Identification notes
Leafy liverwort, or thalloid?
At first glance this liverwort looks a little like a Frullania, with each leaf consisting of a simple lobe and a folded lobule, but all is not as it seems! Neil Bell explains the curious case of Pleurozia purpurea…
Read the Field Guide accountIn the case of Pleurozia, postical and antical (as well as dorsal and ventral) in the way they are used in the literature, refer to developmental homology. As Pleurozia actually appears in the field, the “uppermost” side is actually the ventral side and the “lower” side is the dorsal side. Unlike in true leafy liverworts that have similar-looking lobes and lobules (e.g. Frullania), the larger lobe is developmentally ventral and the lobule is developmentally dorsal. Development is “upside down”, but the habit of the plant in the field effectively reverses this to make it look same as other lobuled leafy liverworts. I suppose you’d expect this, as the function of the lobule (whether that’s simple water retention or trapping of micro-organisms) probably depends on it being protected from the elements “underneath”.
Although the dorsal lobules are underneath in the field, when Pleurozia is grown in lab conditions in a truly prostrate form they are actually uppermost (and rhizoids are produced from the ventral side, which in these conditions is lowermost).
The really interesting thing is that Pleurozia groups phylogenetically with the simple thalloid Metzgeriales rather than the true leafy liverworts using molecular data, and this is corroborated by the apical cell morphology, which is 2-sided (2 cutting faces) just like that of the simple thalloids, but unlike the 4-sided (with 3 cutting faces) apical cells of the true leafies.
So it looks like the whole leafy liverwort morphology (stems with leaves having lobes and lobules) may have evolved independently in Pleurozia and in the true leafy liverworts. Because Pleurozia has developed this morphology using a 2-sided apical cell, it has had to do it in a completely different way. Probably one of the most dramatic demonstrations of evolutionary convergence, a bit like eyes in vertebrates and cephalopods!