DORMANCY IN SEEDS 101 



the still higher and still less favorable temperatures for after-ripening, the 

 catalase content rose much less or actually fell after a small initial rise. 

 While the catalase rises greatly \\dth after-ripening of dormant embryos, 

 the amount of rise is not a strict measure of the progress of after-ripening. 



What is the nature of the changes brought about in seeds ^^^th dormant 

 embryos during low-temperature stratification? First, there is an increase 

 in enzymes, not only hydrolytic and oxidative — lipase, peroxidases, oxi- 

 dases, catalase — but, judging from Pack's finding of a great increase in 

 amino acid, probably proteases also; secondly, there is an accumulation of 

 simple organic materials that can be readily used in building new tissues, 

 sugars, amino acids, etc.; and finally, there is a transformation of insoluble, 

 osmotically inactive substances to soluble, osmotically active ones, i.e., 

 fats to sugars and insoluble proteins to soluble proteins, amino acids, and 

 other nitrogenous organic compounds. The formation of osmotically active 

 substances may account for the free movement of water in after-ripened, 

 rosaceous embryos in contrast to the difficulty of movement in dormant 

 ones. The hypothesis that inhibiting substances may hold embryos in 

 dormancy should not be forgotten, especially since the long stratification 

 in moist medium gives good conditions for the outward diffusion of such 

 substances. Opposed to this hypothesis, however, is the fact that there 

 is a definite optimum temperature for after-ripening of any given dormant 

 embryo and that the effective range of temperature in many cases is very 

 narrow — also that the optimum is very low, 1° C (34° F) in Sorhus. If 

 after-ripening were a matter of the leaching of inhibiting substances, one 

 should expect a Ande range of effective stratification temperatures in which 

 high temperatures are more effective than low. As a matter of fact, high 

 stratification temperatures make many dormant embryos still more dor- 

 mant rather than after-ripening them. It is still harder to see how the 

 continual dwarfishness in the epicotyl portion of the plant can be explained 

 on the basis of inhibiting substances. The dormant slow-gro\\'ing embryos 

 are long in contact with moist peat or actually in aerated water before 

 they are planted in soil; moreover, after they are grown in soil there is 

 opportunity for any soluble inhibiting substance to move from the buds 

 back into the stem and finally to the roots. Perhaps the dormancy in 

 embryos is brought about by organization characteristics of the proto- 

 plasm involving insoluble substances. 



Seeds with Non-dormant Embryos. Many seeds that do not have dor- 

 mant embryos, as sho\\'n by the fact that they will germinate immediately 

 and produce vigorous seedlings if the coats are broken, respond to low-tem- 

 perature stratification. Alisma Plantago-^ seeds germinate readily and 

 with vigor if the coats are broken. They also respond to low-temperature 

 stratification m water, as sho^^^l in Table 11. Barton ^^ '^''d m unpublished work 

 has sho^vn the same to be true for Butomus umhellatus, Scirpus americanus, 

 S. campestris var. paludosus, and Zizania aquatica, all the aquatic seeds 

 she studied in this respect. The same is probably true of many other seeds 



