Stephen Hales, Joseph Priestley By: Monica Heney-Racine and Razan Ali

 Jean-Baptiste Van Helmont, a 15 th century physician, strived to discover whether or not plants obtained all their food from soil.  In order to prove this hypothesis, he performed an experiment that consisted of watching a plant grow for five years.  Independent variable: Amount of time taken for the plant to grow  Dependent variable: Mass of tree, mass of soil  Controlled variable: Amount of water given, amount of air exposed to

 After five years, Helmont observed that the mass of the tree had increased by 74.4 kg, but that the soil’s mass had only increased by 60 g.  He therefore concluded that soil was not responsible for the tree’s increased in mass, the absorption of water was. (1)

An image from Nelson Biology 12

Known best for his Statical Essays. Speculated that plants draw in nourishment from the air, and that light may contribute to a plant’s growth. According to Stephen Hales conducted pioneer experiments on the transpiration, growth, and gas exchanges of plants. Hales explained how water from the soil is transported up the stems and into the leaves, where it gets lost as a water vapour.

 This process was called transpiration.  Stephen Hales proposed that plants extract some of their matter from air.  In evaluating the loss of water from leaves, he had to estimate the total surface area of the leaves on the plant.  He cut off all the leave of this plant, and laid them in five several parcels, according to their sizes.  He then measured the surface of the leaves in each parcel by laying over it a large lattice made with threads, in which the little squares were ¼ of an inch each.

 He multiplied the surface of the leaves by the number of the leaves in the corresponding parcel, which then gave him the area of all the leaves.  Then, he estimated the length of the root system and calculated the corresponding surface area.  Using the values he collected, he compared the flux of water into the plant through the roots with the transpiration from the leaves.

 In one of his works, Hales concluded that plants get some part of their nourishments from the air.  In Hales work Vegetable statics he explains that “… air freely enters plants, not only with the principal fund of nourishment by the roots, but also thro’ the surface of their trunks and leaves…” [10: 153]  Independent variable: amount of lost water from the leaves  Dependent variable: the measured surface of the leaves

 Controlled variables: Size of leaves, size of the squares that have been cut.

 Often given credit for the discovery of oxygen(dephlogisticated air) and its role in photosynthesis. (2)  He lit a candle and placed it in a transparent closed space with a mint plant. He did the same thing with another candle, but did not place a mint plant in the transparent closed space.  After some time, both candles went out. However, the candle that was placed alongside the mint plant was able to burn longer. (3)

 10 days later, he was able to relight the candle that was placed alongside the mint plant(using light from the sun) and it was able to burn perfectly in the air that did not previously support it.  In another experiment, Priestley placed a mouse in one transparent chamber, and a mouse and a mint plant in another transparent chamber.  He observed that the mouse in the chamber that contained the mint plant was able to survive whereas the mouse in the chamber that did not contain a mint plant was unable to survive. (3)

 As a result, Priestley concluded from these experiments that plants restore to the air whatever breathing animals and burning candles remove(OXYGEN). (3)  Independent variable: The presence of a mint plant  Dependent variables: Duration of time the candle can stay lit for, Duration of time the mice can remain alive  Controlled variables: Equally sized transparent chambers, temperature of chambers, type of candle(4)

Photo from the internet ( synthesis%20-%20classics.htm)

 1. Di Giuseppe, Maurice. Vavitsas, Angela. Ritter, Bob. Fraser, Douglas. Arora, Anu. Lisser, Beth photosynthesis p.149, Nelson Biology 12  2. Unknown author, 2012, Oxygen. Received on April 1  3. Ruban, Julien. 2011, The Discovery of Photosynthesis. Received on April 1 sexperiments.html

 4. Unknown author, 2010, Joseph Priestley: Variables, Controls, Errors. Received on April 1  photosynthesis3b/joseph- priestley/variables-and-controls-and-error photosynthesis3b/joseph- priestley/variables-and-controls-and-error 5. Unknown author, 2011, Stephen Hales and the practice of science. Received on April 1 /2011/10/18/stephen-hales/

 6. Unknown author, 2002, Hales, Stephen state.edu/hort/history/108.htm 7. Unknown author, 2012, Stephen Hales 8. Unknown author, Unknown year, History of Plant Physiology Plant-Physiology.html#b