A Quantitative Study of Shoot Development in Vicia faba: II. The Elongation of Internode and Petiole

The elongation of the internodes and petioles of Vicia fabahas been studied in the plumules of normal seedlings, in seedlingsfrom which part or all of the cotyledonary reserve has beenremoved, and in axillary shoots developed on seedlings of variousages. The curves obtained by plotting the lengths of the internodesagainst their plastochrone ages reveal constant differencesin behaviour between plumules and axillary branches. As thegrowth of any shoot progresses a greater contribution to thetotal elongation rate cornea to be made by the internodes fartherremoved from the apex. The length of each internode depends throughout its developmenton its position in the stem as well as on the length of theinternode below. The extent to which the elongation of an internodelags behind that of the one below increases upwards, a relationshipsimilar to that previously shown to exist in respect of thevascularization of the internodes. In plumules the length of petiole corresponding to any givenlength of internode undergoes a decrease followed by an increaseas the plumule is traversed from base to apex, a relationshipexactly oppsite to that previously demonstrated between thevascularization of a leaf-trace bundle and that of the internodein which it runs. The effects of age and of removal of cotyledonary reserves aredescribed, and it is concluded that it is unlikely that thecharacteristic difference in internode length within the limitsof a single shoot are wholly due to changes in the availabiltyof reserve substances.