Final answer:
Without specific data from the student's experiment, it's impossible to answer which organism is the least plentiful. To determine this, one would compare the total counts for each organism from the experiment and identify which has the lowest count.
Step-by-step explanation:
To determine which organism is the least plentiful in the pond water studied, students need to analyze the data collected from counting the number of each type of organism in each 1 mm square of their microscope slides. This process involves comparing the total counts of organisms such as amoeba, volvox, euglena, and paramecium. Since the question does not provide the actual data counts, it's not possible to give a definitive answer without that information. However, when analyzing results, the organism with the lowest total count across all squares would be considered the least plentiful in the pond water. If students found fewer amoebas compared to volvox, euglenas, and parameciums, for example, then amoeba would be the least plentiful organism.
In real scenarios, various factors such as available nutrients, temperature, and predation can affect the population sizes and distribution patterns of these microscopic organisms in pond ecosystems. Typically, a student would tabulate their findings, analyze the data, and then clearly identify which organism was found in the least quantity. One can only offer a general approach to this analysis since specific data is not provided in the question.