DORMANCY IN SEEDS 89 



freezing and tha\ving are not. Although the optimum temperature for 

 after-ripening of Sorhus seeds is nearer 1° C (34° F) than 5° C (41° F), 

 that for other rosaceous seeds with dormant embryos such as some species 

 of Rosa (Fig. 29) is about 5° C (41° F). The best optimum temperature 

 for the stratification of Rhodotypos seeds ^* is about 5° C (41° F). Fluc- 



Figure 29. Rosa rubiginosa seeds: check stored dry; the others in moist sand for six 

 months at the temperatures designated and then planted in a flat in the greenhouse. 

 The picture shows the effective temperature for stratification, that is, 5° C (41° F). 



tuating temperatures ^^ithin a certain range are effective; for instance, for 

 Rhodotypos seeds daily alternating (16 hours at low temperature and 

 8 hours at high temperature) or weekly alternating temperatures (half 

 time at each temperature) of 1° and 10° C (34° and 50° F), 1° and 15° C 

 (34° and 59° F), and 5° and 10° C (41° and 50° F) proved about as effective 

 as, and in some cases better than, the constant temperature 5° C (41° F). 

 Table 11 shows the optimum stratification temperatures,^- the effective 

 range, and the length of time required for complete after-ripening of vari- 

 ous seeds that have been studied at this Institute, with the addition of 

 three studied earlier at the University of Chicago. The three additional 

 ones are: Crataegus mollis, Acer saccharum, and Juniperus species. This 

 table includes seeds with both dormant and non-dormant embryos that 

 respond to low temperatures. This list could be considerably extended by 

 drawing upon literature from other sources. It Mill be noticed that a 

 number of seeds have an optimum stratification temperature of about 

 1° C (34° F), though most of them have 5° C (41° F) as the optimum. A 

 number have 10° C (50° F) as the optimum, and some after-ripen equally 

 well over a rather ^^^de range, 1° to 5° C (34° to 41° F) or 1° to 10° C 

 (34° to 50° F). 

 The degree of sluggishness in the early gro'\\i;h of excised dormant em- 



