PLANT DIVERSITY CLASS
ALLISTER REES
Present-day Climates
Geography: Present, Future and Past
Plants (Part 1):
‘Greening of the Land’ - Origins, Diversification
Plants (Part 2):
‘Icehouse World’ - Carboniferous and Permian
‘Hothouse World’ - Jurassic and Cretaceous
10
9
8
7
6
5
4
3
2
1
Glacial
Arctic
Cold Temperate
Mid Latitude Desert
Cool Temperate
Warm Temperate
Winterwet
Subtropical Desert
Tropical Summerwet
Tropical Everwet
Elevation >1000m
BIOMES AS ORIGINALLY MAPPED BY WALTER & LIETH , 1967
PLANTS (PART 1)
THE GREENING OF THE LAND
Late Silurian (ca 420 Ma) onwards
Do I really want to do this?
Without land plants, there would have been no food
for animals and therefore no terrestrial animal life
(that includes us – just try and picture yourself in a
world without plants….)
Without vegetation, the surface sediments produced by rock weathering
would have been highly mobile
Wind and water would have transported the loose sediments to the oceans
in the absence of rooted vegetation or soils
River flow would have been highly variable, and flash floods
common, producing braided rivers
Vegetation helped bind the sediments together, so rivers flowed more
gradually (‘meandered’) to the oceans
Vegetation also produced soils, promoting even more plant growth
and expansion across previously barren landscapes
THREE PROBLEMS FACED BY PLANTS COLONIZING LAND:
1. The fight against gravity
2. Bodily fluids
3. Reproduction
1. THE FIGHT AGAINST GRAVITY
Rigid support structures needed, using energy + resources
2. BODILY FLUIDS
Fluid distribution (vascular) system needed to
transport water + minerals from the soil
Also need a barrier to prevent water loss (the cuticle) but this also limits capacity to store water directly from
the atmosphere
So, also need roots (these also provide anchorage!)
3. REPRODUCTION
Need to disperse offspring away from the parent
plant to prevent overcrowding
Also need to protect the offspring from drying out
(The reverse reaction is used by animals)
Without land plants, the
oxygen content of the
atmosphere would be much
lower than it is today….
…. so much lower that we
wouldn’t have enough
oxygen to breathe! We all
talk about ‘air’, but what we
really need is oxygen.
Late Silurian (ca 420 Ma)
Small plants (ca 1cm tall)
with simple pore openings in
the surface barrier
These allow gas exchange
but no regulation of water
loss
(Spongeophyton)
(Sporogenites, ca 2cm tall)
(Cooksonia, ca 7cm tall)
Some plants grew a little taller (but still only up to ca 10cm) and had
‘pouches’ at the tips of their stems, containing spores which could be
released and spread by wind
(Sawdonia, ca 90cm tall)
Early Devonian (ca 410 Ma)
Taller plants (ca 50-100cm)
with increased spore dispersal
abilities
(Psilophyton, ca 50cm tall)
BUT at a greater cost of
construction, requiring more
photosynthesis
(Aglaeophyton, ca 50cm tall)
(Asteroxylon, ca 30cm tall)
(Pertica, ca 1m tall)
(~ small leaf)
(~ surface ‘roots’)
Although still small, pretty soon plants were developing:
1) greater structural strength
2) a plumbing system to move fluids up (water) and down (sugars
produced by photosynthesis) the plant body
3) small leaves to increase gas exchange with the atmosphere
Late Devonian (ca 370 Ma)
By this time, large trees with
woody stems, branches and
leaves had developed
They’d also evolved to solve
the leaf gas exchange/water
loss problem, by developing
stomata
They became so efficient that
they contributed greatly to
reducing atmospheric CO2
levels
(Archaeopteris, ca 30m tall)
guard cells
When guard cells have
a lot of water in them,
their curvature
increases, so a hole
opens up between them
(the stoma)
With only little water in
them, they are
straighter and the
stoma closes
stoma
(false-color SEM of leaf
surface of the tobacco plant,
Nicotiana tabacum)
However, this
increased efficiency
created some new
problems….
Because plants had contributed so much to
decreasing atmospheric CO2, they were no
longer just bathed in it, so had to evolve by
increasing the amount of stomata
BUT….
This led to greater water loss, so more efficient
root and stem systems were needed to supply
more water to the leaves
(water transport cells)
(Rhynia)
(Psaronius)
Single conducting (vascular)
strand (Early Devonian)
Multiple strands
(Carboniferous)
Development of roots
small simple stems
main stem + branches
stomata + small
‘stem leaves’
CO2
Level
Time (Ma)
large trees + large leaves
wood + bark
(small trees)
Carbon cycle (1): no vegetation
Carbon cycle (2): vegetation
Estimates of CO2 through time, relative to present day levels
A (very) brief summary of major
innovations….
Estimates of plant species diversity through time
(flowering plants)
(seed plants - conifers, cycads, etc)
(ferns)
PLANTS (PART 2)
ICEHOUSE AND HOTHOUSE WORLDS
Carboniferous through Cretaceous
‘ICEHOUSE WORLD’
Glossopteris
Continental Drift
Alfred Wegener
Locations of certain fossil plants and animals on present-day, widely separated
continents would form definite patterns, if the continents are rejoined.
‘HOTHOUSE WORLD’
JURASSIC – AN ANTARCTIC CASE STUDY
Late Jurassic
Volgian (150 Ma)
CRETACEOUS - A SIBERIAN CASE STUDY
Late Cretaceous
Maastrichtian (70 Ma)
Growth rings in fossil wood
smooth (‘entire’) margin leaves
toothed (‘serrate’) margin leaves
Mid Cretaceous (ca 95 Ma) position
of North Alaska - at ca 80N
Cretaceous coal in North Alaska
represents ca 1/3 of total US
reserves of all ages combined!
Just like Antarctica, these northern
regions of Alaska and Siberia had
diverse thriving forests and much
warmer climates than today
Reconstruction of northern Alaskan forests in the mid-Cretaceous
I’ve shown examples of vegetation history spanning ca. 400 million
years. The key point is that plants have had time to adapt and evolve
to changing circumstances (e.g., geography, climate), but have also
played an active role in determining climate. There were winners and
losers, survival of the fittest, etc. But all of those changes have
occurred gradually.
I leave you with this question: how do you think the human race is
affecting our planet and what sort of future do you envisage? Use
your knowledge and imagination here: 50 years or 50 million years?
It’s up to you. Feel free to come and discuss (GS 334,
rees@geo.arizona.edu).
PLANT DIVERSITY CLASS TOPICS:
Present-day Climates
Geography: Present, Future and Past
Plants (Part 1):
‘Greening of the Land’ - Origins, Diversification
Plants (Part 2):
‘Icehouse World’ - Carboniferous and Permian
‘Hothouse World’ - Jurassic and Cretaceous