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2.1 outline the progress that occurred in the 17th and 18th centuries towards understanding plant growth by identifying: the observation of van Helmont that soil was not primarily responsible for a plants change in mass as it grew Stephen Hales’ proposal that plants extract some of their matter from air the work of Priestley in identifying that plants could ‘restore the air’ used by a candle and his subsequent discovery of oxygen Ingen-Housz’s demonstration of the importance of sunlight for oxygen production by plants Senebier’s demonstration of the use of carbon dioxide during photosynthesis Saussure’s conclusion that water was also necessary for photosynthesis.
outline the progress that occurred in the 17th and 18th centuries towards understanding plant growth by identifying:
- the observation of van Helmont that soil was not primarily responsible for a plants change in mass as it grew
- Stephen Hales’ proposal that plants extract some of their matter from air
- the work of Priestley in identifying that plants could ‘restore the air’ used by a candle and his subsequent discovery of oxygen
- Ingen-Housz’s demonstration of the importance of sunlight for oxygen production by plants
- Senebier’s demonstration of the use of carbon dioxide during photosynthesis
- Saussure’s conclusion that water was also necessary for photosynthesis
- In 1640 Van Helmont weighed 200 pounds of soil and planted a willow sapling weighing 5 pounds in a pot. He added only water to the pot. After five years he reweighed the sapling and found it weighed 169 pounds and the soil weighed 199 pounds. Van Helmont concluded that the tree had gained all its weight from the water he added.
- Stephen Hales grew a plant in a closed system above water. He measured a reduction in the volume of the air above the water of about 14%. This called attention to the fact that air was a participant in the process by which plants lived.
- In 1772 Priestley placed a mint plant in a container and connected it to a second container in which he had placed a candle that had been allowed to burn out (i.e. consume all the oxygen). After several days Priestley successfully relighted the candle. From this experiment he concluded that plants could “restore air which had been injured by the burning of candles”.
- In 1779 Igen-Housz showed that green plants “purified air” only when they were placed in light.
- In 1782 Senebier conducted three experiments. In the first experiment he placed leaves in two beakers: one beaker contained pure (decarbonised) water, the other beaker contained carbonised water from a well. Senebier collected the gas that was evolved and concluded that oxygen was produced by the leaves in the well water but not in the pure water. When the leaves in the beaker of well water stopped producing gas, Senebier replaced them with fresh leaves and found no gas was evolved. Senebier then placed the leaves he had removed from the well water into fresh well water. He found they again produced oxygen. From these experiments, Senebier concluded that it was “fixed air” (carbon dioxide) in the well water that was allowing the production of oxygen and that the leaves themselves were not producing any carbon dioxide for use in making oxygen.
- In 1804 Saussure grew plants in a closed system, measuring the amount of water present initially, the mass gained by the plant and how much water was present at the end of the experiment. He concluded that the plant’s increase in weight could not be solely due to uptake of carbon and minerals but that water was necessary for the photosynthetic production of organic materials.
Useful web sites:
History of Plant Physiology
by John Hanson, Biology Encyclopedia forum
History of photosynthesis
by Paul May, School of Chemistry, University of Bristol, UK.