au ? . 14. 1916 Agricultural Value of Impermeable Seeds 781 



has always depended upon a previous incubation of the seeds at a cool 

 temperature. Usually the longer the previous period of cool incubation 

 the greater has been the effect of the alternation. Alternations from 

 i° to io° or from 20 to 30 had little or no effect. 



In some cases more than 90 per cent of seeds which had previously 

 remained impermeable in wet blotters for several years have softened 

 and germinated in a few weeks with a favorable succession of constant 

 cool temperatures and alternations of temperature. 



The work outlined in this section is of special interest in connection with 

 the following sections. The results here presented would lead one to 

 expect that impermeable clover seeds would lie in the soil without change 

 during either steady cold weather or constantly warm weather and in 

 the fall, when an alternation of warm days and cool nights follows the hot 

 summer months, but that many of them would germinate and produce 

 seedlings at the beginning of the growing season in the spring when a 

 similar alternation follows months of cold winter weather. These results 

 also show that subjection to freezing temperatures is not necessary in 

 order to prepare the majority of impermeable clover seeds for rapid ger- 

 mination. A temperature of about io° C. does quite as well as i°, and 

 either of these temperatures is much more effective under favorable con- 

 ditions than freezing temperatures under conditions which are less 

 favorable. (See p. 776-778 and Table VIII.) 



GERMINATION OF SEEDS AFTER PASSING THE WINTER ON OR UNDER THE 

 PARENT PLANTS IN THE FIELD 



Seeds which had passed one winter on or under the parent plants in 

 the field were gathered the following spring after warm weather had 

 begun. The germinating capacity of these seeds and the percentages 

 which were impermeable were determined and compared with the ger- 

 minating capacity and percentage of impermeable seeds of lots of seed 

 which were gathered from the same stands of plants the preceding fall 

 and stored in the laboratory during the winter 



The red-clover seeds which were gathered in the spring consisted of 

 eight lots from heads which remained intact upon the parent plants and 

 were several inches above the ground, and eight other lots from heads 

 which were embedded in the mud. 



The alsike-clover, sweet-clover, and yellow-trefoil seeds gathered in 

 the spring were all embedded in the mud. A large number both of these 

 and of the red-clover seeds which were embedded in the mud had ger- 

 minated and produced a dense growth of strong green seedlings 

 before they were gathered. Many red-clover seeds had softened, and a 

 few had germinated even in the heads which were several inches 

 above the soil. About half an inch of soil was taken up with the seed- 

 lings which were growing in the soil, and carefully worked over to re- 



