DORMANCY IN SEEDS 127 



In examining all the foregoing discussion on dormancy of seeds, it will 

 be seen that nature secures delayed germination in seeds by a great variety 

 of methods and not by a single method. It is not improbable that dry 

 storage likewise leads to the after-ripening of seeds by several different 

 essential changes in the many kinds of seeds involved. 



Temperature a Factor in Overcoming Dormancy 



We have already spoken of the importance of daily alternating tempera- 

 tures in substituting for light in light-sensitive seeds. There are many 

 other records in the literature of seeds that germinate much better at 

 alternating temperatures than at optimum constant temperatures. This 

 is so important that alternating temperatures are used as a regular pro- 

 cedure in the commercial testing of many seeds. Seeds in nature in the 

 soil experience such a daily alternation of temperatures — warm in the 

 da\i:ime and cool at night. There are many seeds that germinate at tem- 

 peratures at or near the freezing point, and some of them germinate only 

 at such low temperatures. There are other seeds that require high tem- 

 peratures for germination. 



Schroeder and Barton ^"^ found that seeds of some high-altitude alpines 

 (Calochortus macrocarpus, Camassia LeichtUnii, Lewisia rediviva) germinate 

 only at low temperatures. The first germinate best at 5° C (41° F) and 

 will not germinate fully at temperatures much above this. Annual del- 

 phinium seeds do not germinate well at temperatures above 15° C (59° F). 

 These low-temperature seeds can be successful!}^ growTi at temperatures 

 above the low maximum for germination by pregerminating ^^ at favor- 

 able low temperatures before planting at higher temperatures. We have 

 already mentioned the fact that many seeds that need low- temperature 

 stratification will germinate when they are after-ripened right at the 

 optimum stratification temperature. As will be seen in Table 11, this 

 temperature is 1° C (34° F) for several seeds studied. We have also em- 

 phasized the fact that seeds after-ripened at low temperatures may go 

 back into secondary dormancy if put in a germinator at too high a tem- 

 perature. \Miile one generally thinks of physiological processes in plants 

 increasing in intensity as the temperature rises above the freezing point, 

 the after-ripening of seeds goes on fastest near the freezing point and falls 

 off as the temperature rises; and in some seeds the germination proceeds 

 fastest just a little above the freezing point. 



Just as some kinds of seeds are attuned to very low temperatures for 

 after- ripening and germination, others require relatively high temperatures 

 for germination. At the Institute we have tested a number of crops of 

 Amaranthus retro flexus seeds for their temperature requirements for ger- 

 mination immediately after harvest and after various periods of dry stor- 

 age. Immediately after harvest these seeds require a temperature of 35° 

 to 40° C (95° to 104° F) for germination, and they germinate promptly at 

 this temperature. As they remain longer and longer in dry storage they 



