Internodal Growth in Localized Darkness

Abstract

This investigation concerns the growth of the second internode of Helianthus annuus L in localized darkness When the internode was 9-12 mm in length, it was subdivided by india-ink marks into three equal parts. One of these segments (or all three) was wrapped in aluminum foil and kept covered by additional foil as the internode continued to develop and until it became mature The experimental plants as well as a control group without any covers were grown in a greenhouse exposed to a daily light and dark regime It was found that the segments kept in the dark grew at a faster rate and became longer and broader than those of the control plants There was no modification in the developmental pattern nor any basic difference in the duration of growth Evidence from histological studies on the pith indicated that both cell elongation and cell division were promoted in the darkened internodal regions The quantitative changes in gross morphology together with those at the cellular level demonstrated that individual internodal segments show normal dark growth even though the stem tip, the leaves, and adjacent axial regions are simultaneously undergoing development in a daily period of light On the basis of the data obtained in this study, it is concluded that the response to the light stimulus is not transmitted to the dark region but is localized