Plant & Cell 12: 567-579 (1971) Germination of seeds in H.lJptis suaveolens Poit RENATA WULFF and ERNESTO MEDINAl Escuela de Biologfa, Universidad Central de Venezuela, Caracas, Venezuela (Received January 21, 1971) Seeds of Hyptis require long illumination periods to promote full germina- tion, botb light and dark germination being controlled by the phytochrome system. Germination in this species is inhibited both by relatively low (up to 20°C) and high temperatures (45°C). Experiments in which seeds were transferred after different periods from sub-optimal or supra-optimal temperatures to a favourable one, suggest that both high and low temperature inhibition may be explained on the basis of phytochrome action. Thus, a temperature of 20°C induces dark dormancy due, probably, to dark reversion of phytochrome to the inactive form; whereas, at 45°C the dominant process seems to be phytochrome decay. No phytochrome loss has been observed at 10°C. If, however, phytochrome levels are reduced by a particular treatment, no irreversible alteration is produced; seeds simply require longer illumination periods under these circumstances. A low initial concentration of phytochrome could account for the requirement of long illumination periods. Other factors affecting germination in this species, such as alternating temperatures, gibberellie acid and time of storage, are discussed. Hyptis suaveolens is an aggresive annual weed, commonly found in the Central Llanos of Venezuela. Its distribution is tropical and subtropical, normally re- stricted to places where soils have been profoundly disturbed, such as abandoned fields or along roadsides, and in this sense may be considered a "ruderal" species as defined by Walter (1). Distribution of the species is also limited by altitude, and it is not normally found over 500 m. This paper reports the germination characteristics of Hyptis suaveolens seeds in relation to light and temperature responses. As germination is one of the most critical phases for the establishment of a given species in a particular environment, understanding the basic germination mechanisms could help in explaining the distribution of this species. Materials and rn.ethods Seeds of H'yptis suaveolens were harvested at the Biological Station of Calabozo (Edo. Guarico) inJanuary 1969 and January 1970, and stored in plastic jars at 20°C and at a relative humidity of about 40%. Seed age is indicated in each experiment. 1 Present address: Instituto Venezolano de Investigaciones Cientificas, Apartado 1827, Caracas, Venezuela. 567