ONTOGENESIS AND POTENTIAL EVOLUTIONARY PATHWAYS OF FERN SORI

In the light of the hypothesis that the superficial position of sporangia and sori is the primitive condition, different forms of sori are analysed using the methodical framework of the botanical construction morphology. Proceeding according to this method means to compare sori on the basis of their construction including their ontogenesis as well as their developmental potencies for further evolution. Different developmental potencies of the sori depend on the various positional relationships between the sori and the sporophyll, which are caused by differences in sporophyll development. Concerning the construction of their sporangia, the megaphyllous ferns can be arranged in two main groups, one with delicate leptosporangia and one with massive leptosporangia.
In each group, there are possible ways for the origin of marginal sori from superficial initial forms. In both main groups similar developmental processes characterize the step by step transition from superficial singly-arranged sporangia into marginal sori. These processes are: sorus formation, phase differentiation with subblastozonal or blastozonal receptacle formation, costa-relation, and development of special receptacular growing zones and of indusial envelopes. Since all these processes are suitable for demonstrating possible ways to an increase in complexity, marginal sori, especially highly complex constructions such as costa-related marginal sori, cannot be regarded as representing the initial forms of sorus evolution.
Based on the presented morphological results some unconventional systematic conclusions can be drawn.