Further evidence for the existence of two phytochrome systems from two distinct effects of far-red light on lettuce seed germination

Thermo-dormant lettuce seeds can be induced to germinate by small doses of far-red light. The dose-effect curve appears to be an optimum curve. After having been irradiated with a supra-optimal inhibiting dose of far-red the seeds can be reinduced by red light, the threshold red dose for induction being now 104 times as large as it is without the far-red preirradiation. It is concluded that the inductive effect of far-red light is mediated by an uncommon form of phytochrome which is considered to be also present in etiolated Avena seedlings and Sinapis seedlings, and in cells of Mougeotia.