The results from fractional induction treatments give further support to the concept that induction is not an 'all or none' process. It appears that, once a sufficient number of inductive cycles has been given, a condition is attained which is stable but is insufficient to elicit flowering. The existence of degrees of induction is also evident from other types of experiment. In red Perilla, for example, the minimum duration between the donor leaf and receptor plant decreased as the number of SD given to the donor leaf was increased, indicating that the rate of export of a flowering stimulus from the leaf is proportional to the duration of the inductive treatment. The number of days before flower buds appeared on the receptor also decreased with increasing numbers of SD given to the donor, and was similar whether grafts were made immediately after the SD treatment or after the donors received a further 28 LD. The degree of induction of a particular leaf, therefore, appeared to be maintained during non-inductive LD (Zeevaart, 1958). Other experiments with intact plants support the idea that induction varies in degree. For example, in Xanthium, plants given multiple SD cycles produced mature flowers in about 2 weeks whereas, when given a single SD, plants progressed only slowly to flowering over several months (Salisbury, 1963b). The first visible signs of flowering occurred at about the same time in both. The rate of development was not accelerated by reducing the number of buds; it seems, therefore, to be a function of a progressive degree of induction in the leaf and not to the amount of stimulus available at each receptive meristem.
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