Geology Reference
In-Depth Information
Major Events in the Evolution of Land Plants. The Devonian Period was a time of rapid evolution for the
land plants. Major events were the appearance of leaves, heterospory, secondary growth, and the emer-
gence of seeds.
TABLE 21.3
MYA
PERIOD
EPOCH
Flowering plants
Neogene
2.3
Paleogene
Cretaceous
Jurassic
Triassic
Permian
66
146
200
251
299
Cycads
Conifer
-
type seed plants
Ferns
Carboniferous
Seed ferns
Lycophytes
359
Megaphyllous
leaves
Upper
Arborescene Seeds
Progymnosperms
385
Zosterophyllodphytes
Devonian
Middle
Secondary growth
398
Trimerophytes
Major diversification
of vascular plants
Lower
Heterospory
Microphyllous
leaves
416
Pridoli
Rhyniophytes
419
Ludlow
423
428
Silurian
Wenlock
Tracheids
Cooksonia
Llandovery
Upper
444
First land plants
Ordovician
Lower
488
the movement of water and nutrients. The
nonvascular
plants,
such as bryophytes (liverworts, hornworts, and mosses) and
fungi, do not have these specialized cells and are typically small
and live in low, moist areas.
The earliest land plants from the Middle to Late
Ordovician were probably small and bryophyte-like in their
overall organization (but not necessarily related to bryo-
phytes). The evolution of vascular tissue in plants was an
important step because it allowed for the transport of food
and water.
Discoveries of probable vascular plant megafossils and
characteristic spores indicate to many paleontologists that
vascular plants evolved well before the Middle Silurian.
Sheets of cuticle-like cells—that is, the cells that cover the
surface of present-day land plants—and tetrahedral clusters
that closely resemble the spore tetrahedrals of primitive land
plants have been reported from Middle to Upper Ordovician
rocks from western Libya and elsewhere.
The ancestor of terrestrial vascular plants was prob-
ably some type of green alga. Although no fossil record of
the transition from green algae to terrestrial vascular plants
exists, comparison of their physiology reveals a strong
link. Primitive
seedless vascular plants
(discussed later in
this chapter) such as ferns resemble green algae in their
pigmentation, important metabolic enzymes, and type
of reproductive cycle. Furthermore, green algae are one
of the few plant groups to have made the transition from
saltwater to freshwater. The evolution of terrestrial vascu-
lar plants from an aquatic, probable green algal ancestry
was accompanied by various modifications that let them
occupy this new harsh environment. Besides the primary
function of transporting water and nutrients through-
out a plant, vascular tissue also provides some support for
the plant body. Additional strength is derived from the or-
ganic compounds
lignin
and
cellulose
, found throughout a
plant's walls.
The problem of desiccation was circumvented by
the evolution of
cutin
, an organic compound found in the
outer-wall layers of plants. Cutin also provides additional
resistance to oxidation, the effects of ultraviolet light, and
the entry of parasites.
Roots
evolved in response to the need to collect water
and nutrients from the soil and to help anchor the plant in
the ground. The evolution of
leaves
from tiny outgrowths on
the stem or from branch systems provided plants with an
effi cient light-gathering system for photosynthesis.
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