Physiology of Flowering

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Abstract
Since the discovery of photoperiodism in 1920 an enormous amount of data on the photoperiodic control of flowering has accumulated. Most of the work so far has been descriptive and cannot yet be translated into chemical or physical terms. The isolation of the photoreceptor phyto-chrome is a 1st step in that direction. More than 25 years have gone by since the 1st experimental evidence in support of the flower hormone theory was presented, and such evidence has grown steadily, although flower inhibition theories have been advanced occasionally. Extraction and identification of the chemical structure of the hormone are the most urgent problems in this field at the present time. This would result in unlimited possibilities for further research. It is hoped that a recent report about a flower-indue ing extract from Xanthium will lead toward further characterization of the active principle. The principal problem is still the design of a simple and reliable bioassay for the flowering substance. Another approach may be the labeling of the hormone by the use of more specific precursors than C14O2. Obviously, such work should be initiated with the simplest systems available, such as Xanthium and Pharbitis. It will not be profitable to proceed to more complicated cases until one "simple" case has been well worked out. Although there are many similarities among the various reaction types, there are also many differences. As pointed out by Lockhart, the almost universal nature of certain biochemical processes (respiration, photosynthesis) need not necessarily be expected in the flowering process.