Abstract
Dianthus caryophyllus L., a quantitative long day plant, was used for studies of the effect of photoperiod on the different stages of plant development. The anatomical structure of the vegetative shoot apex and the changes during floral transformation are described. These stages were not affected by the various photoperiod lengths, i.e., 9, 13, and 18 hr. The developmental cycle of D. caryophyllus may be divided into 3 stages: vegetative, floral initiation, and floral development. The influence of photoperiod on the duration of each of the 3 stages was different. The length of the vegetative stage was inversely related to the length of the photoperiod (the longer the photoperiod, the shorter the vegetative stage). Daylength had no effect on the period of floral initiation, but did affect the period of floral development (i.e., the longer the photoperiod, the longer the developmental stage). The rate of stem elongation under all treatments increased exponentially until the floral initiation state, and then the plants grown under photoperiods of 13 hr or longer elongated significantly faster than those under the 9-hr photoperiod. After flower initiation was completed, the rate of stem elongation suddenly increased linearly until the flower opened. In contrast to the vegetative shoot, the rate of stem elongation of flowering plants was completely independent of photoperiod, and the rate was the same in the 9, 13, and 18 hr photoperiods.

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