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.
Chlorophyceae a re 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
1) Closed carpel
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.
3) Double fertilisation
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.
5) Companion cells
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.