STUDIES OF DEVELOPMENT IN LONG SHOOTS AND SHORT SHOOTS OF GINKGO BILOBA L. IV. GROWTH HABIT, SHOOT EXPRESSION AND THE MECHANISM OF ITS CONTROL

THE MAIDENHAIR TREE, Ginkgo biloba L., has impressed the naturalist ever since its discovery. Sprecher (1907) in a monograph on Ginkgo included a complete and interesting historical review of its discovery, description, classification and fossil record. The present investigation presents extensive observations on shoot growth and on the general growth habit of Ginkgo. The two well-defined types of shoots, namely, long shoots and short shoots, which occur in Ginkgo are capable of a reversal in growth form. This study attempts to explain the change in shoot expression by experiments on artificially induced reversal, correlated with the studies on auxin production previously published (Gunckel and Thimann, 1949). Also, an attempt is made to explain the over-all change in the growth habit of Ginkgo by observations made on the variations in individual shoot expression over a period of 5 years. MATERIALS AND METHODS.-Observations on shoot expression and growth habit were made throughout the season for 5 years on trees which were 100 (approximately), 35, 15, and 6 years of age in 1940. In 1948, observations were also made on 3-year-old seedlings. These plants were growing under normal field conditions. Also, plants 6 years old in 1940 were held in large pots in a darkened cold room at 1-3?C. during the winter, and brought up each spring to the greenhouse for observation until 1947. We shall consider these plants in three somewhat arbitrary groups based upon shoot expression. Plants considered as "mature" may not be of fruiting age but are only mature in the sense that their shoot expression is rather stable and changes may be induced only with some difficulty. Included under "young" plants are those 6-15 years old, whose shoot expression and growth habit are capable of change under certain conditions, either in nature or induced artificially. Finally there are the "seedlings" up to 6 years old, whose behavior is still more plastic. -In decapitation experiments the terminal buds were removed just below the apical meristem. In the controls the cut surfaces were covered with plain lanolin paste. In the experimental plants the cut tip was covered with a 0.5 per cent indoleacetic 1Received for publication October 15, 1948. The authors' best thanks are due to Mr. William Flemmer of the Princeton Nurseries, Princeton, N. J., and to Mr. Francis Lazenby of the Botanic Garden for so kindly providing seedling material. Grateful appreciation is also due to Mr. Robert Wharton of the Harvard Biological Laboratories for growing the greenhouse plants. 2 This research was made possible, in part, by a grant from the Research Council, Rutgers University. acid-lanolin paste, or with a 1 per cent a-naphthaleneacetic acid-lanolin paste. First applications in all cases were made at the stage when the buds were swelling, or just showing green (Gunckel and Thimann, 1949, Experimental material, stages 1 and 2), and renewed every 3-4 days on a fresh cut surface over a period of 2 weeks. The last application was left on for another 10 days and then removed. Observations were recorded at the end of 3 more weeks, at which time the growth of lateral buds had advanced far enough to determine definitely the form of shoot expression. GROWTH HABIT AND SHOOT EXPRESSION OF MATURE TREES.-In most accounts of the growth habit of Ginkgo the trunk of young plants is described as strongly excurrent and the outline steeply conical, but as the tree gets old it branches and broadens at the top to give a globose outline (Chamberlain, 1935). Further, the staminate trees are often reported to be of the excurrent, conical type while the ovulate trees are of the broader outline and more freely branched type. Observations on a number of trees show that this latter generalization is subject to some wide variations. Fig. 1 shows the 100-year-old ovulate Ginkgo, supposed to have been planted by Asa Gray in the Botanic Garden. This specimen has the typical broad outline described for ovulate trees. Contrast with this, however, the excurrent female tree in fig. 2 which fruited for the first time in 1943. The observation of Chamberlain (1935) that the tree broadens with age is a much better generalization. An analysis was made on trees of varying ages to see if there was a definite correlation between shoot expression and growth habit. Fig. 3 shows a 35-year-old tree, growing in the Arnold Arboretum, which has 21 main branches in all. Careful analysis of all of the terminal buds of not only these main branches but also of the 350400 secondaries showed that all terminal buds on the tree, except 12, were long shoots. Terminal long shoot expression, then, is correlated with the excurrent type of growth. Since this plant was topped by the 1938 hurricane the intact 15-year-old plant shown in fig. 4, also growing at the Arboretum, was analyzed for comparison. The general growth habit and shoot expression was the same, since all of the terminal buds were long shoots except a few on the small secondary branches. By contrast, analysis of the shoots of the 100year-old Gray Ginkgo (fig. 1) shows that there is approximately an equal number of long and short shoot terminals. Trees broaden and fill out, then, as terminal buds become short shoots and lateral buds become long shoots. This change takes place