Distribution of Phytochrome in Etiolated Seedlings.

'I'lle l)lalt p)ignent, phytochrome regulates a large numl)er of l)hysiological processes in higher plaints (14). T'he pigment is nmost readily detected in darkgrown see(llings both because it is normally present in higher conlcenitrationis than in light-grown seedlinigs (8, 11), and because dark-grown seedlinigs contain only smlall amounts of other pigments absorbing ligfit ini the same spectral regionis as phvtochrome. The pigment is l)resent in dark-grown seedlings in the red-absorbing form (IP,.) (8, 11), which is normally stal)le. Following transformiiatioln to the far redal)sorl)ing formll (Pf,.) 1b red light. there is a gradual loss of (letectable pigment reversibility, suggestinig that P,, is labile (8, 11). TlIhe dlisal)l)earance of the labile PI>. is apparently enzymatic (7) an(l is prexvenitedl 1V tenpl)eratures near O)°. A stu(ly of phytochroirie (listrilbution in see(llinigs is usefull for several reasons. First, it allows correlative stuly with respect to the distribution of growth and of various growth substances, e.g. auxini. Secon(l, it allows comparison of pigmzenit distribution with the distril)ution of light sensitivity for a given physiological process, providing a l)otential source of informliationi both on the light-selnsitive l)rocess and(l o thle actioin of phvtochrome itself. Third, it l)rovides informiiationi oni good potential sources of mlaterial both for in vivo stuidies of phytochrome anid for pigmient ptirificatioii. The presenlt paper is concernie(d with the distril)utioln of phytochrome in the etiolated seedIilugs of 6 slecies fairly widely usedI in plalit physiological investigations.