Abstract: A cladistic analysis of the families in the Hamamelidae is made in the present paper. As a monophyletic group, the subclass Hamamelidae includes 19 families, namely, the Trochodendraceae, Tetracentraceae, Cercidiphyllaceae, Eupteleaceae, Eucommiaceae, Hamamelidaceae (incl. Rhodoleiaceae and Altingiaceae), Platanaceae, Daphniphyllaceae, Balanopaceae, Didymelaceae, Myrothamnaceae, Buxaceae, Simmondsiaceae, Casuarinaceae, Fagaceae (incl. Nothofagaceae), Betulaceae, Myricaceae, Rhoipteleaceae and Juglandaceae. The Magnoliaceae was selected for outgroup comparison after careful consideration. Thirty-two informative character states were used in this study. Three principles, namely, outgroup comparison, fossil evidence and generally accepted viewpoints of morphological evolution, were used for polarization of the characters. An incompatible number concept was first introduced to evaluate the reliable degree of polarization of the characters and, by this method, the polarization of the three character states was corrected. A data matrix was constructed by the 19 ingroup families and 32 character states. The data matrix was analysed with the Minimal Parallel Evolutionary Method, Maximal Same Step Method (Xu 1989), and Synthetic Method. Three cladograms were constructed and a parsimonious cladogram (Length= 131)was used as the base for discussing the systematic relationships of families in the Hamamelidae. According to the cladogram, the earlist group differented in the subclass Hamamelidae consists of two vesselless wood families, the Trochodendraceae and Tetracentraceae. This result supports the concept proposed by Takhtajan (1987)and Cronquist (1981, 1988)that the Trochodendrales is probably a primitive taxon in the Hamamelidae. As in a clade group, the Cercidiphyllaceae, Eucommiaceae, Balanopaceae and Didymelaceae originated apparently later than the Trochodendrales. The Cercidiphyllaceae diverged earlier in this group, which implies that this family and the Trochodendrales form a primitive group in the subclass. The Cercidiphyllaceae is either placed in Hamamelidales (Cronquist 1981, Thorne 1983), or treated as an independent order (Takhtajan 1987).This analysis suggests that the Cercidiphyllaceae is a relatively isolated taxon, far from the Hamamelidaceae but close to the Trochodendrales in relation. The Eucommiaceae and Didymelaceae are both isolated families and considered as two distinct orders (Takhtajan 1987, Cronquist 1981, 1988).The Balanopaceae is included in the Fagales (Cronquist 1981, 1988) or Pittosporales (Thorne 1983), or treated as a distinct order Balanopales (Takhtajan 1987 ).Obviously the Balanopaceae and Eucommiaceae are not closely related because of the sole synapomorphy (placentation).In fact these four families are more or less isolated taxa and it is probably more reasonable to treat them as independent orders. Cronquist ( 1981, 1988) places the Eupteleaceae, Platanaceae and Myrothamnaceae in the Hamamelidales, while Takhtajan (1987)puts Hamamelidaceae and Platanaceae into the Hamamelidales and treats the Eupteleaceae and Myrothamnaceae as two independent monofamilial orders. These three families are grouped by more synapomorphies (palmateveined, serrate or lobate leaves, deciduous and anemophilous plants)which may indicate their close phylogenetical affinity. A core group of the Hamamelidae includes ten families, among which the Hamamelidaceae originated earlier than the others, so that it is a relatively primitive family. The Betulaceae, Fagaceae and Myricaceae differentiated later than the Hamamelidaceae. The former two are very closely related, and thus thought to be two neighbouring orders by Takhtajan (1987)or included in the Fagales by Cronquist (1981, 1988)and Thorne (1983). The Myricaceae and Fagaceae are connected in the cladogram by only a single synapomorphy (endosperm absent), and therefore the close relationship does not exist between them. The Buxaceae, Simmondsiaceae and Daphniphyllaceae form an advanced group, in which they are weakly linked with each other by only one synapomorphy (pollen grains＜25μm). The Daphniphyllaceae is closely related to the Simmondsiaceae, but the Buxaceae is rather isolated. The Rhoipteleaceae and Juglandaceae share a number of synapomorphies (axile placentation, endosperm absent, embryo larger, fruit indehiscent) , forming a highly specialized group. The opinion that the Juglandales is composed of the Juglandaceae and Rhoipteleaceae(Cronquist 1981; 1988, Lu et Zhang 1990)is confirmed by this analysis. A contrary point of view, which treated them as two distinct orders by Takhtajan (1987), apparently could not be accepted. The Casuarinaceae was regarded as the primitive angiosperm (Engler 1893), but in fact it is a highly reduced and specialized group. It is united with Rhoipteleaceae and Juglandaceae by four synapomorphies, i. e. placentation type, endosperm absent, embryo large and fruit indehiscent. However, the family presents six autapomorphies, and thus the position of the Casuarinaceae as an advanced family is confirmed by this analysis. Finally a strict consensus tree, which represents the phylogenetic relationships of thefamilies in the Hamamelidae, was given as a result of the analysis.

Key words: Hamamelidae, Phylogeny, Cladistic analysis