According to the well-accepted Florin model of Cordaitales-Coniferales evolution (Florin 1949, 1951, Schweitzer 1963), BSSC in a typical cone should comprise a subtending bract and a secondary fertile shoot in its axil. This spatial relationship is easy to understand and accept considering axillary branching is almost ubiquitous in seed plants (except Cycadales), in which a branch is always in the axil of a subtending leaf (Eames 1961, Esau 1977, Biswas and Johri 1997). Such a compatibility between hypothesis and observation in most seed plants at least partially contributes to the success of the Florin model for cone evolution. Not surprising, such a model is also frequently applied to account for the organization of cones in Cupressaceae, a family in Conifers. According to Farjon (Farjon 2005), as far as back to 1893 Jack has started paying attention to the positioning of ovules in cupressaceous cones. For example, ovules have been seen between bracts in Callitris, Diselma, Fitzroya, Widdringtonia (Plate 16.1-4,6), Juniperus oxycedrus, J. communis, J. brevifolia, J. phoenica (Plate 11.5-6), Libocedrus plumosa, L. bidwillii, Tetraclinis, Cupressus macnabiana, C. guadalupensis, C. arizonica, C. goveniana (Plate 9.2-3)(Farjon 2005). In the meantime, ovules have been seen on the cone apices in Juniperus tibetica, J. squamata, J. satuaria, J. recurva, J. przewalskii, J. pingii, J. indica, and J. angosturana (Farjon 2005). According to Farjon (2005), Jack (1893), Kubart (1905), Noren (1907), Renner (1907), Gaussen (1967), Jagel (2001), Farjon and Garcia (2002) have addressed the morphology of ovules and bracts in cupressaceous cones. Unfortunately, such a consistent inconsistency between cupressaceous cones and the Florin model is largely ignored, probably due to the success and dominance of the Florin model, which appears valid for many coniferous cone. Beyond Coniferales, there are still other cones in gymnosperms including Pentoxyales and Cycadales that appear beyond the coverage of the Florin model. The homology among typical cones of Coniferales and these gymnosperms is still mysterious, hindering a comprehensive appreciation of the evolution of gymnosperms.
Part of the reason of the current academic situation is due to the shortcoming of traditional documenting technique, which cannot demonstrate the morphology and anatomy of cones to botanists and general public. Although paraffin section has contributed much to our understanding of plant anatomy in the past decades, its two dimensional presentation of three dimensional morphology and anatomy requires extra effort and education to correctly conceive the three dimensional relationship among different parts in an organ of interest. Newly developed technologies enable us to visualize and demonstrate the anatomy of plant organs in an easy-conceive way. Micro CT is one of such new technologies. Its application leaves no space for anyone to ignore the anatomical fact in cupressaceous cones. This is the reason we try to apply Micro CT to call for attention to the long-ignored fact about cupressaceous cones.
According to the Florin model for coniferous cones, each basic unit of coniferous cone (BSSC) comprises a subtending bract and a secondary fertile shoot in its axil. This interpretation implies that a secondary shoot and its subtending bract are aligned in the same radius, namely, the secondary fertile shoot opposite the corresponding bract. This implication is well confirmed in most Coniferales, including Pinaceae, Araucariaceae, and Taxodiaceae. This explains the success and wide acceptance of the Florin model. If this implication were not confirmed in the reality, the fate of the Florin model would be quite different. It is interesting that the alignment between ovules and bracts expected by Florin model is not seen in at least above mentioned taxa in Cupressaceae. The existence of such consistent discrepancy between the Florin model and botanical observation undermines the validity of Florin model, or at least reduces the applicable scope of the model.
As documented here, the ovules consistently alternating the bracts in Juniperus are of importance in that they cast doubt over the validity of the Florin model in the Cupressaceae, and, more importantly, prompt new more widely applicable interpretations, at least including the Cupressaceae. Thus a new interpretation for cones valid in wider scope is needed in botany. It becomes more interesting, as you see below, that all cones in gymnosperms may be derived from a single common ancestor, and the Florin model may well be a specialization of a more general model applicable for more gymnosperms.
One of the important studies on plant organ evolution was performed about twenty years by Crane and Kenrick. After careful study of living and fossil plants, Crane and Kenrick came to a hypothesis that the variety of organs seen in living and fossil plants is a result of long time diverted development of reproductive organs throughout the geological history (Crane and Kenrick 1997). In their paper, their hypothesis is exemplified by the provenance of microphylls in lycopsids and interseminal scales in Bennettitalean cones. Actually, the derivation of integument can also be taken as a result of diverted development, as suggested by Benson (Benson 1904) and favored by laters (Walton 1953, Andrews 1963, Taylor et al. 2009). The earliest reproductive organs in land plants are aggregates of sporangia, micro- or mega-, borne on shoot terminals, as seen in the earliest land plants (Taylor et al. 2009). It is conceivable that each of the ancestral female cones may comprise an axis and clusters of ovules (megasporoclads) helically arranged along its sides. This situation may be exemplified by the lax cone of Cycas, in which clusters of ovules are helically arranged around the shoot apex. If each of the megasporoclads in Cycas is reduced into an ovule, a cone with helically arranged ovules/seeds around its axis, just as seen in Pentoxyales, may come into existence. The diverted development (sterilization) of these lateral appendages (ovules), as seen in Pentoxylalean cones, may produce interseminal scales, which surround and protect their fertile peers (ovules) in Bennettitales. The hybridization between axillary branching and the cones of Pentoxyales, turning sterilized ovules into subtending bracts, may give rise to cones seen in Cordaitales and typical Coniferales. Further modification of the secondary fertile shoot may make it a shoot terminating in an ovule with a micropylar tube, as seen in Gnetales. Lacking involvement of axillary branching, sterilization of some ovules into bracts in Pentoxylalean cones might produce the cone configuration documented here for Cupressaceae, namely, some of the former ovules may be sterilized and function as protecting bracts. When clusters of ovules (rather than single ovule) are retained, clusters of ovules dispersed between bracts as seen in Widdringtonia, Juniperus oxycedrus, J. communis, J. brevifolia, Libocedrus plumosa, L. bidwillii, Tetraclinis, Cupressus macnabiana, C. guadalupensis, and C. arizonica become something easy to conceive. When all except the terminal ovule(s) is retained and all others are sterilized into protecting surrounding and subtending bracts, the situation in Callitris rhomboidea (Takaso and Tomlinson 1989), Juniperus tibetica, J. squamata, J. satuaria, J. recurva, J. przewalskii, J. pingii, J. indica, J. angosturana (Farjon 2005), and Platycladus orientalis (Zhang et al. 2000) may come into existence. The significance of this interpretation lies in that it removes the former implicit requirement on spatial relationship between ovules and bracts, namely, the aligned arrangement of ovules and bracts in gymnospermous cones required by the Florin model becomes surplus and unnecessary. Ovules and bracts become independent each other and they have the freedom to combine and coalescence anyway in the new interpretation. Such a great freedom of combination among plant parts makes the great variety of gymnospermous cones easy to appreciate. Thus this lift of unnecessary restriction on spatial relationship between ovule and bract makes a common Bau-plan for gymnospermous cones within reach. If this is the case, botanists will not have to give ad hoc interpretations for various cones and will not have to play ostrich ignoring botanical facts any more. It may well be that the Florin model is a specialization of universal model that is applicable for all gymnospermous cones. If this is true, drawing a conceivable and rational evolutionary roadmap for gymnosperms will be a mission possible for botanists in the near future.