1. The time course of germination of Grand Rapids lettuce seeds has heen followed with different combinations of temperature (3°–35°) and irradiation (red or far-red light). For each set of conditions the following three parameters were determined: (i) the time required for half maximum germination, (ii) the rate of germination during the actively germinating phase, and (iii) the maximum germination attained. In general, as the temperature was lowered, with dark-imbibed seeds, (i) became longer, (ii) became lower, but (iii) became progressively higher. The effect of red light at any temperature was to shorten (i) and increase (ii) and (iii) over the values dark controls. Far-red light exerted an effect opposite to that of red light. Temperatures higher than 25°inhibited (ii) and (iii) under any light conditions. The optimum temperature to the action of red and far-red light is 25°, at which the stimulatory effect of red light and the inhibition of this effect by far-red light are both maximal.2. The growth of the radicles of de-coated seeds of Grand Rapids lettuce shows two phases at all temperatures studied. Phase I is characterized by slow but linear growth which continues until shortly after visible differentiation of the radicle into the hypocotyl and the root. Phase II is a phase of active growth in which the total length reflects mainly the length of the root. The optimum temperature for Phase I is 25°-35°, and that, for Phase II is 25°. In neither phase, and at none of the temperatures studied, is there any effect of red or far-red radiation on the growth of the radicle. The first visible sign of radicle elongation in red light induced seeds, however, takes place at exactly the same time as that of germination.3. Similarities and dissimilarities between the germination and the growth are pointed out, and it is concluded that the two phenomena are different, but proceed at sites closely associated in th
展开▼
