The Growth and Development of Internodes in Helianthus

Internodes of Helianthus annuus L were examined at several stages of development in order to determine the pattern of growth and thereby shed light on the cellular changes by which elongation is achieved. The observations described in this report were made on the first, or lowermost, internode. The growth of an internode from the time of initiation until maturity usually takes place within a 2- to 3-week period and follows the pattern of a sigmoid curve. Elongation is first centered in the basal part and then shifts to loci at progressively higher levels in such a manner that a wave of growth extends from the base in an acropetal direction Although an internode has its origin in the shoot tip, most of its growth occurs many centimeters below this apical region. Histological analyses indicate that growth in all parts of an internode is the result of cell division and cell elongation; the two cellular processes are not limited to specific zones nor to any phase of development. However, the number of cells increases more rapidly in the early stages of development than in the later stages. As an internode grows in length, cell division occurs in progressively longer cells, ranging in length from about 20-30 μ in very young internodes to 250 μ or more in segments of maturing internodes. As an internode approaches maturity, the cells of the supraadjacent nodal region show a marked increase in length, thereby contributing to the wave of growth which extends acropetally through the shoot axis. The elongation of the shoot axis as a whole is orderly in time and in place as a result of the integrated progression of growth in the successive nodes and internodes.