CHAPTER SIX
THE
TERRESTRIAL PLANTS
CHAPTER SUMMARY
a. From Chlorophytic ancestor.
b. Osmotic pressure with change from saline -> brackish -> fresh water.
c. Dehydration prevention cell walls.
d. Subaqueous. Upper part is photosynthetic lower is nutrient gatherer and assimilation is by diffusion.
e. Improved energy capture by elevating upper part with stems.
f. Development of a rooting system.
g. Improved energy capture by using leaves which increase the surface area for photosynthesis.
h. Development of a trachea for nutrients and as a bonus improved support.
i. Increased size especially height to get above competition.
3. Classification
Kingdom Plantae
The primary classification of animals is based upon structured similarity but the plants are based mainly upon the reproductive structures and processes. Whereas Animalia derive their energy and build up their tissues by feeding upon plants or other animals, the Plantae, with the aid of the green pigment chlorophyll, can absorb energy. With the absorbed light energy plants are able to transform water and carbon-di-oxide into substances such as cellulose, starch, and sugar and to synthesize from such compounds, and from nitrogen absorbed from the soil, such highly complex molecules as proteins.
These are the simplest plants that do not have any root system, such as the liverworts and club mosses. When plants adopted a terrestrial life during the Lower Palaeozoic Era they evolved an epidermis [to prevent excessive evaporation of water], a rhizoid [rooting hairs] to absorb water and nutrients from the soil, and resistant membranes around their spores [reproductive bodies]. The Bryophytes show these characteristics.
These are simple rootless plants with a trachea but without leaves. They are the simplest of the truly land plants and the early representatives lived in swampy environments.
These have true roots, stems and leaves. Known from the Silurian Period until the present day they are particularly abundant in the Pennsylvanian and Mississippian periods.
The present day Horsetail is typical of this ancient plant family which was particularly important in the Pennsylvanian and Mississippian periods.
These include the ferns are were particularly prominent in the Upper Palaeozoic and Mesozoic eras.
These have been important member of the northern and southern temperature climatic zones since the late Palaeozoic Era.
The Ginkgophyta including the extant Maidenhair tree are deciduous gymnosperms.
The true Cycads have separate male and female plants and only a few species are extant. They were very important in Gondwanaland during the Triassic and Jurassic periods, declining during the middle part of the Cretaceous Period. The related Cycadeoidea Bennettitaleans are distinct in that they do not separate into male and female plants.
The flowering plants dominate the present landscape and include about 50 aquatic families. For example: Najadaceae, found in the USA, includes 13 genera and 60 species living in the fresh, estuarine and marine environments. Hydrocharitaceae occur in the fresh and marine environments. They are often referred to as the Angiosperms.
3.Evolutionary development
The Bryophyta, Psilophyta and Lycopodophyta.
Dominance of the Arthrophyta, Pterophyta, and Coniferophyta.
Raise of the Ginkgophyta, Cycadophyta and Magnoliophyta [early Cretaceous].
Grass commenced at end of Cretaceous and began to dominate by the Miocene Epoch.
THE RAISE OF THE TERRESTRIAL PLANTS
The earliest photosynthetic organisms were the photo-bacteria. However, when the algae developed [eucaryotic protista] the diversity of life forms that obtained energy by radiation increased dramatically. Many phyla of protista evolved such as the brown algae (Phaeophyta), red algae (Rhodophyta) and green algae (Chlorophyta) amongst others. These algal groups are particularly distinctive in the types of pigments they contain. In the Chlorophyta the major pigment is chlorophyll which is used in the photosynthetic process. The general relationships between the chlorophyta and the plants indicates that the chlorophyta gave raise to the true land plants (Kingdom Plantae). It is probable that the early chlorophyta developed large aquatic communities living in fresh water - just as they do today. However, in order for them to evolve into true plants important modifications had to take place.
The first change was from saline to brackish to fresh water. Physically this is an osmotic pressure problem. The final stage was adjusting to using rain water and the use of roots. The change to utilizing fresh water probably took place in the chlorophyta and thus the earliest land plants did not have this problem.
The initial problem was one of water storage. Probably the first plants developed immersed in water. Photosynthesis using direct sunlight was possibly the driving force causing plants to become sub-aqueous i.e. with part of their structure above the water level. In order to do this they had to develope a supporting structure of stiff tissue. At the same time the development of a primitive vascular system [an internal tubular system for carrying nutrients and water] allowed efficient metabolism.
The earliest true fossil plants belong to the Division Psilophyta. They consist of horizontal stalks (rhizomes) which grow in damp soil. Coming off from the rhizomes are short vertical leafless stems which have branches and sporangia. The rhizome later developed into a rooting system but initially it was a simple stalk that absorbed nutrients and water. There is some evidence that in these earliest plants a trachea existed (find fossilized woody [lignin] tubes with cellulose in the center of the stalks when looked at under a microscope). Thus rather than diffusion an actual transport of nutrients took place via xylem tissue (upward movement of nutrients and water) and phloem tissue (downward movement of manufactured food) that were formed of elongated hollow cells.
The development of a method of overcoming dehydration was another major problem. This was done by developing cell walls. These cell walls do not occur in animal cells. In plants they consist of layers of lipids (waxes) that form a protective coating around the cell. The outer surface of the plant developed a thick covering of cells that were full of lipids and this formed an epidermis which prevented excessive evaporation. This epidermis along with the lignin and cellulose, acted as a support structure allowing the plants to grow taller. The rhizome developed not only into a root system for gathering nutrients but also as an anchoring system.
The Division Bryophyta is a group of leafless plants that do not have true roots (e.g. liverworts[ which show sexual differentiation], and club mosses) but have rhizoid, epidermis and reproduction spores that have a thick cell coating (called an exine).
During the Late Silurian and Lower Devonian a root system and primitive leaves developed and there was an improvement in the trachea. The earliest forms with true root, stems and leaves, were the Division Lycopodophyta. From the early Devonian on plants truly began to conquer the land. Initially the Lycopodophyta, then the Division Arthrophyta [e.g. the present day Horsetail plant which was very abundant and huge in the Pennsylvanian and Mississippian forests], and then the Division Pterophyta (Ferns) which also were extremely abundant in the Middle Paleozoic. From the Division Pterophyta the Division Coniferophyta arose. These are the evergreen conifers and have dominated the colder regions of earth since the late Paleozoic.
During the Mesozoic Era a number of new divisions arose such as the Division Ginkgophyta, and Division Cycadophyta. However, the most important development was the development of the Division Magnoliophyta [the flowering plants or Angiosperms]. From the early Cretaceous until today this Division increased and the flowering plants dominate our landscape today.
After the origin of the flowering plants the only major evolution development was the evolution of grass at the very end of the Cretaceous Period. Grass was rare during the Paleocene Epoch but took over by the Miocene Epoch, eventually invading the fresh, brackish and saline marshlands and even invading the marine environment.
Once the hurdle of dehydration and adequate nutrition was overcome the plants were able to dominate the terrestrial landscape. The real major changes from an evolutionary viewpoint were related to better reproductive methods.
The seedless spore bearers
The simple Bryophyta have reproductive cells [spores] that were all of one and the same appearance (called Homospores). These spores develop into small gametophytic haploid phase plants with antheridia [male] and archegonia [female]. Flagellated sperms are produced by the antheridia and swim to the archegonia causing fertilization and the formation of a zygote. In the archegonia the small plant originates (sporophytic diploid phase) which then produce sporongia with homospores. The Psilophyta, Anthrophyta and Pterophyta similarly produce homospores. In the more advanced plants such as the Lycopodophyta some primitive forms only produce homospores but in general two kinds of spores are produced: microspores and megaspores. These spores are borne on the leaves. In the Anthrophyta the homospores are borne on cone-like structures separated from the leaves.
The flowerless seed bearers
The next stage was the development of the seed producing plants that did not have flowers. Out of the Pteridophyta came a group called the seed ferns, which produced protected seeds on their leaves [Glossopteris, Pecopteris, Neuropteris]. The Coniferophyta are the naked seed bearers with their seeds on cones.
The flowering seed bearers
The Magnoliophyta carry enclosed seeds.
Classification
Phylum Bryophyta
Phylum Psilophyta
Phylum Lycopodophyta
Phylum Arthrophyta
Phylum Pterophyta
Phylum Coniferophyta
Phylum Ginkgophyta
Phylum Cycadophyta
Phylum Magnoliophyta
Evolutionary development
The raise of the land flora
The first vascular land plants are found in the Silurian System.
End of Paleozoic.
The cause of the massive extinctions of marine organisms during the Permian Period affected the terrestrial biocoenosis to a lesser degree. The movement of the flora inland provided a new food source for the vertebrates and the early dinosaurs [and mammals] evolved as they followed the food source inland. Plant groups that could not adapt the the rapidly changing conditions were eliminated. Fresh water loving forms such as many of the Arthrophyta and Bryophyta became extinct. Within Gondwanaland the tree fern Glossopteris was prominent.
The Coniferophyta and Ginkgophyta that had become important in the Late
Paleozoic Era evolved rapidly and they began to dominate the temperate and
higher latitude climatic zones during the Paleozoic - Mesozoic
transition periods.
Mesozoic.
By the Triassic the Coniferophyta [Cheirolepidiaceae], Ginkgophyta e.g. Baierophyllites, and Seed Ferns e.g. Caytonia, Denkania, Dicroidium, and Glossopteris.began to dominate, especially inland. Cycads such as Pterophyllum, and Zamites, and the related Bennettitales became abundant in the Triassic. Another extinction period during the late Triassic terminated the phylogeny of the Glossopteridians as it removed many of the remaining marine families such as the Conodonta, most marine reptiles and the labyrinthodont amphibians,. As Laurasia separated from Gondwanaland the northern temperate zone became the dominated by the conifers e.g. Araucarioxylon and the southern continent by seed ferns such as Dicroidium
Important Coniferophyta of the Mesozoic Era included Araucaria, and Pinus, in addition to forms such as Cypress and Redwoods of the present day.
Important Cycadophyta of the Mesozoic Era included Bowenia, Cycas, Leptocycas, Microcycas and Zamia .
The Ginkgophyta peaked during the Jurassic and Cretaceous periods and were probable radically influenced by the evolution of the earliest flowering plants which are clearly present in the Cretaceous System but probably had their origin during the Jurassic Period.
As the Magnoliophyta underwent an explosive burst they competed with the other major phyla. They have dominated the flora since the Cretaceous Period and the development of the larger carnivorous dinosaurs such as Tyrannosaurus has been related to the abundance of plant food supply for the herbivorous forms. The herbivores expanded with the food supply and the carnivores [approximating a 2:1 relationship] grew with their food source e.g. Ankylosaurus, Homalocephale, Triceratops, Pachycephalosaurus and Lambeosaurus.
Numerous plants have been found in the Cretaceous Period prior to the mass extinction of the dinosaurs, that can be directly related to modern flora. These include: Alnus, Aralia, Cornus, Dryophyllum, Ficus, Laurus, Magnolia, Metasequoia, Palmus, Pinus, Populus, Podocarpus, Quercus, Salicaceae, and Tetrastichia.
The vertebrate terrestrial fauna evolved alongside the plants. The amphibians were eased out once the dinosaurs commenced their evolution during the middle part of the Triassic Period. These early herbivorous dinosaurs such as Heterodontosaurus, Lesothosaurus, and Plateosaurus found an abundant food supply with little, if any, competition. The Coniferophyta probably dominated the diet of many of the terrestrial vertebrates of this time. However, the abundance of the fast growing, ground level foliage supplied by the Lycopodophyta, Pterophyta and Arthrophyta were an abundant source of food for herbivorous vertebrates foraging in the wetlands.
Cenozoic.