How Do We Classify the Living World

7/28/2019 How Do We Classify the Living World

1/5

How do we classify the living world? Page 1

Notes on Biology

How do we classify the living world?

Life used to be simple! Early scientists classified organisms as either Animal or Plant.Animals moved, had nervous systems and showed heterotrophic nutrition, among

other features. Plants, in contrast, were photosynthetic with a cell wall enclosing the

cytoplasm. Bacteria and fungi aren't usually photosynthetic but as they have cell walls

they were looked upon as being plant-like. (We now know their cell walls are really

quite different.) In this simple system of plants and animals, "plants" which had no

recognisable shoot/root/leaf regions were said to have a body termed a thallus and

were termed Thallophytes. In this earliest classification system bacteria, fungi and

algae were all put into this grouping.

There is, however, a more fundamental divide than all that. We now know that based

on cell structure we can divide organisms into prokaryotes and eukaryotes. Theeukaryotic cells (e.g.of plants, animals and fungi) have a nucleus, internal membrane

systems (i.e. organelles like mitochondria) and distinctive 80S ribosomes. The bacteria

show the radically different prokaryotic organisation without internal membranes, with

naked DNA in place of a nucleus, with 70S ribosomes and with different flagellar

structure.

Today we recognise;-

Monera (Prokaryotes) - with their radically different cell structure, namely the

bacteria including the blue-green bacteria (once called blue-green algae).

Plants - photosynthetic (mainly terrestrial) eukaryotes with cell walls

Animals - motile, heterotrophic eukaryotes, the cells of which are not surrounded

by cell walls

Fungi - non-motile, cell wall-bound, spore-bearing eukaryotes with a saprophytic

or parasitic mode of heterotrophic nutrition

Click here for more on fungi

A fifth kingdom is also recognised by many scientists ;-

Protista - defined really as none of the above! It comprises eukaryotic

microorganisms and their immediate descendants, viz. protozoa, slime molds,

algae.


2/5


Taken together these constitute the Five Kingdom Classification of Whittaker

(1969)

Cavalier-Smith (2004) has proposed a Six Kingdom System

Prokaryote EmpireBacterial Kingdom

with their radically different cell structure, including the blue-green bacteria (once

called blue-green algae).

Eukaryote Empire

Plant Kingdom - all land plants as well as green and red algae

Animal Kingdom - motile, heterotrophic eukaryotic organisms, the cells of which

are not surrounded by cell walls

Fungal Kingdom - non-motile, cell wall-bound, spore-bearing eukaryotes with asaprophytic or parasitic mode of heterotrophic nutrition

Click here for more on fungi

Protozoan Kingdom- eukaryotic, motile unicells

Chromist Kingdom - includes brown algae, golden algae, yellow-green algae,

diatoms, water molds

Many botanists have a problem with the idea that algae, especially green algae, are not

considered plants. This six kingdom system at least places green algae within the plantkingdom. A similar view unites all Green Plants in the Tree of Life Project. In fact,

the Tree of Life Project which is an international collaborative project recognises that

while Plants, Fungi and Animals are clear Eukaryotic evolutionary lines, unravelling

the interrelationships of the other Eukaryotes ('Protists') is very much a work in

progress. Part of this story is the endosymbiont hypothesis of how plastids have

evolved and been passed on within these groups.

An Eight Kingdom system has also been proposed. If only monophyletic groups an

ancestor and its descendents should be accepted as forming a kingdom then these

groups may be quite unlike traditional kingdoms. You could end up with animals andfungi in one kingdom! The question is How many Kingdoms?

In this course, we will not worry too much about classification systems and will focus

on green algae and survey other major algal groups in order to get a better


3/5


understanding of how land plants evolved.

What is a plant?

All the features of plants can readily be understood in the context of their autotrophic

mode of nutrition. If plants simply transform light energy into chemical energy then all

that is required is for the plant to be anchored firmly in one place with a maximum

surface area to capture sunlight. There is no need for plants to move or to have

sophisticated nervous systems such as animals need to enable them to find food. (By

analogy, have you ever seen a solar water heater that moved? Its not an attribute that

householders feel to be necessary!)

Far from being boring and just sitting there and doing nothing plants by their subtlety

are quite intriguing.

Because they cannot move, plants must be extremely responsive to

environmental cues and have their life cycle in perfect synchrony with the

seasons. Plants must tide over inhospitable dry seasons or winters as seeds, bulbs

etc or in the case of some trees in a "bare-bones" leafless state. A plant cannot

simply pick up itself like an animal and get out of the rain!

Because they cannot move and because it is their lot in the web of life to be eaten

by animals, plants have a unique mode of development. This involves plants

growing by meristems at their tips. Plants also have remarkable powers of

regeneration resulting from the fact that plant cells are totipotent. Any single

plant cell can regenerate an entire plant with all its cell types - a feat no animal

can match. A limb lopped off an animal is certain disaster. For a plant it just

regenerates the missing portion!

Plants use the cheapest material around to generate size - water and carbohydrates

(Wiebe, 1978). They have a unique means of attaining stature with the most basic of

skeletal materials - no bones, no expensive proteins!


4/5


Each plant cell comprises a protoplast with a central vacuole which makes the cell

much bigger than it would otherwise be. The protoplast is enclosed by a cell wall

based on polysaccharides. The tendency of the protoplast to take in water (by osmosis)

is counteracted by the cell wall which restricts this expansion. This mutual antagonism

between protoplast and cell wall makes each cell a turgid little box. It is this simple

construction principle that allows herbaceous plants with little or no strengthening

material to rise several feet into the air relying on this system of one turgid box on top

of the next!

The earth today is dominated by the flowering plants. Our purpose in this course is to

understand how these flowering plants came into being. We can only understand this

by studying the plant groups that came before. This course is, therefore, a brief survey

of the plant kingdom from an evolutionary perspective.

PLANT CLASSIFICATION SYSTEMS

There are several plant classification systems and little agreement, e.g. some botanists

believe the ferns are a class within the Division Pteridophyta, others consider them adivision of their own. In this course, rather than use a formal classification system we

will simply refer to these groups by their informal names;-

Algae

Bryophytes

Pteridophytes

Gymnosperms

Angiosperms

There are a few other higher categories of classification you may come across in your

reading.

Lower plants usually include algae and Bryophytes, while higher plants refer to

Pteridophytes, Gymnosperms and Angiosperms.

The word Cryptogams literally means "hidden wedding" and alludes to the fact that


5/5


the sex life of these plants (algae, Bryophytes and Pteridophytes) was once not

understood. Phanerogams ("open wedding") are the seed plants - the gymnosperms

and angiosperms.

Thallophytes is an outmoded term for plants whose body is not differentiated into

root/stem/leaves but is termed a thallus. Algae fall into this category (and fungi did too

when they were considered to be plants).

Vascular plants or Trachaeophytes are those with vascular tissue (xylem &

phloem). Embryophytes (all but algae) are plants that bear an embryo and are

synonymous with land plants.

Documents

How Do We Classify the Living World