裸子植物一新科——竹柏科

竹柏科Nageiaceae是根据叶无中脉而具多数近平行细脉和雌性生殖器官接近原始的枝条状结构，从裸子植物罗汉松科Podocarpaceae中分出的单属新科。 新科内有2个组，5个种，分布于太平洋西海岸的东亚、南亚近海山地至新几内亚等南太平洋岛屿区域。 该科植物具有2个以上叶迹和无中脉的多脉叶类型，在现存的裸子植物中十分特殊，仅与少数几个系统位置较为孤立或系统位置不甚清楚的类群有一定相似之处，而与其他大多数具中脉叶类型的类群几乎没有任何过渡和联系。 古植物学证据表明，自古生代以来，各不同地质时期均有这种多脉叶类型化石存在，同时，在某些古裸子植物化石中，也存在接近Nageiaceae所具有的枝条状雌性生殖器官类型。 据此推测，在裸子植物系统发育中，可能存在一条以叶具多脉为标志的M-演化线(multinerved-leaved evolutionary line)，这条演化线可追溯至古裸子植物科达类甚至更古老的类群。

Nageiaceae—A New Gymnosperm Family

A new monotypic gymnosperm family, Nageiaceae D. Z. Fu, is sepa-rated from Podocarpaceae. It is characterized by having multinerved leaves with-out costae, and primitive shoot-like female reproductive organs (female strobili).The new family contains a single genus consisting of 2 sections, 5 species and isdistributed along the western coast of the Pacific, from low coastal mountains ofeastern and southern Asia to the Phillipines and Papua New Guinea.     The first species in the Nageiaceae was described as an angiosperm, Myricanagi Thunb. (1784), but it was soon recognized to be a gymnosperm belongingto a new genus, and was renamed as Nageia japonica Gaert. (1788). The genericname, Nageia, however, has seldom been used, and the members of Nageia havegenerally been treated as an isolated section of Podocarpus in the Podocarpaceae.When revising the Podocarpaceae, De Laubenfels (1969) established a new genusDecussocarpus based on Nageia, but several years later (1987) he revived the oldgeneric name, Nageia. Page ( 1988,1990)considered Nageia to be a valid genericname and redefined it as a natural genus.     The distinctive,broadly lanceolate, multinerved leaves (without costae) of Nageiaare rather unusual in gymnosperms,only being similar to those of Agathis in theAraucariaceae, their leaves are also similar to each other in anatomy. For example,there are many  single vascular bundles arranged parallelly,  between whichoccur sclerenchyma cells in the mesophyll. Apparently,leaves in Nageia are rathersimilar both externally and internally to paleogymnosperm cordaitean leaves, andsclerenchyma cells found in Nageia might be the remains of straps between veinsin cordaitean leaves. In addition to leaf characters, the large and nearly roundpith of the young shoot in Nageia appears to be a reminiscent of the large pith incordaitean stem.    The female reproductive organs (female strobili ) in Nageia are shoot-like.The female strobilus  has  a  sterile terminal  bud,  and  several  opposite  orsubopposite sterile scaly bracts on its axis; two opposite megasporophylls arefound near the axis apex and both have an anatropous ovule which is almost en-tirely covered by the megasporophyll; a bract is partly adnate to the lower backof the megasporophyll;mature arillate seeds are 1-2 or occasionally 3 in number;the axis becomes woody when the seeds mature,  but in some species (N.wallichiana) the upper part of the axis becomes fleshy (in the shape of a receptacle),in which no distinct boundary was found between the fleshy receptacle and thewoody part, and both have the same scaly bracts or traces.     Many characters in Nageia are distinctly different from those in Podocarpus.Leaves in the Podocarpaceae have distinct midribs; in Podocarpus, the reproduc-tive organ, which was generally thought to be similar to that in Nageia, has no ter-minal bud,  and  its  bract  is  entirely  free  from  the  lower back  of themegasporophyll, the fleshy receptacle is derived from both the axis and the sterilebracts (except the lowest two), and the female strobilus at the seed stage has a sec-ondary stalk.     The multinerved leaf in Nageia can rarely   be found in most of the livinggymnosperms except in some rather isolated groups, such as Araucariaceae,Ephedraceae,Ginkgoaceae and Welwitschiaceae. Paleobotanical evidence shows thatmultinerved leaves have been found in all of the geological ages from the Paleozoicto the present, and such a shoot-like female reproductive organ as in Nageia wasfound in some paleogymnosperms. It is very difficult to determine the systematicpositions of these fossil plants because of lacks adequate material of reproductiveorgans or even lack of complete vegetative organs. The vascular system andleaf characters of gymnosperms are considered to be very  conservative, and thefact that the common leaf shape and venation exist in both fossil and livinggymnosperms could imply  that there exists a multinerved-leaved evolutionaryline ( M-line )  in  gymnosperms,  which  could  be  traced  back  to  thepaleogymnosperm cordaitean plants or even older ones with multinerved leaves.The different types of the female strobili (female reproductive  organs) of living gymnosperms, regardless of having one or only several seeds without a typical cone or many seeds with a cone, might have been derived from shoot-like or spike-like female reproductive organs possessed by their common ancestor.The fossil eviden ce shows that the typical cone similar to those of living gymnosperms first appeared in the Jurassic, much later than the single-seeded fossil plant without cones. The seed fossil appeared in the late Devonian Period. It is very difficult to infer the relationshipsamong living gymnosperms, which are hardly derived from one another. But ananalysis of the strobili,  including the axis structure and position,  number,morphology and degree of adnation of the phyllomes on them, would be helpfulto the study of their phylogeny. It is evident, therefore, that the gymnospermswith leaves having a midrib might also have a rather long evolutionary course,but notransition between the midrib and multinerved patterns of leaf venation has so farbeen found in both living and fossil plants.    Finally, it is noteworthy that the Nageiaceae are distributed along the westerncoast of the Pacific, where many primitive representatives, both in gymnospermsand angiosperms, still survive. This would be advantageous to the considerationof Nageiaceae as a primitive representative, or a descendant of fhe paleogymnos-perm cordaitean plants.