Evolution, classification & naming of plants
Evolution of plants.
The earliest forms of life, bacteria, appeared about 3,500 – 3,800 million years ago and they make up
the baseline Kingdoms Archaebacteria and Eubacteria.
These are the Prokaryotes – microscopic, unicellular organisms with no nuclear membrane, no mitochondria,
circular DNA and they reproduce asexually by binary fission (mitosis).
Mainly bacteria and blue-green algae (Cyanobacteria) they are still the most abundant form of life.
At some stage the bacteria became able to obtain energy by photosynthesis.
It is likely that multi-cellular organisms – the Eukaryotes – appeared when some bacteria developed a
symbiotic relationship within a host bacterium.
They evolved about 2000 mya.
Their cells have linear DNA in a membrane-bound nucleus.
The great diversity among the Eukaryotes followed the appearance of a sexual form of reproduction or
meiosis where genetic information can be resorted among the chromosomes.
This eventually led to the formation of 3 new Kingdoms – the plants, animals and fungi.
Plants are multi-tissued organisms that are immobile, can manufacture their own food and have
cellulose in their cell walls.
They have complex reproductive organs and alternating vegetative and reproductive generations.
The earliest life on earth appeared about 4250 million years ago (mya).
Land plants began developing from green algae about 500 – 600 mya or earlier then began to
diversify from about 430 mya.
Flowers (modified leaves), only seen in the angiosperms, appeared around 130 – 200 mya.
Botany is the study of plants.
Classification of plants.
About 1.8 million organisms have been scientifically named and it is estimated that the total
number of living organisms could be around 9 million.
Classification is the sorting of these into groups based on shared characteristics such as
similarities in their form, reproduction or genetics.
There are many different methods of classifying plants. Common ones are:
1. By Clades.
A clade is a group that consists of a common ancestor and all its lineal descendants.
A cladogram is a diagram of the whole family tree.
The common ancestor may be at any level and may be extinct or extant, known or unknown.
Each branch splits into smaller branches reflecting evolutionary history as populations evolved independently.
Initially cladograms were morphologically based but increasingly they are now more genetically based.
2. By the presence or absence of specialised vascular tissue.
The vascular tissue is the phloem and xylem, which transport sugars, water, and minerals throughout the plant.
a.) Non-vascular plants are the Bryophytes – the liverworts, hornworts and mosses that
reproduce by spores formed in capsules.
b.) Vascular plants are the Tracheophytes.
the Pteridophytes – Ferns, Club mosses etc. have spores in specialised sporangia.
the Gymnosperms such as cycads and conifers have naked seeds in a cone.
the Angiosperms or flowering plants have seeds enclosed in an ovary.
3. By rank.
This is the classification most commonly seen.
Ranks reflect plants common features and/or ancestry.
The International Code of Nomenclature for plants says the most important ranks are Kingdom,
Division, Class, Order, Family, Genus and Species which is usually the smallest
group that can be consistently identified.
The Plant List and the Angiosperm Phylogeny Group (APG) use family as the highest rank.
The ranks may change as new information about relationships is discovered.
Any number of other ranks can be placed anywhere below the rank of Kingdom.
In most cases organisms in the higher ranks arose earlier than those in lower ranks.
Ranks are relative and depend on the particular classification scheme used.
For example liverworts have been classified as a family, class, order and division.
As new information emerges the ranks can change.
Naming of specimens.
Naming is the allocation of a unique name to a specimen be it a bacterium, a fungus, a lichen or a plant.
The rules are laid out in the “Codes of Nomenclature” issued by the International organisations for each Kingdom.
The rules are basically similar for all Kingdoms with minor differences which are not discussed here.
1. All names must have 2 parts – the genus name and the species name.
The first letter of the genus is capitalised and the rest is in lowercase.
2. Names are in Latin.
3. In hand written work the name is underlined.
4. In printed publications the name is in italics.
5. For the first use in an article etc. the genus and species names must both be given in full.
If the name is repeated in that article the genus name can be shortened to its initial and a
full stop e.g. Agaricus bisporus for the first use and then A. bisporus for later uses.
Species names are usually descriptive of the specimen e.g. colour, shape e.g. Asparagus densiflorus
or the person who first described it e.g. Asparagus macowanii.
6. Cultivars, from selection or hybridisation, are shown with an ‘x’.
If both parents are known the ‘x’ goes between the 2 names e.g. Magnolia x soulangeana.
A formally named hybrid has the ‘x’ before the species name which is written with
the first letter capitalised and in inverted commas e.g. Citrus limon “Lisbon”.
7. Varieties within a species are sometimes put in a separate category – subspp. for a sub-species,
var. for a variety and f. for a form.
8. Name endings denote the rank.
Division names end in …phyta in Botany (…mycota in Zoology).
Class names end in ..opsida.
Order names end in …ales.
Family names end in …aceae.
Abbreviations used include:
incertae sedis – the classification is uncertain or unknown.
s.l. or s. lat. – sensu lato (in the broad sense).
s.s. or s. str. – sensu stricto (in the narrow or strict sense).
sp. – species or spp. (plural). Used when the specimen/s are only identified to the genus level.
syn. – synonym. Used where a specimen has previously been known by different names.