MORPHOLOGY OF ANASTOMOSES IN THE DICHOTOMOUS VENATION OF CIRCAEASTER

Abstract

Anastomoses are very infrequent in the dichotomous venation of Circaeaster agrestis, and no examples of a leaf with more than a single anastomosis were found in a comprehensive study of herbarium and cultivated specimens. The anastomoses are classified into three principal morphological types. In Type I, the adjacent convergent branches of two vein‐dichotomies become confluent for a very short distance and then separate and extend without branching into corresponding teeth of the lamina margin. Except for the short area of union between the two xylem strands, Type I anastomoses resemble vein approximations. The latter represent the close juxtaposition, without xylem‐fusion, of the branches of adjacent vein‐dichotomies. In Type II anastomoses, the inner branches of two neighboring vein‐dichotomies converge and by means of a permanent union, form a single excurrent vein which terminates in a marginal tooth. The distinctive feature of Type III anastomoses is the formation of a very short “commissural vein” which interconnects the closely adjacent branches of two vein‐dichotomies. Cleared leaves revealed that the two veins which fuse in Type II anastomoses may have equally well‐developed xylem strands or one of the branches may be vestigial in structure or entirely devoid of tracheary elements. The xylem of the “commissural vein” in Type III anastomoses consists of one or several tracheary elements which often produce a rather tenuous connection between the two veins. A brief discussion is given of some of the morphogenetic questions posed by the sporadic anastomoses in Circaeaster. The significance of anastomoses with reference to the phylogeny of the dichotomous venation is also considered. Although the anastomoses seem morphologically comparable with the vein‐unions in certain primitive members of the Pteropsida, these resemblances are probably indicative of parallel evolution rather than of the actual nature of the venation in the remote ancestors of Circaeaster.