114 
WM. CROCKER 
occur. Of the seeds of this class Crataegus mollis has been most 
fully studied. Both the optimum conditions'^ for after-ripening and 
much of the nature of the changes'^ involved, have been worked out. 
After-ripening occurs most rapidly at 5° C, but the structures 
surrounding the embryo influence greatly the rate of the process at 
this temperature — with the naked embryo 3 or 4 weeks are required, 
with the testa only intact 3 or 4 months, with both testa and stony 
carpel intact more than a year. Investigations indicate that the 
coats retard the after-ripening at least in large part by limiting the 
oxygen and water supply to the embryo. The possibility is not 
excluded, however, that they hold in some inhibiting substances. 
Regarding the changes involved in the after-ripening of the seed 
Eckerson finds that in the dormant condition the hypocotyl is slightly 
basic or neutral to phenolphthalein, while the cotyledons, capable of 
immediate growth, are quite acid. As after-ripening progresses the 
hypocotyl becomes more and more acid. The acid reaction of the 
hypocotyl seems to produce general physical-chemical conditions 
favorable for water absorption, enzyme formation and action, and, 
through these, for growth. In accord with this certain acids greatly 
hasten after-ripening. Apple seeds behave qualitatively as Crataegus, 
but quantitatively the dormancy is much less persistent. 
A study of several more representatives of this physiological type 
of seed is needed in order to know how generally the findings for the 
haw hold for the group. 
There is no doubt that every one of the five types of dormancy we 
have described is represented by many different seeds. It is also 
likely that combinations of two or more types of these show up in a 
given seed. While in general the embryos are primarily responsible 
for the dormancy in the first and fifth types and the coats for the 
second, third and fourth types, every one shows a more or less complex 
interaction between the coats and the embryos. 
Secondary Dormancy. — It is a rather generally observed fact that 
some seeds capable of immediate germination can be thrown into a sec- 
ondary dormancy by a period in a germinator lacking some one condi- 
tion necessary for germination or involving a substance inhibiting germ- 
ination or one hardening the colloids of the coats. A few cases may 
be cited. Gassner" produced dormancy in the light requiring seeds 
2^ Davis and Rose. Bot. Gaz. 54: 49, 1912. 
Eckerson. Bot. Gaz. 55: 286. 1913. 
