Megasporangium, nucellus, ovule and presemen: their nomenclature and homology

Palaeobotanic studies of remains of megasporangiate organs of early Cycadophytic forms of Palaeozoic age, carried out in the last few decades, have contributed a sufficient amount of new data to enable the complete reconstruction of the phylogenetic history of the ovule. The evolution of the ovule took place as a continuous (orthogenetic) process in which nine successive phases can be distinguished. The morphological evaluation and homologisation of parts of the ovule and its accessory organs is complicated by the ‘ontogenetic’ changes taking place between the initiation and the shedding of the ovule (in the more primitive presemen stage of evolution) or between its initiation and the completion of the fertilisation process (in more advanced ovules). In spite of this difficulty the homologisation of certain parts is clear and unambiguous at both the pre- and post-fertilisation stage. Our increased knowledge has strengthened the case for a pteridosperraous affinity of the so-called water-fems appreciably, so that some, or all, of their morphological features and their life-cycles may serve as a yardstick for the conditions prevailing in the pteridosperms currently regarded as quite extinct since Palaeozoic times.