According to the biological species concept, species in the same genus should be more genetically distinct than individuals within a species due to reproductive isolation. However, in the widespread moss Physcomitrium pyriforme, we have evidence of three distinct genetic clusters unrelated to geographic isolation. We hypothesize that P. pyriforme contains cryptic species -  taxa- morphologically indistinguishable but genetically different. We aim to confirm whether the three clades are morphologically distinct in microscopic characteristics including spore and stomatal size. Using a controlled cross-experiment, we also aim to test for reproductive isolation across clades. We crossbred three clades from eastern North America in a growth chamber to test for the production of hybrid offspring. The lack of offspring would suggest that P. pyriforme has cryptic species under its genus. An alternative explanation is that species are not cryptic but are instead defined by microscopic characters including size differences in spores and stomata. Each genetic cluster having different stomatal characteristics of size, density, and count would support the description of new species of the common bladder moss.