolution of non-vascular plants by the middle
to late Ordovician Period (~450-440 m.y.a.) on the basis of fossil spores.
These first non-vascular, "amphibious" plants, were limited primarily to
lowland, wet areas, of temperate to tropical latitudes. Still most land
areas were barren.
By some 420 m.y.a., seedless vascular plants evolved which were bigger
and more diverse than their ancestors. These seedless vascular plants required
water for sperm to reach the eggs and were still limited in their distribution
to wet climates. These seedless plants grew to tree size, reaching 10 or
more meters by the late Devonian (~370 m.y.a.). Lowland swampy environments
had abundant seedless plants.
After the 1st land plants evolved (approximately 450 m.y.a.) they underwent
rapid expansion and radiation with great forests by Middle Devonian
Period. (385 m.y.a.). Although these forests were limited in their extent,
vast quantities of organic material were deposited and preserved in swampy
environments. Through time and burial, this organic material was converted
to coal, forming the first significant coal deposits of Earth history.
At a similar time (late Devonian, ~375-360 m.y.a.), seed bearing gymnosperm
plants (non-flowering) evolved. The development of the seed allowed plants
to proliferate and spread to drier areas of continents. Gymnosperms became
the dominant plant type between ~290 to 145 m.y.a. and are still common
During the final days of the dinosaurs (Cretaceous), the final chapter
of plant evolution began. This was the evolution of flowering plants (angiosperms).
During the Cretaceous Period, angiosperms began overcoming non-flowering
plants as the dominant land plants. Since this time they have become the
dominant land plants.
For most of the history of planet Earth, landmasses have been barren
of life. Even with the advent of vegetation, plants first clung to lowland
wet regions. Much later, with the arrival of gymnosperms, they spread to
drier regions. Only within the last 100 m.y. did the planet's plant world
begin to resemble the Earth we know.
1. Prior to plant colonization of land, how would geologic processes
(such as erosion) have differed from today?
2. Why are land plants important as eventual providers of a fossil fuel
3. In what type of environment are plants preserved as fossils?
4. When did upland and higher latitude areas become forested? By what
type of plants?
IV. PLANT EVOLUTION
TRACHEOPHYTES (VASCULAR PLANTS) - 3 groups differing in adaptation
for reproduction - 3 periods of evolutionary expansion
A. FERNS & RELATIVES - dominated through most of the Paleozoic
1. no seeds
2. no effective way to prevent gamete desiccation - require moist environment
B. GYMNOSPERMS - conifers and other groups, first appear in Carboniferous,
dominant land flora by Triassic.
1. seeds and pollen
C. ANGIOSPERMS - flowering plants - first appear in early Cretaceous
1. flower and enclosed seed
Phylum Tracheophyta Subphylum: 4 seedless; 1 seed-bearing
FOUR MAJOR TYPES OF SEEDLESS PLANTS
Reproduction of seedless plants: primitive reproduction, gametes not produced
on large leafy plant but on separate small leafless plant - called gametophyte.
See text page 370 (Box 13.3) for an illustration of the reproductive history
of a seedless vascular plant. Compare with the illustration of the reproductive
history of a primitive seed plant (gymnosperm).
From the illustration note: - alternation of sporophyte and gametophyte
plants. - need moist soil for gamete protection.
A. Psilopsids -
most lack roots and leaves,
simple vascular system,
abundant only in Devonian.
Seedless Trees - leaves and strong root systems; evolved from small herbs
- dominant trees of Pennsylvanian
huge trees of coal forests (up to 30m).
Arose in the Early Devonian (small), large by Late Devonian
Lepidodendron - scale trees
Sigillaria - scale trees
During the Carboniferous, coal swamps with abundant trees of this type
accumulated, forming the greatest coal deposits of all geologic time.
Elongate leaves emerged directly from trunks and branches, leaving distinctive
leaf scars (shown in class).
Living club moss a representative
In many, sporangia in clusters called cones, male and female cones on separate
plants (advantage for variability)
D. Pteridopsida - Introduction
to the Pteridopsida
joint stemmed plants (scouring rushes)
Calaminites - common in Carboniferous
Equisetum (horsetail) - only living representative
Seedless trees overshadowed in Late Carboniferous and Permian by gymnosperms.
see fossil Sphenophyta
5 classes gymnosperms: seed ferns, cycads, ginkos, conifers, angiosperms
Huge trees in Carboniferous
over 10,000 living species
Reproduction of gymnosperms
A. Seed Ferns
seed not enclosed (exposed), e.g., woody scales of pine cone
great advantage over seedless ancestors: reproduce without external moisture
retain gametophyte stage but no sporophyte plant
See text page 370 (Box 13.3) for an illustration of the reproductive history
of a primitive seed plant (gymnosperm). Compare with the illustration of
the reproductive history of a seedless vascular plant.
From the illustration note: - Seed bearing plants bear male and female
cones. - The pollen fertilized seed is protected and may survive dried
conditions for a period of time before growing. This adaptation allowed
plants much wider distribution that possible by seedless plants.
B. & C. Cycads & Ginkgo
First plants to bear seeds
First in lower Carboniferous to Late Devonian
Common through Permian extinct in Early Cretaceous
only major division to go extinct declined rapidly with rise of other gymnosperms
First in Carboniferous
Pines, firs, cedars arose in Early Mesozoic
Dominant vegetation where water sparse, poor soil, cold
RISE & DOMINANCE OF FLOWERING PLANTS ANGIOSPERMS
Very dominant group today - 96% of all vascular plant species
Success the result of development of flower and enclosed seed
specialized flower reproductive structure: - both seed and pollen producing
colored leaves to attract pollinators
Double fertilization: one pollen grain fertilizes ovary, and a second triggers
vigorous growth of the nutritious fruit, nut, or kernel.
seed covering: soft, fleshy, edible, aid in seed dispersal
other dispersal mechanisms
exhibit great diversity
geologic range: few in Triassic and Jurassic; dominant since Cretaceous
record of Angiosperms