636 GERMLNATION AND DORMANCY 



understood. It is entirely ineffective with seeds of some species. Seeds of 

 carrot and timothy, for example, germinate just as well at constant tem- 

 peratures as when temperatures are varied. In general this method of treat- 

 ment is used principally with seeds in which dormancy is inherent in the 

 embryo. 



4. Light. — In the previous discussion light was mentioned as one of the 

 conditions essential for the germination of certain species of seeds. Light 

 may be considered, therefore, as a means of breaking the dormancy of such 

 species. In some of the species other environmental factors can be substituted 

 for light. In seeds of Vero?uca lotigifolia (one of the commonly cultivated 

 speedwells), for example, light improves germination at low temperatures but 

 the seeds germinate equally well in total darkness at high temperatures. In 

 seeds of Kentucky blue grass exposure to light is effective in improving 

 germination at both intermittent and constant temperatures. No satisfactory 

 explanation of these facts has yet been suggested. 



5. Pressures. — Seeds of sweet clover (Melilotus alha) and alfalfa {Medi- 

 cago satlva) showed greatly improved germination after being subjected to 

 hydraulic pressures of 2000 atmos. at 18° C. (Davies, 1928). When the 

 pressure was applied for periods of from 5-20 minutes the germination of 

 the seeds was increased by 50-200 per cent. The effect of the pressures 

 persists after the seeds have been dried and stored and is undoubtedly due to 

 changes in the permeability of the seed coats to water. 



Longevity of Seeds. — The life-span of seeds varies from a few weeks 

 to many years, depending upon the species and the environmental conditions 

 to which the seeds are subjected. The silver maple {Acer snccharhium^ may 

 be cited as an example of a species which has short-lived seeds. When the 

 seeds of this species are shed in June their water content is about 58 per cent. 

 Once their moisture content drops below 30 to 34 per cent the seeds die 

 (Jones, 1920). Since this often happens within a few weeks in nature, seeds 

 of this species soon perish. The seeds of the majority of crop plants are 

 relatively short-lived under the usual storage conditions, generally remaining 

 viable for only one to three years. The life-span of such seeds can often be 

 increased several fold by keeping them under suitable storage conditions. 



At the other extreme there are a few authentic records of seeds which 

 have lived for more than a hundred years. Bequerel (1934) succeeded in 

 germinating in 1934 seeds of Cassia bicapsularis which had been collected 

 in 1 819, and seeds of Cassia fiitiltijuga which had been collected in 1776. 

 These are both South American species of legumes. Viable seeds of the 

 Indian lotus {Nelumbo niicifera) have been found buried under layers of peat 

 and soil in Manchuria of such depth that they must have been at least 120 



