Abstract
When the vine of the chinese yam corrtinues its growth beyond the extreme of its support, it com-mences to hang downwards. The authors have noted that aerial-tubers are formed when the vine is in an inverted position and that the formation is restricted within the part hanging downwards. Based on the above mentioned fact, we have con-ducted experiments in order to clarify the 'effect of direction' of the vine upon the aerial-tuber formation. The results observed may be summarized as follows 1. When the vine is growing vigorously upwards, generally no aerial tubers are formed. Whereas when the vine is inverted artificially, aerial-tubers begin to develop in the axils of the leaves. In this case, the size of aerial-tubers increases by steps and mark-edly as the inverted tip is approached, which denotes that there is no correlation between the size of the aerial-tuber and the leaf area of respective node. Moreover, when the vine is trained in a wave form directing the vine upward and downward alternately, the aerial-tuber formation is restricted to the downward parts of the vine. From these results, it is conjectured that gravity has an important effect upon the formation of aerial-tubers.
2. Chemical analysis of leaves reveals that the rate of flow of carbohydrates from the leaves is more sluggish in the 'inverted vine than in the upward growing vine.
3. To further elucidate our point additional experiments with common potatoes were conducted. When experimental potato plants were allowed to grow downwards in a hanging position a similar aerial-tuber formation occurred.
4. In order to block the translocation of carbo-hydrates from leaves, the tuberous root was removed at an early stage. In this case, aerial tubers are formed not only in the hanging but also even in the upwards growing vine. Chemical analysis of leaves indicated a high accumulation of carbohy-drates in the leaves of treated plants.
5. The aerial-tubers can readily be induced to form in the axils of the leaves by stem cutting or leaf bud cutting; presumably because of the car-bohydrate accumulation.
6. From these results, it is evident that the accumulation of carbohydrates in the above-ground part is the causal factor which induces the forma-tion of aerial-tubers of chinese yam, and in the open field, the accumulation of carbohydrates is induced naturally by dangling of vine.
