Female Cone Development In Cupressaceae

The female cone in conifer has received considerable attention not only for its systematic significance, but also for its intriguing morphological nature. In a series of detailed studies of fossil and living conifers, Florin (1954) came to the most widely accepted conclusion that the female cones of most conifers are compound structures, in which the seed scale represents a modified branch borne in the axil of a bract scale. Many researchers were in favor of that "in modern conifers, evolutionary advancement in the ovulate cone is shown by the various degrees of fusion between the ovuliferious scale and its subtending bract. Throughout the Pinaceae, the bract is only basally adnate to the scale, whereas these structures are more or less completely united in members of the Taxodiaceae and Cupressaceae."Female cone development in some conifer species was extensively investigated, most of which supported Florin's theory. Nevertheless, the results were not consistent with the interpretation that the ovuliferious scale and bract were completely united in the female cone of Cupressaceae. The family Cupressaceae including about 20 genera, widely spread in the Northern and Southern hemispheres. As to such a large family, the relevant results derived from the limited species were not adequate for a conclusion. Therefore, it is essential to examine the female cone development in a wide range of species.Ontogeny and vascular system of female cones from 7 northern genera of Cupressaceae (Calocedrus, Chamaecyparis, Cupressus, Fokiena, Juniperus, Platycladus, Thuja) were investigated in detail by means of SEM together with conventional sectioning technique. The main results are briefly summarized as follows: 1) The bracts were arranged in the decussated manner similar to those in leaves.2) No, ovuliferous scale was observed in all of the species investigated.3) In the axil of the bract, the ovules were developed simultaneously or in acropetal order.4) In Juniperus and Cupressus, the uppermost pair of bracts were fertile or sterile. While in other genera the uppermost 1-2 pair(s) of bract were sterile.5) In Cupressus, which have global cone with large number of ovules, 2-3 series ovules initiated in a basipetal sequence in the axil of each bract6) Female cone development revealed the transition from several lateral ovules to the single terminal ovule in Juniperus.7) In all the genera studied, three types of vascular system in female cones could be recognized. (1) In Juniperus, Cupressus, Chamaecyparis and Fokiena with global cone and peltate scale, radial branching of the strands formed two vascular arcs: an adaxial series of strands with inverted orientation of the xylem and an abaxial system comprising normally oriented bundles. (2) In Calocedrus and Platycladus, only a single series of inverted vascular bundles were generated in the adaxial side of the bract scale; (3) There was no inverted vascular bundle developed in the bract scale of Thuja occidentals.The ontogeny of ovulate cone revealed that the female cone in Cupressaceae was a compound structure. In the axil of the fertile bract, the ovules were initiated on a broad swelling, which represented the aborted secondary branch system.No ovuliferious scale was developed in Cupressaceae, which indicating that the evolutionary trends of female cone in conifers may not be the progressive fusion between the ovuliferious scale and the bract. The absence of the ovuliferious scale in Cupressaceae and some genera in Taxodiaceae implied that the ovuliferious scale had been lost from some Taxodiaceae in evolution.Taking the evolutionary trends in the characteristics of female cone into account, Fokienia was considered as the most primitive genus, while Juniperus was regarded as the most advanced one in all the genera studied. The rest genera were considered to be intermediate between the two genera mentioned above.Based on the characteristics of female cone in Curessaceae, together with results derived from earlier investigations, we believed...