ANASTOMOSES IN THE VENATION OF GINKGO BILOBA

ARNOTT, HOWARD J. (Northwestern U., Evanston, Ill.) Anastomoses in the venation of Ginkgo biloba. Amer. Jour. Bot. 46(6): 405-411. Illus. 1959.-Although the vasculature of the lamina of Ginkgo biloba has been described as open dichotomous and devoid of anastomoses, vein unions were found in a survey of 1065 leaves collected from both long and short shoots of 11 trees. When studied by directing a strong light through the lamina and by clearings, 9.9% of the leaves possessed 1 or more anastomoses. Long shoot-leaves showed 13.4% anastomoses while short shoot-leaves showed 8.2% anastomoses. Multiple anastomoses were found in almost half of the leaves bearing anastomoses. In the 105 leaves having vein unions, 163 anastomoses were counted. These anastomoses could be grouped into 4 types which are easily distinguished by the number of dichotomies involved and by the presence or absence of dichotomies above the point of vein union. Other deviations were found from the "normal" venation pattern; these consisted of unconnected veins, veins anastomosed marginally but unconnected basally, and veins ending a considerable distance from the margin. It was speculated that the anastomoses found in Ginkgo biloba are of a simple, archaic type and are apparently analogous to the anastomoses in the leaves of certain ferns and in the leaflets of various cycads. The evolutionary significance of these anastomoses must be assessed by a broad study of venation patterns in the seed-plants. THE INQUIRING MINDS of nineteenth-century investigators probed many basic morphological problems. The foundations for the study of the venation systems in the leaves of angiosperms were laid by Von Ettinghausen (1854a, b, 1861, 1865) during this period. Through his prodigious efforts several magnificent monographs on venation patterns were produced. There has been a resumption of interest in venation patterns in recent years due to the work of Goebel (1922), Troll (1938), Foster (1950a, b, 1951), and others, who have shed new light on the foliar vasculature of plants. The bizarre venation patterns of such plants as Quiina (Foster, 1950a), Touroulia (Foster, 1950b) and Lacunaria (Foster, 1951) have added new interest to the study of venation patterns. Even more recently, Foster (1952), Pray (1955a, b) and Slade (1957) have considered foliar venation from an ontogenetic standpoint. In the present paper some interesting aspects of the venation system of Ginkgo biloba will be presented. That anything novel could be discovered about the foliar vasculature of Ginkgo, long used as a typical example of a seed plant with open dichotomous venation, was quite astonishing. The discovery of 2 vein anastomoses in a cleared leaf suggested the desirability of a comprehensive survey of the frequency and nature of vein unions in the leaves of Ginkgo. The results of this survey show that several rather distinct "types" of anastomoses occur despite the repeated statements in the literature that vein unions are not present in the foliar venation of this remarkable gymnosperm. It seems probable that these anastomoses have been observed before, but that they were doubtless dismissed as anomalies. 'Received for publication December 17, 1958. The author wishes to express his appreciation to Professor Adriance S. Foster for reading the manuscript anid for his many valuable comments and criticisms. The nodal anatomy (Gunckel and Wetmore, 1946; Bailey, 1956), the open dichotomous vena. tion (Sprecher, 1907; Florin, 1937; Arnold, 1947), and other characteristics, such as motile sperms, have given Ginkgo, along with the cycads, an unrivalled primitive position among living seed plants. Ginkgo has been traced back in the fossil record to the Triassic (Arnold, 1947). The general nature of the open dichotomous ven. ation of the leaves of Ginkgo biloba is well known from both living and fossil specimens of this species. Fankhauser (1882) has excellent illustrations of the leaf and of its venation. Sprecher (1907) states that as in Botrychiumr, Adiantum, Trichomanes, etc., the foliar venation of Ginkgo is devoid of anastomoses. Florin (1937) emphasizes that anastomoses are never present ("Anastomosen fehlen stets," 1937, p. 3).2 According to Arnold (1947, p. 276): "In all cases the tips of the veins are free at the leaf margin. There are no cross connections in the Ginkgoales." The vascularization present in the leaf of Ginkgo biloba is constituted as follows: 2 traces arising at a single gap, but coming from independent portions of the eustele, extend up through the petiole (Gunckel and Wetmore, 1946); at or just below the expansion of the lamina each of these traces dichotomizes. The veins produced by these dichotomies fork repeatedly, until each ultimate vein ends freely near the leaf margin (cf. Arnold, 1947, p. 274-276, for descriptions of the venation of both living and fossil examples of Ginkgo). MATERIALS AND METHODS.-Shoots were collected from 11 trees growing on or near the campus of the University of California, Berkeley. The tree and the 2What appears to be an anastomosis (type "C") can clearly be seen in the upper right leaf in fig. 1 (p. 4) of Florin (1937).