microbes also aided in the weathering of rock into soil, helped to prevent soil
erosion, and provided the nutrients more advanced plants needed for survival.
The first land plants comprised algae and seaweedlike plants. They
resided just below the sea surface in the shallow waters of the intertidal zones
(Fig. 78). Primitive forms of lichen and moss lived on exposed surfaces. They
were followed by tiny fernlike plants called psilophytes, or whisk ferns, the
predecessors of trees. They were among the first plants to live onshore. These
simple plants lived semisubmerged in the intertidal zones, lacked root systems
and leaves, and reproduced by casting spores into the sea for dispersal. The
most complex land plants grew less than an inch tall and resembled an out-
door carpet covering the landscape.
By the late Silurian, all major plant phyla were in existence. Except for
simple algae and bacteria, the early land plants diverged into two major
groups. The bryophytes, including mosses and liverworts, were the first plant
phylum to become well established on land. They have stems and simple
leaves. However, they lack true roots or vascular tissues to conduct water to
the higher extremities and therefore are required to live in moist environ-
ments.They reproduce by spores that are carried by the wind for wide distri-
bution.The earliest species occupied freshwater lakes in the late Precambrian.
The pteridophytes, or ferns, were the first plant phylum to develop true
roots, stems, and leaves. Some present-day tropical ferns grow to tree size as
they did in the geologic past. The whisk ferns, which appeared at the end
ofthe Silurian and became extinct near the end of the Devonian, probably
gave rise to the first club mosses, horsetails, and true ferns.The true ferns are
the largest group both living and extinct and contributed substantially to
Figure 78Evolution of
plants from the ocean to the
land:(1)fully submerged;
(2)semisubmerged;(3)
fully terrestrial.SILURIAN PLANTS