The female reproductive organs of conifers and taxads

TLDR: The present article deals with the later history of the morphology of the female conifer cones, and finds that the Palaeozoic cordaites and conifers furnished the principal clue to the interpretation of the true conifer cone of Mesozoic and more recent age.

Abstract: SUMMARY 1 The morphology of the female conifer cones has long been a matter of dispute. In 1900 an account of the theories put forward so far was given by Worsdell. After a brief characterization of the situation at that time, the present article deals with the later history of the subject. 2 At the turn of the century there were rivalling concepts of the nature of the so-called ovuliferous scale in the conifers: (a) the Excrescence or Ligular theory of Sachs-Eichler, (b) the general Brachyblast theory of Braun, Caspary, celakovský and others, (c) van Tieghem's modification of the Brachyblast theory, and (d) the Foliolar theory of Delpino-Penzig. 3 In the subsequent three decades the discussion proceeded on the same or similar lines. The Excrescence theory retained a strong position until the end of the nineteen-twenties, but, as before, some morphologists professed the general Brachyblast theory. Herzfeld and Wettstein considered that the axillary conifer strobilus had one or more reduced carpels (megasporophylls) which were used up in the formation of terminal ovules, as well as an ‘ovuliferous scale’ consisting of secondary outgrowths from the strobilar axis. In Goebeľs opinion these outgrowths were instead produced by the megasporophylls. Doubts were expressed of the unity of the true conifer group, but Eames showed that the apparently widely different female cones of the Pinaceae and Araucariaceae are homologous, and that the Araucariaceae, Taxodiaceae and Podocarpaceae exhibit complete transitions by fusion and reduction from types with distinctly compound strobilar units–each with an ‘ovuliferous scale’ in the axil of a bract–to others of the most simple form. In contradistinction to the true conifers, the genus Taxus has no compound strobilus, and its ovule is a direct continuation of the axis of the fertile short shoot (Dupler). 4 In the first half of the period after 1930 opinions differed as much as ever, although the general Brachyblast theory now prevailed over the Excrescence theory and other concepts. Chadefaud believed the ‘ovuliferous scale’ and the bract to represent between them a carpel derived from a prototype analogous to the pinnate megasporophyll of Cycas. Hirmer interpreted the ‘ovuliferous scale’ and the bract as formed by a serial splitting of one single member. Lanfer supported Goebeľs views of the terminal position of the conifer ovules on reduced megasporophylls, and of the nature of the ‘ovuliferous scale’. In Hagerup's opinion the female cones of most true conifers are compound and have a short secondary axis developed axillary to each bract. This axis was supposed to carry two transversal prophylls, fertile (megasporangial) or sterile, and a varying number of sterile leaves; and the megasporophyll, with a megasporangium on its upper side, to constitute the integument of the ovule, and be homologous to a lycopod sporophyll. 5 Florin (1938-45) found that the Palaeozoic cordaites and conifers furnished the principal clue to the interpretation of the true conifer cones of Mesozoic and more recent age. Primarily, the fertile seed-scale complex in the axil of each bract was a radially symmetrical short shoot (strobilus) with several sterile scales and one to a few uniovulate megasporophylls; the ovules were terminal in position. The later types of cones have arisen by the reduction and transformation of this primitive organization, which in the majority of cases has differentiated the strobilus into a proximal fertile part facing the cone axis and a distal sterile part (‘ovuliferous scale’), while its anterior sector facing the bract became totally suppressed. Exceptionally, no ‘ovuliferous scale’ at all was developed, and the strobilus became wholly fertile. The ovular integument is a continuation of the megasporophyll, and appears to arise out of two transversal primordia at its apex. The taxads differ from the true conifers by their simple strobili being placed axillary on reduced vegetative shoots. Their ovules are seated terminally on the strobilar axis itself; megasporophylls are accordingly absent. The living and extinct genera which have previously as a rule been considered coniferous represent therefore two separate subdivisions of the gymnosperms–the true conifers, and the taxads. 6 Earlier concepts of the morphology of the female cones of the conifers, and of the nature of the integument of their ovules, are in part or wholly untenable.