Carboniferous coal deposits. Most reproduced with spores, but the extinct
seed ferns bore seeds.
The spermatophytes are the higher plants that produce seeds and include
the gymnosperms and angiosperms. Gymnosperms are conifers that bare seeds
on exposed scales or cones. They ranged from the Carboniferous to the pre-
sent and covered vast areas in harsh climates. Angiosperms are flowering
plants, whose seeds often develop in a fruit.They originated in the Cretaceous
and range in size from grasses to huge trees.The higher plants are represented
by about 270,000 modern species.
The next major evolutionary stage was the development of a vascular stem
using channels to conduct water from a swamp or from the moist ground
nearby to the plant’s extremities.The strengthening of the stems enabled vascu-
lar plants to grow tall.The early club mosses, ferns, and horsetails were the first
plants to make use of this system. When roots evolved, plants could survive
entirely on dry land by drawing water into their stalks from soil moistened by
rain. By the end of the Silurian 400 million years ago, 2-inch-long plants had
water-conducting vessels that allowed them live totally out of water.
The lycopods, which included the club mosses and scale trees (Fig. 79),
were the first plants to develop true roots and leaves.The branches were arranged
in a spiral shape, and leaves were generally small. Spores were attached to modi-
fied leaves that became primitive cones. The scale trees, so named because the
scars on their trunks resembled large fish scales, grew upward of 100 feet or more
high and eventually became one of the dominant trees of the Paleozoic forests.
Many plants were still quite primitive and restricted to wet habitats.They
had no roots or leaves and resembled branching sticks creeping along the
ground. More advanced plants grew as high as 10 feet tall and had substantial
root systems that burrowed into the ground as much as 3 feet deep.The deep-
rooted plants lived along streams that seasonally dried up, requiring them to
tolerate prolonged arid conditions. Since the environment was so harsh, hav-
ing large roots helped the plants withstand such severe conditions.The deep
roots helped control erosion, allowing the buildup of fertile soils on the land,
which encouraged additional plant growth.
The burgeoning vegetation also produced a substantial drop in atmos-
pheric carbon dioxide concentrations, changing a steaming greenhouse cli-
mate to one more hospitable for the first animals to populate the land. The
plants shared the land with a variety of arthropods. They were joined by
amphibious fish, which began to make short forays onto the beach to prey on
abundant crustaceans and insects. Freshwater invertebrates and fish began to
inhabit the lakes and streams.
During their first 50 million years on dry land,plants displayed increased
diversity and complexity, including root systems, leaves, and reproductive organs
employing seeds instead of spores.When true leaves evolved, plants developed aHistorical Geology