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If we could suddenly transport ourselves back to the Coal Age, or Carboniferous Period, as it is called by geologists, probably none of the plants in those magnificent rain forests would be familiar to us. Even the dinosaurs and other large reptiles feeding on them would seem strange, except to those who have visited museums to see the reconstructed plants and the animal skeletons. Yet these lush forests of trees 90 or 100 feet high and three feet in diameter were the progenitors of our present spore-bearing vascular plants, the ferns and their allies. These were the plants from which nearly all the coal we burn had its origin.
In the ensuing ages as the climate of the northern hemisphere became cooler and drier, many of these large spore bearing trees became extinct, and their extinction, in turn, may in part have been responsible for the extinction of the giant reptiles who fed upon them. Through all the struggles for existence, a few remnants of these plants survived and remain with us today.
All that we know about previously existing forests comes to us from various fossil evidence such as carbonization, imprints, petrifications, or casts. Coal represents carbonized plant material, but few structures are preserved to identify the plants in the coal. Odd structures called coalballs, found near coal formations, have furnished some of our best material for the study of plants known during the Coal Age. It is not known how coalballs formed. Leaves which fell into slow moving streams or lakes were covered with clay or sand and left permanent imprints showing the perfect outline and details such as vein patterns. These soil particles later became compacted to form shale or sandstone in which the fossils may be discovered today. Some stems, roots and cones have become petrified, meaning that most of all the organic material has been replaced gradually by dissolved mineral substances such as silicates or carbonates. Petrification is an extremely slow process that depends upon plants being kept free from fungi and bacteria which cause rotting. Plant structures which are to become petrified usually are covered by silt or volcanic ash and are thus protected from decay. Some trees which were covered by mud or volcanic ash did rot out. In the cavity, dissolved material seeped in. Thus a cast was made showing fine details of bark, leaf scars and other external features, but no internal structure was preserved.
Unfortunately, a complete plant consisting of stems, leaves and cones is never preserved in a single locality. Cones may be petrified in one area, stems somewhere else. Leaf imprints are never found with petrified stems. Stem casts are preserved under different circumstances.
Now let us examine briefly some of the major plant groups among the fern allies, most of which are native to California.
The earliest fern ally appeared about 300 million years ago and specimens of it are well preserved as petrified stems in chert of Rhynie, Scotland. These earliest plants did not even have leaves, just forking stems with a simple vascular system and 4 spores borne at the top of the stem. A living remnant of this oldest group, Psilopsida, is the genus Psilotum, not native to California, but to the tropical areas of the world.
The Club Mosses or Lycopsida form a major group among the fern allies with two of the four known genera growing in the Sierra Nevada. The living plants represent only a remnant of a large group once numbering hundreds of species. During the Coal Age many were large trees; our present species are mostly low growing erect or creeping plants. These plants have roots, stems and small leaves, usually spirally arranged. The SPORANGIA, clustered at the stem tips in CONES, are not attached to the stem, but to the top of the leaf, designated a SPOROPHYLL, or spore-bearing leaf. Some cones are borne on special stalks, others are difficult to distinguish from a vegetative branch.
Lycopodium, one genus not represented in the Sierra, but found further north, as well as in tropical areas, has HOMOSPORUS cones, in which all spores are the same size, similar to our common ferns. Selaginella, a genus in our area, is HETEROSPORUS, having two kinds of spores in each cone: MICROSPORES, very small ones, and MEGASPORES, larger ones. The third common genus is Isoetes, which always grows in damp places and is often submerged in lakes or quiet streams. Isoetes in no way resembles the other genera. It looks much like a sedge or grass with long slender leaves which have large air spaces inside. The stem of Isoetes is condensed into a storage tissue called a CORM. The outer leaves actually are MEGASPOROPHYLLS, each leaf bearing one sporangium with its numerous MEGASPORES at the base. The inner leaves are really MICROSPOROPHYLLS, each bearing one sporangium with thousands of MICROSPORES.
The life cycle of Club Mosses is similar to ferns with alternating spore-bearing and sexual generations. In those genera which are HETEROSPORUS the microspores form male plants, the MEGASPORES form female plants. These sexual plants are small and sometimes not green, often parasitic.
The extinct Club Mosses, which were present as trees and shrubs in the Carboniferous Period or Coal Age, became important components in Coal. One example of an extinct genus is Lepidodendron, meaning scale tree, a large tree with dichotomous branching stems and roots. Many casts and preservations of this tree show the leaf scars the whole length of the stem. Leaves of the Club Mosses were either scales or long and narrow.
The last major group is the Horsetails or Sphenocsida with a single living genus, Equisetum. Its 25 species are found in all parts of the world except Australia and New Zealand. The jointed hollow stems have a reduced whorl of leaves at every node, fused into a collar. Most of the photosynthesis occurs in the green stems. In some species the stems are unbranched, in others whorls of smaller jointed branches occur. The spores are borne in cones at the ends of the stems. Again in this group the alternating spore and sexual generations occur.
Among extinct Horsetails the whorls of leaves were not always fused into a collar as in living forms. In Annlaria (illustrated p. 61) the wedge-shaped leaves are not fused to each other. Some of the extinct forms had a cambium which was the reason for much larger stems and roots than in our present forms.
Our living fern allies represent a remnant which was able to survive the severe climatic changes on this continent and in other parts of the world, and have adapted themselves to their present environments.
[click to enlarge]
Fig. 6 Carboniferous forest composed of now extinct species of Horsetails, Club Mosses, Ferns, and related plants as reconstructed from fossils. Courtesy of Chicago Natural History Museum.
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