Stephen P. Broker
In this section the major divisions of plants and the basic characteristics of each type of plant will be listed. Some comparisons will be made between extinct species and present day forms. But first, some mention should be made of our understanding of the origin of life on earth and the major events which must have taken place prior to the arrival of the land plants.
The oldest known life consists of forms of bacteria, blue-green algae, and green algae, all of which probably resided in salt water environments. Earlier this summer an announcement was made of the 1977 discovery of the oldest known biological life. U.C.L.A. paleobotanist J. William Schopf and an international team of scientists had delayed announcement of the find for three years, while extensive tests were run on these filaments of cells to determine their age, chemical makeup and microscopic appearance. The studies convinced the group that this cellular life existed little more than one billion years after the earth’s formation. The findings also suggest that a period of organic evolution took place much closer to the origins of the planet than previously realized. The 3.5 billion year old cells are near-identical in appearance to some present day bacteria, and more primitive forms must have preceded them.
Much additional evidence of life from approximately half a billion to three billion years ago has been gathered in the past 15 years, most notably by Elso S. Barghoorn of Harvard and by Schopf. The 3.2 billion year old chert of the Fig Tree formation in Transvaal, South Africa contains spherical microfassils named
Eobacterium
. Chert (a flintlike or quartz-like rock) from the Gunflint formation in western Ontario, dated to 2.0 billion years ago, contains filamentous structures resembling present day blue-green algae. This is believed to be early evidence of photosynthetic activity. These above-mentioned microfossils were found chiefly during the examination of stromatolytes, dome-shaped layered deposits of material found in various regions of the world, which are a type of organosedimentary structure. The stromatolytes were formed by the action of early life. They are the indirect evidence of early life, rather than consisting entirely of fossilized life. They do, as mentioned, contain some actual remnants of life. Stromatolytes are comparable in form to the algal mats produced under certain conditions today.
Bacteria and blue-green algae have a prokaryotic organization. Their cells lack true nuclei, because of the absence of a nuclear membrane. The cellular unit of plants and animals is the more advanced eukaryotic cell, in which nuclear material is set off from the cytoplasm of the cell by a nuclear membrane. Eukaryotic cells first appear in the fossil record in the 0.5 billion year old Bitter Springs charts, also in Australia’s Northern Territory. These cells are evidently photosynthetic and they most closely resemble the green algae. The eukaryotic organization achieved 500 million years ago led to rapid evolution of multicellularity in plants and animals, and most significantly to life on land. By 400 million years ago a number of distinct plant types had begun to appear. Since then each group has experienced diversification and varying degrees of success. There have been dominant plant types throughout this time, and there have been a number of extinctions. Some of these developments are listed below.
Thallophytes
:
algae
These eukaryotic plants, which ore classified primarily on the basis of color (green. goldenbrown. brown, red) lack true roots, stems, and leaves. Their aquatic environments hove o stability not offered to plants living on land. The niches occupied by today’s algae ore
in many ways the same as those filled by the green algae of 500 million years ago.
fungi
The decomposers of the plant world, fungi have been found in the fossil record in deposits as early as the Rhynie charts of the Middle Devonian. As with the algae, much remains to be learned about the early forms of these non-green plants and their phylogenetic histories.
Bryophytes
:
mosses
and
liverworts
These plants possess a greater specialization of tissues than is found among the thallophytes. True roots do not exist, but the mosses and liverworts have distinct leaf-like forms. Their fossils are quite rare.
Tracheophytes
:
club mosses
Once the dominant plants of the forest (Carboniferous Period), the club mosses, also called lycopods or lycopsids, are reduced today to five genera and approximately 900 distinct species, all small and herbaceous. They possess true roots, stems, and leaves, as is true with all tracheophytes. The most common living members are
Lycopodium
and
Selaginella
, which enjoy widespread distribution. The club mosses are characterized by scale-like leaves and a dichotomous branching of the stem. During the Carboniferous the most conspicuous forms were the arborescent lycopsids, especially
Lepidodendron
and
Sigillaria
. The herbaceous
Lycopodites
and
Selaginellites
were also common plants of the period. Tree-like forms had a two-layered bark, which is well preserved in molds and casts. Their evolutionary significance lies mainly in the advances made in branching and in the development of a more complex rooting system, an example of differentiation of plant tissue. Their remains are one of the chief ingredients of coal.
horsetails
These lower vascular plants, also called sphenopsids, are even more reduced in diversity today than the club mosses.
Equisetum
is the only living genus. The leaves are reduced in size, and the stem and cones of horsetails are characterized by the presence of ridges (nodes and internodes). The arborescent horsetails, especially
Calamites
, were dominant in the Carboniferous lowland swamps, and herbaceous forms (e.g.,
Sphenophyllum
) were common. They faced the same Permian extinctions which greatly reduced the diversity of the club mosses. The present day plants continue to grow in moist habitats.
pteropsids
, including:
ferns
These plants range from the commonly known hernaceous ferns of Connecticut forests to very large tree-like species, usually of more tropical climates.
Psaronius
is a 20 to 25 foot toll fern of the Carboniferous. A separate category of fern-like plant was the seed fern; these plants bore foliage typical of a fern, but they also bore seeds, a gymnosperm-like characteristic. Seed ferns were present during the late Devonian and persisted until the Permian.
Medullosa
is a common example from the Carboniferous. True ferns are a group of plants which appeared fairly early in the history of land plants, and they continue to adapt successfully to the earth’s environments.
gymnosperms
: the
cycadeoids
Cycadeoids were present during the early Triassic and survived until the end of the Cretaceous, when this form of gymnosperm became completely extinct. They had large, fern-like leaves, which grew from a thick, barrel-shaped trunk. During processes of preservation, leaves almost always became dissociated from trunks, and are found separately as fossils today. The trunks bear two types of scars, from the leaf bases and from the reproductive organs. Yale’s Peabody Museum of Natural History has an outstanding collection of these rare fossils, most collected from the Black Hills of South Dakota by G.R. Wieland. These and other gymnosperms are seed plants.
cycads
Fossil leaves of cycads are essentially indistinguishable from those of the cycadeoids. These two plant types, which also have similar short, thick trunks, are classified separately on the basis of reproductive anatomies (particularly cone structure). Some present day cycads are arborescent, an Australian species growing as tall as 60 feet.
ginkgos
These plants were present during the Permian, or perhaps as early as the Carboniferous. One species remains alive today, native to remote forests of China, and planted widely as an ornamental tree. Leaves of these plants are fan-shaped, sometimes deeply lobed, and with a characteristic venation. Because the ginkgo has remained largely unchanged in appearance for many millions of years, it is referred to as a “living fossil.”
conifers
The conifers have been present on earth since before the Carboniferous, and they are a group of plants which once had a greater diversity than is evidenced today. The seven present day families include pine, spruce, fir, hemlock, juniper, cypress, bald cypress, and the largest forms of life on earth, the Sequoias (
S. gigantea
the largest plants;
S. sempervirens
the tollest). Conifers are usually evergreens and their leaves appear as needles. They ore more prevalent in temperate parts of the world.
angiosperms
Recognized as the most advanced plants on earth, the angiosperms contribute the reproductive structure of the flower to the seed habit of life. These “covered seed” plants enclose their ovules in hollow ovaries. They are abundant and exceedingly diverse plants, with more than 200,000 species known to exist. Angiosperms first appeared as long ago os 135 million years, and they began their present dominance 70 to 100 million years ago. The two basic forms of angiosperms are the monocotyledons and dicotyledons, which are distinguished by the number of embryonic leaves, the arrangement of vascular bundles in stems and roots, leaf venation, and groupings of petals in the flowers. Angiosperm families are grouped primarily on the basis of floral structure.