Cytology. Blue-greens resemble
bacteria and their cells are usually less than 10um, whereas greens range from
10-100um. Blue-greens have no internal membrane system except for a
photosynthetic membrane, they are prokaryotic. Greens are eukaryotic and have
all the usual membrane bound organelles; nucleus, golgi etc., and their DNA is
organised into tightly packed chromosomes, while blue-green DNA is naked.
Blue-greens contain chlorophyll A and carotene, greens contain chlorophyll A
and B and carotene.
Morphology. Blue-greens occur in single cell, grouped cells, and filamentous forms; but compared to greens their size range is very small. Greens have a very varied morphology ranging from unicellular, flagellate Chlamydomonas, to motile, colonial Volvox, non-motile Pleurococcus, filamentous Spirogyra, to larger parenchymatous forms such as Ulva.
Reproductive characteristics. Because the blue-greens are prokaryotic, they reproduce asexually by cell division, fragmentation, or spore production. Genetic recombination has been observed, but the mechanism is unknown. Greens reproduce both sexually and asexually. Asexually by fission, mitosis, and zoospore production. Sexual reproduction is iso-, aniso-, or oogamous.
Allogenic succession is driven by external influences which alter conditions e.g., salt marsh to woodland. Autogenic succession of species is driven by processes within the community itself e.g., shallow pond to bog to scrub.
Primary succession occurs on land that is new and has never had a flora and fauna e.g., glacier retreats, lava flows. Secondary succession occurs on land that has been cleared e.g., by fire, of flora and fauna, but which still has viable seeds and spores in the soil.
Around 3000 million years ago
(MYA) in the early Precambrian, the only organisms were prokaryotic. Eukaryotes
did not evolve until 1200-1900 MYA, the dates are uncertain, but anyway there
was at least 1000 million years of only prokaryotes.
It is thought that eukaryotes evolved from prokaryotes by a series of steps involving symbiosis between various prokaryotes to form a large eukaryote containing various plastids. This might explain the size difference, prokaryote cells are usually 1-10um, while eukaryote cells are usually > 10um Prokaryotes do not have the distinct membrane-bound nucleus that eukaryotes have. Prokaryotes generally reproduce by binary fission (a very simple method), whilst eukaryotes reproduce by various forms of mitosis. This means that the DNA of eukaryotes is highly organised. It is wound around histones and organised into chromosomes, and when the cell divides, centrioles pull on copy of each chromosome into each half of the dividing cell. Meiosos means that sexual recombination is possible in eukaryotes, this may be why they have been more successful, as there is more scope for mutation. Prokaryotes evolve only by point mutation, and their DNA is naked.
Some prokaryotes are strictly anaerobic, perhaps a legacy from when there was no O2in the atmosphere. Whereas most eukaryotes are aerobic, the few anearobic ones appear to have been later evolutions from aerobic eukaryotes.
Both may have flagella, but the prokaryote flagella are always simple, whilst the eukaryote follows the 9 + 2 pattern. Prokaryotes do not have the various membrane-bound organelles that eukaryotes do e.g. chloroplasts, golgi, mitochondria, these plastids perform specific tasks.
|Cell wall||Yes||Yes in plants, no in animals|
|Nucleus||Lacks nuclear envelope||Bounded by nuclear envelope|
|Chromosomes||One, DNA||Multiple, DNA & protein|
|Endoplasmic reticulum||No||Nearly always|
|Glycosomes||No||No in animals, often in plants|
|Plastids||No||No in animals, often in plants|
|Cilia/flagella||Simple||Complex (9 + 2)|
|Centrioles||No||No in plants, yes in animals|
Diversity can be measured by a number of indices e.g., Simpson's. Species richness simply means the no. of species in a given area. Diversity takes into account the proportion of each species. For example a community might have a richness of 20, but only a very small diversity if 1 species was dominant and accounted for 90% of the total. Another community with 5 species would have a higher diversity rating if each species accounted for around 20% of the total.
A plant benefits if its seeds are dispersed, and it loses if its seeds are rendered non-viable.
Benefits by dispersal, a seed may escape predation, fire, pathogens, competition from the parent plant by transportation to a site suitable for seedling growth. Most seed-bearing plants that rely on dispersal by animals make their seeds attractive in some way to the animal.
Elaiosomes are lipid rich and are eaten by ants. The ants generally do not eat the seed, though some do get harmed. The ants transport the seed to their nest, so the seed escapes predation, fire and pathogen attack, and it is placed in a nutrient-rich medium. Myrmecochory is most commonly found in the early flowering herbs of the understorey of N. temperate mesic forests. In one study one third of such plants in a USA wood had elaiosomes. Myrmecochory also occurs in sclerophyllous vegetation in the Mediterranean-type ecosystems, e.g., 1500 species in Australia, and 1200 species in the S. African fynbos.
Elephants and Acacia spp. 90% of acacia seedlings germinate in piles of elephant dung. As the elephant eats many seeds do get crushed, but many pass through its gut where the high pH kills predators and pathogens. Then the seeds pass out the other end with a nutritious pile of dung. Most seeds left uneaten are directly under the parent tree and are eaten by weevil larvae.
Rhino and Indian grasses. Rhinos defecate in open places where there is no danger of ambush. They eat grasses, the seeds of which need light to germinate. So the rhino deposits them on nice open ground away from their parent and, like the elephant, in a dollop of dung.
Losses of the interaction of seeds with animals is that seeds often get eaten and not passed out with a dollop of dung.
Harvester ants collect seeds, mainly grasses, which are not equipped with elaiosomes. Most of these seeds are eaten. Plutarch first recorded how the ants bit off the radicle to prevent germination. However in the moist environment of the ant's nest some do geminate. These are taken outside and placed on refuse piles. It is this act that gave rise to the erroneous belief that ants plant seeds. The seeds grow quite happily in the refuse pile.
In fact in most cases of seed predation there are mistakes where the seed is left to germinate. The Brazil nut relies on the mistake of the agouti for its very survival. Jays and other birds bury nuts, but forget the location of a few. The cost to the plant is that it must produce an excess of seeds to rely on the mistakes. But an oak tree need only have a few of its 30 000 acorns germinate each year to be regarded as a success. The only time seeds really do seem to be lost is through attack by boring insects such as weevils. these insect live inside the seed and once established the seed is killed.
|Both are radially symmetrical|
|Both have their body parts arranged symmetrically around the mouth|
|Both have a gelatinous body|
|All Cnidaria have nematocycsts; only 2 species of Ctenophore have|
|Ctenophores have comb plates; Cnidaria do not|
|Ctenophores are never colonial; many Cnidaria are|