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Plants of the Coal Age
Nature Bulletin No. 330-A   February 1, 1969
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

PLANTS OF THE COAL AGE
IN the preceding bulletin we told about the early part of the Age of Ferns when there were many low-growing kinds as well as the tree ferns, some of which were quite tall, with typical fronds and crowns of all sizes and shapes, but all reproducing by means of spores -- just as ferns do nowadays. In addition, the Seed "Ferns" appeared: the first plants that ever spread pollen or bore seeds. In appearance, except for primitive pollen sacs and seed sacs, they resembled the other ferns.

The Age of Ferns included what is known as the Pennsylvanian Period which began about 250 million years ago and lasted for some 35 million years. This was the first of two periods in the earth's history when luxuriant tropical vegetation blanketed the land from pole to pole and there were enormous accumulations of plant material which later became coal. The climate all over the earth, apparently, was very warm and humid with no seasonal changes. In addition to the ferns, tree ferns and seed ferns, there were giant club mosses as much as 100 feet tall with trunks three or four feet in diameter, treelike horsetails, and a new type of plant distantly related to our evergreen conifers: large trees that bristled with enormous needles like sharp green swords. The first mosses and liverworts also appeared. The vegetation was all green, yellow or brown. There were no flowers, no birds, no butterflies or bees. There were cockroaches, gigantic dragonflies and small reptiles but the world must have been strangely silent. It was during this period, the Coal Age, when the greatest deposits of plant material were made, which eventually were transformed into beds of bituminous and anthracite coal that circle the northern hemisphere from near the North Pole to the South Temperate Zone.

Then the earth's climate changed and there were serious disturbances of the earth's crust. The southern hemisphere was covered probably six times by glaciers advancing from the South Pole and, for the first time plants had to retreat. Some managed to "migrate" toward the equator and then returned toward their former ranges as the glaciers melted back, but some became extinct. The tree ferns, giant club mosses, and the tall horsetails all disappeared. Only the smaller hardier forms of ferns and horsetails survived. Then the climate became much drier all over the earth. Two new groups of plants developed: the first conifers and the cycads. The Age of Ferns, which has lasted almost since the dawn of plant life on land, was ended. For the next 95 million years or so, the vegetable kingdom had a new ruler.

The Age of Cycads, although it produced coal beds throughout the earth, was dominated by scrub forests because it was not so moist. Even the conifers, destined to be the world's tallest and biggest trees, seldom reached 60 feet in height. The Petrified Forest in Arizona consists of fossilized pines that grew in the Triassic Period. The cycads had woody stems or trunks that almost never branched, and crowns of palmlike or fernlike leaves at the top. The stems of some kinds were ringed by the remains of old crowns that died down to become new additions to the trunk, and they grew to be 50 or 60 feet tall. Many odd dwarf kinds covered the ground. One had a trunk only two feet high and just as wide. Leaves and fruiting bodies projected everywhere from its bark, so that its fossils look like overgrown honeycombs. There are still two small species of cycads native in Florida, and six in Mexico. In the Jurassic Period which followed, two important developments occurred in plants. The earth's climate was changing again.

Extra ! Extra ! Read all about it in the next bulletin.


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