Rhynia major.It has a prostrate axis with rhizoids, the vertical axis branches dichotomously. Both have a central vascular strand of thickened cells. The main axis has stomata and a thick cuticle. It has no leaves. The terminal sporangia release spores by splitting longitudinally. It reaches a height of about 50cm. The gametophyte is believed to be Lyonophyton. Asteroxylon also reached about 50cm in height. It was covered in leaf-like scales, but as the vascular tissue did not enter the scales, they cannot really be called leaves. Unlike R. major the vascular tissue was not circular, but star shaped and extended out towards the scales. It had axillary sporangia, so growth of the main axis could continue, i.e. apical growth was not disturbed. Trimerophyta and Rhynia are seedless. Asteroxylon may have had the ancestor of leaves or needles. Progymnos had more elaborate branching and vascular system, some may have had fern-like leaves, others had branching systems resembling conifers, they also had pith, most were homosporous, but some were heterosporous, some were relatively large. They lacked the seed habit that gymnosperms and seed ferns have
Ancestral green algal
cells were photosynthetic, aerobic, and probably had a motile stage in
their life cycle.
There were various types of prokaryotic cell, and it is
believed that a series of symbiotic relationships between various combinations
of these cells led to all the eukaryotic life we see today.
An ancestral green algal could have evolved in the following manner.
As photautotrophic respiration increased the O2 content of the atmosphere built up making the air toxic for anaerobes. If an anaerobic heterotroph ingested, but did not digest an aerobic heterotroph, then it would survive. The aerobe would gain by having a controlled environment inside the anaerobe. The aerobe would function like a present day mitochondrion. Result - an aerobic heterotrophic eukaryote.
Mobility. An amoeboflagellate organism could have joined the above. Similar to today's slime moulds. The mobility would have given greater access to food, an advantage for a relatively large organism.
Photosynthesis. Photoautotrophs were responsible for increasing the O2, so must have been quite common. If one formed a symbiotic relationship with the mobile aerobic eukaryote, it could function as a chloroplast, and the whole thing could be classed as an ancestral algal cell.
Statistical analyses of real food webs have revealed some general patterns. It was believed the maximum food chain length would be around 5, as over 90% of energy is lost at each step, but in practice much longer lengths have been found. 5 is average for Ythan Estuary in Scotland. 11-15 is the maximum for tropical rainforests. The prevalence of omnivory was thought to be rare as it would cause extinction. In practice 30% in Ythan Estuary and 90% in a valley in Arizona were omnivorous interactions. There is no relationship between s and c.
Are found mainly in fresh water, though some are marine, and some terrestrial in moist situations e.g. north side of a tree trunk. They have chlorophyll a and b as well as xanthophylls. Their food reserve is usually starch and their cell wall components are mainly cellulose and hemicellulose. Their reproduction is varied, both sexual (iso-, aniso- and oogamy) and asexual. Their morphology is also very varied ranging from flagellate, unicellular Chlamydomonas to motile colonial Volvox, non-motile Pleurococcus, filamentous Spirogyra, to larger parenchymatous forms such as Ulva. It is believed that land plants are descended from green algae.
The 5 unique characteristics of angiosperms are:
1)Ovule enclosed in maternal tissue 2)Growth of pollen tube penetrates maternal
tissue 3)Double fertilization 4)8 nucleate embryo sac 5) Companion cells formed
from the same mother-cell as sieve tube
Perhaps the most important advantage of an enclosed ovule is protection from herbivorous insects eating the flowers and leaves. The ovules are protected till they are fertilised, and after fertilization while they enlarge and develop. Later the ovary (fruit) might aid in dispersal by being attractive to animals, which eat it and distribute the seeds; or it may burst open scattering the seeds. Distribution was vital to take advantage of changing conditions that occurred when Panagea broke up.
A closed carpel also stops self-pollination, encouraging outcrossing, leading to genetic variation and a greater chance of mutation; another great advantage during changing conditions. There is also some protection from desiccation; again an advantage in the drier conditions that followed when the monsoonal weather patterns ceased.
2)Pollen tube penetrating maternal tissue.
As mentioned above this is an incompatibility mechanism, preventing self-pollination and the entry of unsuitable pollen, i.e. pollen from another species. The increased outcrossing increased genetic variation and form enabling angiosperms to colonise new areas and evolve relatively quickly, coping with the rapid changes.
Leads to the production of endosperm or swollen cotyledons, these are food reserves, and contain growth regulating hormones. The originally triploid endosperm undergoes repeated mitosis. The endosperm of some seeds is very abundant, e.g. cereals; and situated next to the embryo there is no impediment to the transfer of metabolites, so enabling rapid development compared to gymnosperms. A great advantage when colonising new ground.
Companion cells have dense cytoplasm and a prominent nucleus, while the sieve tube has no nucleus. It is not known what advantage this may have conferred on angiosperms, but it was probably physiological; possibly better transportation of photosynthate . The angiosperms became dominant at a time of relatively rapid change and diversification of habitats. It is their ability to change and diversify rapidly, both phenotypically and genetically that enabled them to spread and become dominant.
Most of its life is spent as a haploid mycelium where the individual compartments may contain one or more nuclei.
Sexual production of spores
For plasmogamy to take place the mycelium forms a coiled ascogonium containing a long thread-like hyphae (trichogyne). Antheridia are produced in a similar way. When the trichogyne contacts a suitable antheridium the cell walls dissolve and the contents of the antheridium enter the ascogonium. A small strand of dikaryotic mycelium containing 2 haploid nuclei is produced (ascogonius hypha), the nuclei fuse to become the diploid ascus initially, then undergo meiosis and mitosis to give 8 haploid nuclei in the ascus. At maturity the ascus bursts releasing the spores. Asexual reproduction takes place when the vegetative mycelium produces conidia containing haploid spores. Most reproduction is asexual, sexual usually takes place in times of environmental stress as the developing asci can provide protection during cold etc. and release spores under more favourable conditions.
Basidiomycotina often form relationships with trees. Their spores are produced from fruiting bodies usually above ground. These fruiting bodies are dikaryotic, formed from 2 primary mycelia fusing, usually by "clamp connections". The 2 nuclei at the hyphal tips of the gills fuse (karyogamy), then undergo meiosis, resulting in 4 daughter nuclei. The cell forms 4 extensions (basidiophores) into each of which a haploid nucleus migrates. These spores are pushed out and germinate to form haploid hyphae.
Continental islands usually suffer some extinctions as they are supersaturated at separation. Once equilibrium is reached extinctions usually balance immigration. Extinctions are few at first because there are few species and plenty of resources, increasing later as more species go extinct and resources and niches are occupied.
Insects do not regulate their water balance by sweating or panting, fluid loss is regulated mainly by the rate of secretion of the Malpighian tubules and absorption in the rectum. When they have excess water a diuretic hormone is released which increases the volume of urine entering the gut. When water is in short supply due to environmental conditions, or to the type of food intake, insects are able to survive by absorbing water fro the rectal lumen, even against a concentration gradient. Some insects, e.g. locusts can decrease their respiratory loss six-fold to minimize water loss.
Salvilia molesta is a floating aquatic fern native to South America, but spread accidentally since 1939 into many tropical rivers, lakes and canals. Its growth is favoured by warm, nitrogen rich water, and it can spread vegetatively. It has no natural enemies outside South America, so rapidly became a serious pest completely blocking waterways and disrupting the livelihood of people who depended on the water for transport, irrigation and food, especially fishing, rice and sago palm. The problem was especially bad in the Sepik River in Papua New Guinea where some villages had to be abandoned as the locals could no longer make a living. By 1980s 2000 km2 of water surface was covered. This was too expensive for manual, mechanical or herbicide removal. Biocontrol was the answer.
In Brazil the fern is controlled by a weevil, but it appeared that certain Salvinias have their own species specific weevils. So the Salvinia had to be correctly identified, then its specific weevil identified. This took until 1978. Previously the Salvinia had been mis-identified. Once the correct weevil had been found and released success was rapid. In Papua New Guinea the villagers were able to re-occupy their villages and live their lives as they had done before. In Sri Lanka the economics of bio control were studied in detail and a cost/benefit analysis was done. Returns for financial investment were 53:1, and labour investment was 1678:1. the team responsible for the ecological research leading to the bio control were awarded the UNESCO Silence prize in 1985. It was recognised that taxonomists had made essential contributions by establishing the true identity of the salvinia and the weevil. The weevil in question was new to science.
Garadoume is located in a river valley, the river is dry for part of the year. the problems faced are lack of water and soil erosion. Bankettes, low stone dykes, were built on the ground raised by a plough along the contours of a very gentle slope. The bankettes were made every 40 metres, and they help to catch silt etc. Crops can be planted within 15 metres of the bankette catchments area. Across the valley floor windbreaks were planted every 100 m to stop topsoil erosion and provide fuel wood. Crops can be grown between the windbreaks. The bankette terraces were made by women's groups. Along the contours of the very gentle slope crescent-shaped mounds of stones help to increase infiltration of water and the trees planted in the depressions help to stabilise the soil. The whole programme was very labour intensive and the locals were involved at every step. Most of the work and organisation was carried out by women. The latest danger is that gravel and sand from the dry river bed is being taken by a big construction company. The women say this makes the river flow faster and therefore there is less water infiltration. They have the support of the local chief, and have dug deep pits in the riverbed to make it impassible to trucks. All of this work has cemented a good community spirit and given the women a feeling of power and achievement.
Genetic drift - Random changes in a small gene pool due to sampling errors in the propagation of alleles.
Gene flow - Change in gene pools due to immigration or emigration of individuals between populations.
Mutation pressure - Change in allelic frequencies due to net mutation.
Nonrandon mating - Inbreeding or selection of mates for specific phenotypes (assortative mating) reduces the frequency of heterozygous individuals.
Natural selection - Differential reproductive success increases frequencies of some alleles and diminishes others.