86 



GROWTH OF PLANTS 



tute for stratification and has advantages over nature's method in that 

 the seeds are well covered with soil. 



For a long time it was assumed that after-ripening in stratification was 

 brought about by freezing or by freezing and thaAving. This is almost, 

 although not quite, entirely wrong. The changes involved in low-tempera- 

 ture after-ripening of seeds occur in the main at temperatures above 

 freezing, ranging from 1° to 15° C (34° to 59° F), for various sorts of seeds. 



Figure 26. Excised embryos of control seeds of Sorbus ancwparia showing the develop- 

 ment of the cotyledon in contact with the moist filter paper for 21 days. 



These are temperatures at which essential metabolic changes occur within 

 the living tissues of the seeds. The exceptions which may involve freezing 

 or freezing and thawing are seeds of certain water plants, like Alisma,^^^ 

 as well as some others in which the seed is held dormant entirely by the 

 coats and in which freezing and thawing rupture or weaken the coats. 

 Even such seeds are generally after-ripened by low- temperature stratifica- 

 tion a little above freezing. It will be best to discuss the two different 

 physiological groups of seeds that respond to low-temperature stratifica- 

 tion under separate headings: (a) seeds with dormant embryos, and (6) seeds 

 with non-dormant embryos held dormant solely by the coats. 



Seeds with Dormant Embryos. Because the embryos are dormant in 

 this type of seed it must not be assumed that the coats are not important 



