Systematic Position Of Cercidiphyllaceae Based On Morphological Data

Cercidiphyllum japonicum Sieb. et Zucc. belongs to the familyCercidiphyllaceae, which has one genus and two species, C. japonicum and C.magnificum (Nakai) Nakai native to East Asia. C. magnificum is native to Japan. Thefeatures of floral and leaf morphogenesis, the variation of perforation plate of vessels,the contact types between vessels and xylem parenchyma cells, the organic anatomy ofC. japonicum were described in the present paper. Based on above research, wediscussed the systematic position of Cercidiphyllaceae. The observation results asfollows:1. Morphogenesis and Development of Female FlowersThe pistillate inflorescence is a spike with and two to six (to eight) flowers congestedlyand oppositely arranged in pairs on the axis. Each of the flowers contains a so-called"bract" and a carpel. From the process of morphogenesis and development of femaleflowers, the "bracts" primordia of the first pair are crescent-shaped, truncate at theapexes, not conduplicate, no stipule primordium observed at the base. Finally, theybecome lanceolated with a few narrow triangular teeth in the upper margin and somehairs in the lower margin. This process is different to the morphogenesis anddevelopment of vegetative leaf, so we considered that the so-called "bract" of C.japonicum might be the tepal of the pistillate flower in morphological nature. The carpelprimordium of the pistillate flower is crescent-shaped when it occurs. Then it becomescone-shaped because of the fast growth of the apex in the later developmental stages. Alongitudinal concave appears in the center part of the carpel in abaxial side. Thisconcave is the early conduplication of the carpel. But the top part of the carpel remainsflattened, not conduplicated. The carpel encloses from the base to the top to form theventral slit. When the conduplicated carpel encloses completely, the unconduplicatedpart and the upper part of the suture becomes an inclined ovate plane. Thenceforth theinclined plane elongates and differentiates into a stigmatic region with short unicellularpapillae. That makes the stigma tissue of all carpels in an inflorescence be outwards.This position might benefit the wind pollination of the plant. We did not find any traceof other floral organs in the morphogenesis of pistillate flower. Therefore we considered that the unicarpellate status of extant Cercidiphyllum might be highly reduce andadvance characteristics that makes the extant Cercidiphyllum isolated from both fossilCercidiphyllum-like plants and its extant affinities.2. Morphogenesis and Development of Male FlowersThe male inflorescence is zygomorphic, spike. The outermost of two flowers arecontains a so-called "bract" and four to six stamens. The third one contains oneso-called "bract" and two to three stamens. Each of the other flowers has only onestamen. The "bract" primordia of the first pair are crescent-shaped, truncate at theapexes, not conduplicate, no stipule primordium observed at the base. Finally, theoutmost of two "bracts" become peltate with some long teeth in the upper margin. Thethird "bract" is relatively simple. The developmental process of male flowers are similarto female flowers and also different to the development of vegetative leaf, so weconsidered that the so-called "bract" of C. japonicum might be the tepal of the maleflower in morphological nature. The stamen pdmordium of the first pair of flower issemi-ring's protuberance when it occurs. Then some concaves appear in theprotuberance towards the tepal and becom four to six stamens. The morphogenesis ofthe third flower is similar to the first pairs. The male flower with one stamen also hasone stamen primordium when it occurs. The stamen primordium is semi-hemispherical.Anther is longitudinal dehiscence.3. Morphogenesis and Development of the LeafOn the long shoot of C. japonicum, two leaf primordia occur oppositely. The leafprimordia are triangular in shape and round at the apexes. The quick growth of themargin of the primordia makes the margin involuted and conduplicate. Two smallprotuberances appear at the base of each side of the primordia. The small protuberancesare the primordia of the stipules. After that, the base of the stipule primordia expandsslightly but the apex grows quickly that makes the stipule to be lanceolate and someteeth appear in the margin later. With the development of the leaves and the stipules, theapexes of leaf primordia enlarge to form a gland-tooth. When the gland-tooth continuesto develop, the conduplication of the leaf becomes more drastically. Some teeth occur inthe margin of the leaf primordia at the same time. But all gland-teeth become similar insize and shape and have many small and irregular dints on the surface after the leavesshoot out. When the leaf tends to be mature, the gland-teeth become saccate in shape,sticking on the margin of the leaf, and smooth on the surface. The stipules fall off shortly after the young leaf shoots out from the bud. The leaf development on the shortshoot is similar to that of on the long shoot.4. Vessel Elements in Secondary XylemVessel is angular in transverse section, long and small in diameter. There are two contacttypes between vessel element and xylem parenchyma cells: one is vessel-vessel whichcontains end wall-end wall, end wall-lateral wall and lateral wall-lateral wall; another isvessel-xylem parenehyma cells which contains lateral wall-rays and lateral-woodparenchyma. The end wall of vessl elements contain scalariform perforation plates andtransitional types which contain the scalariform perforation and reticulate perforation inone perforation plates. The lateral-walls have typical scalariform perforation plates,reticulate perforation plates and reticulate-scalariform perforation plates in which theperforation pits were both reticulate and scalariform. The appearance of the perforationplates of lateral wall enhence the water's transverse transportion which makes the plantmore suitable for circumstance.5. Anatomical StudiesThe primary structure of root is triarch, other structures are similar to mostdicotyledon. The vascular bundles are ring arranged in the primary stem, the confines ofvascular bundles is not very clearly. The rays in the secondary wood are heterogeneous,multiseriate or uniseriate; wood parenchyma is diffused, and other structures are similarto most dicotyledon. The node is unilacunar and 3 traced in the short shoot; in the longshoot, the node is trilacunar and 3 traced. The vascular bundles of petiolar come throughseparate-combine-separate rule from adaxial side to abaxial side. The epidermic cells ofleaf are irregular. The stomata consist of two guard cells and some radiately elongatedsubsidiary cells, the gland-teeth are saccate in shape, sticking on the margin of the leaf,and smooth on the surface. On the receptacle of the pistillate inflorescence, the node ofoutmost tepals has two to three vascular bundles, and the inner tepals have one to threevascular bundles. The carpel has two vascular bundles at first, then those two vascularbundles become offshoot and four vascular bundles have been formed. The two vascularbundles of abaxial side come into ventral slit and the adaxial one enter into the dorsalside. Stigmatic has short unicellular papillae. On the receptacle of the maleinflorescence, the tepal has two vascular bundles and the filament has one vesselbundles.6. Systematic Position According to the foral morphogenesis, vessel element and morphologicalcharacters of Cercidiphyllaceae, then compared with former research results by otherscholars, we considered that Cercidiphyllaceae might have not close relationship withTrochodendraceae, Tetracentraceae and Eupteleaceae which based on morphologicaltaxonomy. But it seems to show closer relationship with Hamamelidaceae,Daphniphyllaceae and Alangiaceae. This result inosculates to the standpoint ofmolecular systematics. So we considered that it is reasonable to put Cercidiphyllaceaein Saxifragales. But Cercidiphyllaceae might be a primary taxon in woody dicotyledonsin core eudicots else. At the same time, the floral structure and morphogenesis ofCercidiphyllaceae are different from other relative families, those characteristics makethis family highly unique and isolation in phylogeny.