Chinese Senna (Senna obtusifolia) is a common plant found in many parts of the
world. It is traditionally used to treat a number of bacterial infections.
Some scientists decided to investigate the antimicrobial properties of extracts from
S. obtusifolia leaves.
The following method was used.
• Dried leaves were crushed in sterile water, to produce an extract.
• The extract was pipetted onto discs of filter paper.
• Other discs of filter paper were soaked in sterile water to use as controls.
• All of the discs were then dried.
• Plates were prepared using agar containing the bacterium Salmonella typhi.
• A disc containing the extract and a control disc were placed on each of three
agar plates.
• The agar plates were incubated for 48 hours.
• The diameter of the zone of inhibition was then measured for each disc as
shown in the diagram.
This method was repeated using three organic solvents, A, B and C, to obtain extracts
from the leaves, instead of using sterile water.
Name the independent variable in this investigation.