The flowering response of short-day plants, such as Xanthium occurs after they have been exposed to a period of darkness which exceeds some minimum duration. We suspect the participation of at least three fundamental processes during this dark period: the action of the photo morphogenetic pigment system (phytochrome), biological time measure ment, and the synthesis of flowering hormone. Action of the phyto chrome system probably combines with some other time measuring mechanism to determine the critical night : the period of darkness which is just infinitesimally too short for the initiation of flowers. Following the critical night, flowering hormone is synthesized in the leaves and subsequently exported to the buds (for reviews, see Hilt .man 1962; Salisbuey 1961, 1963A). Time measurement is especially interesting. At present we have virtually no information about the process except that it is relatively temperature independent (Long 1939, see review in Hillman 1962), and that it may be slowed by the presence of the cobaltous ion (Salis bury 1959 and 1963B). It has been suggested by workers at Beltsville, Maryland (e.g. Borthwick and Hendricks 1960, and Hendricks and Borthwick 1963) that time measurement results from the metabolic conversion of one form of the pigment phytochrome (F-phytochrome, sensitive to far-red light) to the other form (R-phytochrome, sensitive to red light). After darkness begins, the time required for this conversion might constitute the critical night. Some degree of temperature insen sitivity could be achieved by a feed-back between the rate of phyto chrome conversion and the effectiveness of phytochrome in a given process, as both are influenced by temperature. Thus F-phytochrome may inhibit the flowering process most effectively at higher temperatures but also convert most rapidly at higher temperatures. It has been suggested by Bunning (e.g. 1960) and others (e.g. Hamner, 1960, 1963) that time measurement in flowering is related to an internal oscillator such as that apparently in control of other bio
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Feb 1, 1964
HANS MOHR 
