The Plant Kingdom
We can view the evolution of plants in two dimensions. The first is the variety of plants seen today - see Figure 1 **.
These range from the simplest - the
prokaryotic cells of blue-green algae - to the most complex and varied group -
the angiosperms - the flowering plants.** Figure 1 was devised as a way to reduce the number of words in the essay to below the limit specified for the assessment. A picture can replace a thousand words!
We will meet these groups in turn as we consider the second dimension, that of time. The general trend is from simplicity to complexity, but this is overlaid by other changes as species have 'adapted' to meet new conditions, and as they have found advantages in the inherent variation which takes place within communities and populations.
What we see today are the successful journeys. Some are a struggle to survive, while others have found an easy road where the qualities of the organism fit well with its environment. The Angiosperms - the flowering plants - are one such success story, but we must start our journey a long way before they appear.
Geological time - Part 1
Our time dimension begins when the earth was formed some 4,700 million years ago - see Figure 2.
The largest portion of geological time - the 'Pre-Cambrian' - was once thought to be entirely devoid of 'life'. Recent work has taken our knowledge well back into this era and shown that fossils are rare because the organisms of the day were not readily preserved.
Life - whatever our definition - began in the seas. The first 'life forms' consisted of carbon compounds created 'inorganically' which would naturally form chains and membranes. At some stage such membranes formed into simple cells, the precursors of the prokaryotes. There was no free oxygen, and energy was obtained by breaking down organic material anaerobically using a process such as fermentation. Such organisms would be analogous to our present day bacteria, and signs of them have been found in rocks up to 3,600 million years old.
Further evidence is found at 3,000m years in stromatolites, calcareous structures now found associated with blue-green algae. By then the organisms had developed more sophisticated chemistry, using free oxygen, produced by photosynthesis, to obtain energy more efficiently by 'respiration'. Changes in these sediments show that oxygen was present in significant quantities about 2,000m years ago .
These early cells would have replicated by binary fission, as do modern prokaryotes. This provided little scope for diversification, and little change is found over a period of 2,000m years - almost half of the age of the earth.
At about 1,500m years larger micro-fossils are found, suggestive of modern eukaryotes - more organised cells with a true nucleus. Life was still restricted to the sea, but now had the potential for more widespread variation. These fossils are preserved only because of fortunate circumstances - such as the formation of flints - and until the end of the Pre-Cambrian both the animals and the plants lacked significant hard parts. Up to this time, the difference between animals and plants is largely one of interpretation. As in the modern Euglena, features of plants - photosynthesis - and those of animals - mobility - appeared in the same organisms. Only as organisms became multi-celled and more complex did such features separate. The plants became less motile, but certain stages of their life cycle - the gametes - often retained this facility, and it is still found in algae and the lower plants.
Geological Time - Part 2
From the Cambrian onwards, geological time is divided on the basis of the fossil record - see Figure 3.
Unfortunately for the botanist,
this record is primarily that of animals and the names of the Palaeozoic,
Mesozoic and Cenozoic - Ancient life, Middle life, and New life - are based on
changes in the animal communities.No significant change in plants occurred until the Ordovician when fragments found in Poland and the Appalachians suggest that plants had made the move from the sea to land. The first well preserved examples are from the Silurian of Australia. These were spore bearing plants with a cuticle and a primitive vascular system, some resembling the Club Mosses which became abundant in the Devonian.
The move to land would have had significant effects on the plants since the higher light intensity would increase the efficiency of photosynthesis. This would change the demands for raw materials - water and carbon dioxide - and increase the levels of carbohydrates and waste products to life threatening levels. The plants thus had greater potential, but also needed the variation caused by adaptive evolution to survive.
Waste products no longer diffused away. Some, such as cellulose and lignin were used instead to thicken cell walls. When such cells died, their walls persisted and were the basis of the vascular system which offered evolutionary advantages including physical support and the movement of water and food.
The plants diversified through the Devonian, became increasingly woody, and left increasing numbers of fossils. These include possible Bryophytes, but given the nature of their modern equivalents, it is hardly surprising that they are not well preserved. The first significant fossils therefore represent the Tracheophytes - the spore bearing Lycopods or Club Mosses; Sphenopsida, related to modern Horsetails; and Filicineae, the true ferns. Their origins, possible from early bryophytes, remain obscure and no modern intermediate has been found.
These spore bearing plants formed the Carboniferous coal swamps which also included the first 'seed' bearing plants - the Pteridospermophyta. Though resembling ferns, they produced nut-like seeds rather than spores. Together with the first conifers - the Cordiates - at the end of the Palaeozoic the seed bearing plants emerged as a dominant group.
At the end of the Carboniferous, and into the Permian, the floras of the northern and southern hemispheres diverge and imply contrasting climatic conditions. In the southern hemisphere the flora is typified by Glossopteris, in which the reproductive parts are partially enclosed in a structure not unlike the carpel of the angiosperms.
The few fossil plants found in the Triassic - the first period of the Mesozoic - show no significant change other than in the cycadeoids which reproduced by 'pseudo flowers' - unlike surviving cycads which have true separate male and female flowers. The cycadeoids became extinct in the Cretaceous and appear to have been an early experiment on similar lines to the later angiosperms.
The Jurassic has an abundant flora including conifers, cycadeoids and ferns, and includes the first sign of angiosperms as questionable pollen grains in a Jurassic coal at Brora in Scotland. Flower-like structures are found in the Caytoniales of the Yorkshire Jurassic, but the first undisputed angiosperms do not appear until the lower Cretaceous - a mere 130m years ago - when they rapidly became widespread, possibly favoured by climatic changes and marine incursions which modified the pattern of ecological niches.
Reasons for Success
The success of the angiosperms is based on the ability to adapt and to make use of a wide range of external factors - their 'plasticity'. These abilities include:
