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Abstract-Phytochrome is the photoreversible chromoprotein that controls many aspects of plant growth and development. Phototransformations of the red absorbing form (Pr) and the far red absorb ing form (Pfr) involve initial photoreactions followed by dark relaxation reactions. Techniques for the study of intermediates of phototransformation and the present picture of intermediates involved in the phototransformations of Pr and Pfr are outlined. The molecular natures of the phototransformations are reviewed in relationship to knowledge of the chemistry of the chromophore and apoprotein. The significance of phytochrome intermediates in understanding the physiology of phytochrome controlled responses is discussed

Phytochrome was found by direct spectrophotometry to be present in whole dry seeds of cucumber. This pigment is spectroscopically different from the pigment found in etiolated plants. It shows the phenomenon of inverse reversion; in darkness, the red-absorbing form (Pr) reverts slowly to the far-red-absorbing form (Pfr). This may explain why 75% of the "dry-seed" phytochrome is in the Pfr form.After imbibition, total phytochrome in the seeds starts to increase. The newly-formed pigment is all in the Pr form and has properties similar to those of "classical" phytochrome of etiolated plants. The relationship of this newly-formed phytochrome with control of germination is presently not known. The dry-seed phytochrome remains unchanged during imbibition and appears to retain its capacity for inverse reversion. This may explain the requirement for continuous or intermittent far-red irradiation in the suppression of germination of cucumber seeds. A similar form of phytochrome may be responsible for control of germination in other seeds which are similarly affected by far-red radiation.