Geology Reference
In-Depth Information
The earliest known vascular land plants are small Y-shaped
stems assigned to the genus
Cooksonia
from the Middle
Silurian of Wales and Ireland. Together with Upper Silurian
and Lower Devonian species from Scotland, New York State,
and the Czech Republic, these earliest plants were small,
simple, leafl ess stalks with a spore-producing structure at
the tip (
Figure 21.31); they are known as
seedless vascular
plants
because they did not produce seeds. They also did not
have a true root system. A
rhizome
, the underground part of
the stem, transferred water from the soil to the plant and an-
chored the plant to the ground. The sedimentary rocks in
which these plant fossils are found indicate that they lived in
low, wet, marshy, freshwater environments.
An interesting parallel can be seen between seedless
vascular plants and amphibians. When they made the tran-
sition from water to land, both plants and animals had to
overcome the problems that such a transition involved. Both
groups, while successful, nevertheless required a source of
water in order to reproduce. In the case of amphibians, their
gelatinous egg had to remain moist, and the seedless vascu-
lar plants required water for the sperm to travel through to
reach the egg.
From this simple beginning, the seedless vascular plants
evolved many of the major structural features characteris-
tic of modern plants, such as leaves, roots, and secondary
growth. These features did not all evolve simultaneously,
◗
◗
Figure 21.31
Cooksonia
The earliest known fertile land plant was
Cooksonia
, seen in this fossil from the Upper Silurian of South Wales.
Cooksonia
consisted of upright, branched stems terminating in sporangia
(spore-producing structures). It also had a resistant cuticle and produced
spores typical of a vascular plant. These plants probably lived in moist
environments such as mudfl ats. This specimen is 1.49 cm long.
Protolepidodendron
Dawsonites
Bucheria
◗
Figure 21.32
Early Devonian Landscape Reconstruction of an Early Devonian landscape showing
some of the earliest land plants.
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