5.2 Applications and skills

5.2.3 Recognising plant phyla and making dichotomous keys

More than 2000 years ago, Aristotle divided all living things into two groups: animals and plants. Linnaeus, despite a lifetime of categorising, continued to think of all the species on Earth as belonging to one of these two groups. It’s not difficult to understand why. As humans, we instinctively understand that some things, called animals, move around, and some things, called plants, grow up from the ground and stay put – this is the most natural classification system there is!

All organisms in the kingdom Plantae:

 Only some plants:

  • reproduce asexually (that is, every individual is genetically identical)
  • create fruit and flowers – these are the sexually reproducing plants.

Plant similarities are grouped between four main phyla: Bryophyta, Filicinophyta, Coniferophyta and Angiospermophyta.

Comparison of plant phyla

Phylum

Body plan

Vasculation

Reproduction

Environment

Bryophyta
(mosses and liverworts)
See Figure 5.2.3c

Leaves, no real roots
Height: 2–5cm

None – plants absorb water through leaves

Asexual – through spores contained in capsules

Damp environments
necessary for water absorption

Filicinophyta
(ferns, hanging moss, horsetails)
See Figure 5.2.3d

Leaves, roots and woody stems
Height: up to 15m

Roots, stems and leaves, with water-conducting tissues

Asexual – spores produced under leaves

Varied

Coniferophyta
(all cone trees, e.g. pine, cedar, fir trees; shrubs)
See Figure 5.2.3e

Typical tree body plan; some have needle leaves to reduce water loss
Height: from very small to >100m

Complex conducting tissues
(xylem and phloem)

Sexual: pollinated by wind; seeds held in cones on branches

Evergreen forests

Angiospermophyta
(all flowering plants, e.g. orchid, apple)
See Figure 5.2.3f

Typical plant body plan, diverse shape of leaves and flowers/fruits
Height: from very small to >100m

Complex conducting tissues
(xylem and phloem)

Sexual: pollinated by wind or animal, seeds contained in fruits or nuts

Deciduous and tropical forests

 

Dichotomous keys

Imagine you stumble upon a plant you’ve never seen before. How would you go about identifying it? The simplest way would be to compare the features of the unknown plant to a published description.

A dichotomous key is a chart or set of instructions that maps out the features of an unknown organism. It does this by separating groups of organisms, one feature at a time. It is called ‘dichotomous’ because at each step the organism can either be in the group or not. An example is shown below.

sample of a dichotomous key

Figure 5.2.3a – Sample of a dichotomous key
A dichotomous key can be arranged as a branching spider diagram or as a series of questions and ‘go to’ instructions. In general, the second format is more useful in a field guide.

Activity: Making a dichotomous key

Instructions

  1. Follow the key in Fig 5.2.3a to identify the five plant species shown. Make sure you understand the principle of using a key for identification.
  2. Examine the 12 species shown in Fig 5.2.3b. Working with a partner, design a spider chart key to identify the species.
  3. Rewrite the key in the form of a series of questions and ‘go to’ instructions. This is your final dichotomous key.
  4. Exchange your dichotomous key with another group. Follow the new key to identify the species.

Dichotomous key activity

Figure 5.2.3b – Dichotomous key activity

Further reading

HL students will learn more about plant vasculation Pages 9.1.1 and 9.1.2 and sexual reproduction Page 9.1.4 later in the course.

Tip

If you don’t know the name of something, it helps to describe it as accurately as possible, using scientifically appropriate language, on an internet search. It’s like a reverse glossary search!

BryophytaFigure 5.2.3c – Bryophyta

FilicinophytaFigure 5.2.3d – Filicinophyta

ConiferophytaFigure 5.2.3e – Coniferophyta

AngiospermophytaFigure 5.2.3f – Angiospermophyta