9.2 Applications and skills

9.2.4 Nature of science: Analysis and deduction

Scientific knowledge depends on observable evidence gathered in a number of ways, and especially by:

  • direct observation of phenomena in the natural environment
  • experimental manipulation of the biotic or abiotic conditions
  • building model systems
  • analysis and deduction.

Plants lend themselves very well to experimentation and experimental manipulation. Much of what we know about the biochemistry of plants is a result of hypothetico-deductive experiments on living organisms, rather than the more modern, technological approach of biochemical assay.

Analysis and deduction in the discovery of auxins

Long after that other theory was published, Charles Darwin (Figure 9.2.4c), prolific naturalist and generally curious gentleman, also hypothesised the existence of a chemical messenger responsible for phototropisms. He published his work in 1880 in a book called The Power of Movement in Plants. Unlike the theory of evolution by natural selection (see Page 5.2.4), Darwin’s work with plants is a great example of the deductive nature of science.

darwin's experiment
Figure 9.2.4a – Darwin's experiments with grasses
Darwin’s experiments with grasses: analysis and deduction were used to determine which plant part initiates phototropism

At the end of his analysis, Darwin deduced the existence of a chemical message located in the apex of the coleoptile.

Over the next 40 years, many scientists used similar experiments to learn more about the nature of this mysterious chemical. In the 1920s, Frits Went ((Figure 9.2.4d) named this chemical ‘auxin’, which comes from the Latin word for growth.

auxindiagram

Figure 9.2.4b – Summary of plant physiology experiments on the nature of phototropisms

Activity

Try this – the hypothetico-deductive method:

  1. Formulate a purpose for each of the experiments shown above. (there should be one unique question for each experiment).
  2. Fill in a table (an example has been done for you).

Scientist

Manipulation

Observation

Conclusion

Darwin

Cut off tip of coleoptile

No bending

Tropism initiated at the apex

 

 

 

 

Manipulating biotic relationships to improve techniques – aphid stylets

Transpiration rates can be easily measured since transpiration is a physical process requiring no energy input. See: Page 9.2.2

Translocation, on the other hand, is an active process occurring only in living phloem tissue. Scientists have used a variety of methods to measure translocation rates, but the most significant is the use of aphid stylets.

  • Aphids (Figure 5.2.4e) are small insects that feed opportunistically on phloem sap through sucking mouthparts called stylets.
  • A stylet has a diameter of approximate 10–15µm, and is inserted very precisely by the aphid into a single sieve tube element.
  • Stylets can be separated from the aphid using laser incision after the insect is anaesthetized with carbon dioxide gas.
  • The intact stylet then acts as a micropipette, and flow rates can be measured by collecting liquid exuded through the tube.
  • Data collected from stylets embedded in various parts of the plant give scientists a clear picture of the locations of sources and sinks.

The process of excising an aphid stylet

More recently, flow rate data from aphid stylet exudation experiments have been combined with tracer data from C14-labelled sugars to get an even clearer picture of the mechanisms of translocation in the phloem. This would not have been possible without an organic chemistry technique for synthesising C14-labelled carbon dioxide.

Charles DarwinFigure 9.2.4c – Charles Darwin

Frits Warmolt WentFigure 9.2.4d – Frits Warmolt Went, Caltech

TOK

  • Counterfactual history is a form of historiography that tries to answer ‘What if…?’ questions. Whenever you learn about an important experiment or discovery in the history of science, consider the following:
    • Could this discovery have occurred at any time?
    • What was special about the social, political or scientific conditions at the time of this discovery?
    • Was this discovery inevitable?
    • How do you know?
  • Science is collaborative. Do you think Went and Darwin worked in collaboration? Is collaboration involved when plant physiologists use C14 sugars made by organic chemists to study translocation

anatomy of aphidFigure 9.2.4e – External anatomy of an aphid