and 7 days in the bean hypocotyl (2). It is clear that the total amount of energy required to initiate these responses at low light intensities is higher than that required by the human eye since the response of the eye to low light intensities takes a fraction of a second. The photoreceptor for photomorphogenesis in plants is the chromoprotein phytochrome (P). The phytochrome pigment system exists in 2 forms — PFR — the far-red-light-absorbing form which exists after irradiation with red light and PR - the red-light-absorbing-form. The relationship between these two forms of phytochrome is shown in Fig. 2. Figure 2. The relationship between phytochrome and red-light or far-red- light illumination. Note that destruction of PFR to an unknown occurs in light. Plants respond to PFR. Developmental responses in plants are regulated by the levels of PFR; as far as is known, PR does not play a regulatory role in development (3). The way in which phytochrome acts to influence developmental events in plants is not known; however, a considerable amount is known about the relationship between FPR formation and the initiation of the developmental event. It is generally agreed that there is a roughly logarithmic relationship between PFR and the developmental response (U,5). Furthermore, for
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