|Octoblepharum capsule and stoma|
Sporophytes may use stomata in gas exchange for photosynthesis to partially support itself, but bryophyte stomata are also involved in drying and dehiscence of the capsule. The best example is the pseudostomata of Sphagnum that do not form a pore and are the first cells to dry, changing the shape of the capsule to release the spores. This role in drying the capsule to gradually release spores can explain why stomata are mostly located at the base of the capsule, or why some mosses have very few or no stomata at all.
Stomata of tracheophytes are essential for sustaining their lifestyle and the absence of stomata is recorded only in aquatic plants. This is not the case for bryophytes and taking into account the distinct function of bryophyte stomata, the widespread records of mosses that do not have stomata is not surprising. In our recent study (Renzaglia et al. 2020), we found that stomata are absent in species of 74 genera (40 families!) that accounts for over 60 independent losses. In terms of development, stomata are relatively easy to lose; interrupting one or several key genes for stomata formation can lead to stomata-less capsules. Unfortunately, not a single nor combination of environmental or phenotypic characters can explain the absence of stomata. Capsules of aquatic or semiaquatic mosses always lack stomata, but this explains very few losses, and reduction of the sporophyte can lead to smaller guard cells and perhaps a reduction in the number of stomata but not complete loss. In the last decade, the study of bryophyte stomata has greatly advanced our understanding of stomata evolution, each study adding a piece to the puzzle, but some of the oldest questions as of why some bryophytes do not have stomata remain unsolved.
Renzaglia, K.S., Browning, W.B. and Merced, A., 2020. With Over 60 Independent Losses, Stomata Are Expendable in Mosses. Frontiers in Plant Science, 11, p.567. doi:10.3389/fpls.2020.00567