The hypothesis tested was that the “M” strain of spores could not send a signal for
growth when in the petri dish.
This lab began with a basic review of the life cycle of a fern, and an
exploration of the life stages involved. Sketches and noes were then made of the fern life
cycle. Next, groups of four or five were made, and the instructor passed out petri plates
containing both “M” and “W” strains grown at medium density for observation.
Observations and data were recorded, and from this the hypothesis given above was
developed. Next, the instructor provided a complete set of “M” and “W” strains, which
were grown at three different population densities of Low, Medium, and High Density,
with approximately nine, 38, and 150 gametophytes per plate respectfully. From these
three plates, a data set was collected by removing the lid of each plate, and placing them
underneath the dissecting microscope individually. From there, the different shapes of
each spore were taken into account, and the number of each shape in each dish was
recorded. The shapes were found to be “mitten-shaped” and “light bulb-shaped.” From
there, a graph was constructed in the lab workbook in Table 6-2, page 77. The dependent
variable was the average percentage of light bulbs, while the independent variable was
density. The graph was then examined, and the class discussed causation for the results.
The experiment was then set up, with varying filtrates being combined with varying
spores in petri plates to test the hypothesis. “M” filtrate was combined with “W” spores
as the experimental control, water was put with “W” spores as a manipulated variable,
“W” filtrate was put with “W” spores as a positive control, and finally “M” filtrate was