Stephen P. Broker
Paleobotanical work of this century has provided many clues to the evolutionary history of the plant kingdom. A detailed description of phylogenies for the major divisions of plants is beyond the scope of this unit. It is important, however, to identify some of the most significant trends in the evolution of plants, any one of which could be explored to a degree in the classroom. The following trends in evolution are judged to be of greatest significance as plants attained higher levels of complexity.
a)
method of energy use
heterotrophic and anaerobic life forms leading to autotrophic and aerobic life forms. It is generally believed that the earth’s atmosphere did not have high levels of oxygen until the evolution of photosynthetic plants. Early life forms must necessarily have existed under anaerobic conditions. They were heterotrophic forms of life, meaning that foods and necessary nutrients were obtained externally, most likely through processes of fermentation. Later, autotrophic organisms developed which could produce their own food, through the process of photosynthesis. A consequence of autotrophic life is development of aerobic life.
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b)
cellular organization
Prokaryotic cells, with less organized nuclear material leading to eukaryotic cells, with true nuclei; unicellular life forms leading to multicellularity.
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c)
ecological niches occupied
aquatic (marine and freshwater) forms leading to the development of the land plant. The principal obstacle to overcome was the tendency to dry out in a nonaqueous environment. Anatomical advances, such as a thickening of cell walls and a system of transporting water and minerals throughout plant tissues, were required for successful development of the land habit.
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d)
differentiation of plant tissues
branching; evolution of roots, stems, and leaves; development of the seed. Early land plants lacked branching. Greater complexity of growth was attained by plants which had a dichotomous pattern of branching, two equal axes being formed at each fork. Ultimately, trilateral branching off a main stem evolved. The evolution of the seed is considered to be one of the most significant advances of the land plants.
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e)
plant size
the herbaceous habit leading to the arborescent habit. It is not to be assumed that larger plants are more complex or advanced plants. It is true, in the plant world as well as the animal world, that many life forms have followed a trend toward larger and larger representatives. The club mosses and horsetails are examples of plants which achieved gigantism and dominance, only to be followed by extinction of the large forms.
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f)
mechanisms of reproduction
spore size; asexual reproduction leading to sexual reproduction. Early land plants were homosporous; they had reproductive spores of one type, one size. An important advance in the move toward seed production was heterosporous reproduction, where two distinct reproductive cells, of differing sizes, were produced by plants. The development of sexual reproduction and of the seed allows for increased efficiency of reproduction and the increased possibility of genetic variability.
g)
coevolution with animals
The angiosperms have undergone an accelerated evolution during their 135 million year history largely as a response to the pressures exerted by insect predators. They have adapted to life with insects not only as a means for defense as is the case with the conifers, for example but also in ways which have increased the reproductive advantage and dispersal advantage of these plants. The wide variety of flower types among the angiosperms, and the extremely elaborate ways in which some species have coevolved with particular species of insects (orchids and wasps, for example) give ample evidence that they have attained a degree of success in exploiting their environments that has never been attained by other plant types.
The two sections that follow will be most useful in the teaching of plant evolution because of the attention that they give to the concept of change over time, and to the methods of fossilization of plant tissues. It is suggested that emphasizing the differences between two ecologies widely separated in time will be more effective in teaching biological change than would the in-depth tracing of any plant phylogeny, particularly with high school students. Paleobotany and plant anatomy can become very involved disciplines, and the knowledge required to fully appreciate the modifications undergone by plants throughout their evolutionary histories is beyond the level of most high school students. Class time which concentrates on some comparisons of plant forms and ecological niches occupied by these plants, as well as on methods of locating and studying preserved plants, will be most productive in this study of evolutionary biology.
The Lowland Swamp Forest of the Carboniferous and the Present Day Northern Deciduous Hardwood Forest
A very effective way to apply the concepts of time and change to the study of the evolution of plants is to have the student make direct comparisons between the floras of two widely separated geologic periods. The vegetation of the Upper Carboniferous (Pennsylvanian, 300 million years B.P.) is well known for the eastern United States, and it is fairly easy to make comparisons between the flora of this period and the flora of a modern Connecticut forest. The disturbances of man in Connecticut have eliminated essentially all virgin forest in the state, but much is known about these former hemlock-hardwood forests.
The two forests should be compared on several different levels: in terms of climate, environment, and anatomical features of representative plants. Six genera of plants which characterize the Carboniferous are:
Lepidodendron
(an arborescent lycopsid);
Calamites
(an arborescent sphenopsid);
Sphenophyllum
(an herbaceous sphenopsid);
Cordaites
(a gymnosperm);
Psaronius
(a tree fern); and
Medullosa
(a seed fern). (See Appendix, Figure 3). These genera represent a cross section of the flora common to the Carboniferous, and they are the plants which were dominant and abundant during the period.
For their similar abundance and dominance in Connecticut’s deciduous hardwood forests and for the similarity in the niches they fill in the forest, the following present day species are selected for the comparison:
Tsuga canadensis
(hemlock);
Fagus grandifolia
(American beech;
Pinus strobus
(White pine);
Kalmia latifolia
(Mountain laurel);
Oxalis montana
(wood-sorrel) ; and
Osmunda cinnamomea
(Cinnamon fern). Either
Acer saccharum
(Sugar maple) or
Quercus borealis
(Northern red oak) can be substituted for the beech for the convenience of using a more familiar species.
During the Pennsylvanian, which lasted 45 million years, North America experienced a succession of events in which land masses would rise and seas would abate, to be followed by subsiding lands and advancing seas. Inland seas occupied large portions of North America. In the eastern part of the continent as well as throughout the world the lowland swamp predominated. The lush vegetation of these swamps has been transformed over time into the rich coal deposits of Pennsylvania and Appalachia. Carboniferous plant fossils are numerous in layers above and below the coal, preserved by submergence.
The great height of the soft-wooded trees, the large leaves of many of these plants, and the excellent preservation of very delicate leaves all suggest that the climate of the Carboniferous was warm and moist. The climate is believed to have been tropical or semi-tropical, in marked contrast to the temperate climate and the distinct alternating seasons of today. Freezing winters didn’t occur in the Carboniferous.
Significant ecological differences exist between these two widely separated periods. As mentioned, the most obvious difference is that the Carboniferous forest was a lowland swamp and the deciduous forest is on higher, drier land. The Carboniferous trees (
Lepidodendron
,
Calamites
,
Cordaites
) grew in close proximity with standing water. The herbaceous sphenopsid
Sphenophyllum
may well have grown partly submerged. An important similarity, however, between the two floras is in the niches occupied by each of the plants.
Lepidodendron
(up to 100 feet tall) and
Cordaites
(50-100 feet tall) in particular formed a broad open canopy in the forest, comparable to the heights achieved by hemlock (50-100 feet), beech (75 feet), and white pine (60-100 feet), The deciduous forest probably has a tighter canopy. Intermediate levels of growth (the understory) are formed by
Calamites
(to 30 feet),
Psaronius
(20-25 feet), and
Medullosa
(12-15 feet) in the Carboniferous and by the present day example of Mountain laurel (15-20 feet).
Sphenophyllum
was a creeping, semierect plant which grew in dense clumps. Similar niches are occupied today by Wood-sorrel and by the ferns. These common features of the plants, which are all photosynthetic plants and hence producers, should be emphasized.
It is in anatomical detail that the two floras are markedly different. The coal swamps were totally lacking in deciduous trees, which had not yet made their appearance on earth. These are the dominant plants today. The most wide-spread plants of the coal swamps were the arborescent club mosses and horsetails and the gymnosperm
Cordaites
. Today it is the flowering plants. Some specific differences follow.
Lepidodendron
species were the most imposing plants in the Carboniferous forest. These “scale trees” (arborescent lycopsids) enjoyed dominance for a period of millions of years, then became extinct during the Permian. The only surviving club mosses are herbaceous plants, growing a few inches high.
Lepidodendron
had dichotomous branching, two forks of equal size always forming. The bark of this tree was two-layered and thick. Growth rings are not found in the trunk and branches.
Calamites
was another thick-barked tree. An arborescent sphenopsid, it too found extinction in the Permian. The trunk, stems, and rhizomes were jointed and had vertical ribs, a feature found today in the herbaceous
Equisetum
, the only surviving horsetail genus.
Calamites
bore aerial roots, above-ground structures found today usually among tropical trees.
Lepidodendron
,
Calamites
, and
Sphenophyllum
(the herbaceous sphenopsid) were spore-bearing plants.
Psaronius
, a tree fern, and
Medullosa
, a seed-fern, were the plants most similar in appearance to a representative plant in today’s forests, the modern fern. In many respects the fronds of all three plants bear strong similarities. What is noticeably different is that
Psaronius
was a tree-like plant, unlike any of our ferns in Connecticut. and
Medullosa
belonged to a now-extinct group of plants which combined the general appearance of ferns with a seed-producing habit.
Cordaites
was an early gymnosperm. This tree closely resembles some of the pine family members. However,
Cordaites
had broad, strap-shaped leaves, not needles. Also different is the method of producing seeds, in racemes, not cones.
The Stiles Clay Pit of Hamden
The Quinnipiac Valley has extensive deposits of red clay, laid down between 11,000 and 12,000 years ago during the close of the last glacial period. Our region of Connecticut is famous, in fact, for its New Haven clay, which has been quarried for brick manufacture for more than 125 years. Up until the last several years this area was the site of a major quarrying operation. Clay pits were dug in the meadowlands bordering the Quinnipiac River, and the Stiles Corporation brickyards with their kilns were located nearby. Many of the earlier buildings in town were built of the locally manufactured bricks.
Bordering the marshes along State Street, from New Haven to North Haven, the deposits of clay lie between 15 and 20 feet below the present land surface and well below sea level. The clay varies in thickness, due to differential erosion of the deposits more than 7,000 years ago, but various estimates of the thickness of the clay range from a minimum of 3 to 20 feet to a maximum of more than 75 feet. Close examination would reveal that the clay actually is made up of many distinguishable layers, called varves. Each varve was laid down during post-glacial time in the course of one year, the clay material that precipitated out in the winter freezes being different in appearance from the summer silts. For this reason it is known as varved red clay. There is reference in the literature to a count of 364 varves in one 17 foot thick section of exposed clay.
In order to get to the clay, workmen had to remove the covering layers of material, which were deposited in more recent millenia. In the 1800s and early 1900s they dug manually; steam shovels were used later. The upper deposits are the ones of interest to the paleobotanist, because they are rich in compacted, semi-fossilized Plant matter. The outdoor laboratory of the Stiles clay pits has received the attention of the Yale scientific community since the early 1900s. As quarrymen labored for the important clay deposits, gealogists and ecologists studied the exposed overlying strata and their contents. At certain depths the following macroscopic plant matter has been uncovered: leaves of many types, twigs, nuts and seeds, cones, stumps and lags, some of them quite sizable, and remains of salt marsh plants and peat. Microscopically, many types of pollen are found. A cursory examination of the sediment is all that is needed to realize that there are several distinct layers present and that the geological conditions under which organic and inorganic materials were deposited varied. A summary of the geological events of this region follows.
Between 10,000 and 15,000 years ago, in late-glacial and post-glacial times, a freshwater lake had formed at the mouth of the Quinnipiac River. As the glacier slowly retreated, a mass of ice broke off, became stagnant, and dammed up river water as well as its own melting water. These deep, muddy waters were responsible for the thick buildup of the clay. During the gradual warming period which ensued, the glacial ice eventually disappeared and the ponded waters were restored to a river system. There may have followed several thousand years during which silt and sands were deposited, only to be eroded away along with some of the underlying clay.
Today a four foot layer of coarse sandy material lies immediately above the red clay. It has been dated, through radiocarbon testing of an entrapped log, to 7,000 years before present. There is no indication that this sand was deposited under salt water conditions. Rather, the freshwater river, moving at a rate fast enough to carry along fairly heavy sand particles is responsible for these deposits. Logs and stumps are found throughout this layer and also in the four foot layer of finer gray sand above, apparently deposited by more slowly-moving waters. There are also layers in which nuts and seeds are common, or thicknesses of matted leaves. These important fossil-bearing strata are now located 10 to 18 feet below the surface. It is this material which shows the early stages of fossilization.
Found in this “basal alluvium” are the remains of wood, leaves, and reproductive structures of beech (the predominant species of tree), hemlock, hickory, pine, oak, and buttonwood. The logs present are not in their natural positions but instead were washed into the area, where they sank and became buried. The remnants of shrubs and of a number of herbaceous plants are also contained in the sandy layers. These findings are suggestive of a shallow stream or river environment, surrounded by a beech forest. In the upper portion of the fine sandy layer maple is found, indicating that the region changed over to a freshwater marsh for a period of time. A log in this upper layer of sand is radiocarbon-dated to 3560+80 years B.P.
All material lying above the sand is peat, and it forms a layer approximately 10 feet thick. The peat contains remains of
Spartina
and
Phragmites
, grasses. This is salt water tidal marsh vegetation, and it indicates that for the last 3,500 years the region was at sea level, and that the tidal conditions had killed off all freshwater trees, shrubs, and herbs. The peat accumulated slowly from succeeding growths of marsh grasses. The balance between sedimentation and submergence of the land (or a rising sea level) had shifted to conditions which persisted until recently.
When man intervened and constructed a dike and railroad bridges downriver, the now-protected area changed over to a freshwater marsh habitat, characterized by the growth of
Typha
(cattail). It is now referred to as an estuarine freshwater marsh. The same species growing 3,000 to 7,000 years ago in the area of the Quinnipiac River are found in the surrounding area today. A walk through the thickets that lie south of the clay pit turns up wild grape vines and raspberry canes, whose seeds are preserved in the clay pits. Beech trees, maples, and oaks also make up the thickets.
Today the New Haven clay is no longer quarried in Stiles clay pit. A series of quarried and abandoned pits are found to the east of State Street, filled up with water. The most recent site to be dug is now being used for landfill, meaning that it is a garbage dump. It will eventually revert to marshland or be filled in above sea level. Presently below sea level, the pit is protected by a dike from the nearby tidal waters and it is continually being pumped of water that seeps in from the uplands, just as it was pumped during the brickmaking period. One expanse of the pit remains where a 14 foot high cross section of the peat and sandy layers can be seen. The brick clay now lies out of view.