MECHANICS OF DORMANCY IN SEEDS 
119 
and others), belonging to this class, hold their vitality for years in the 
soil.49 
While we are discussing light as a forcing agent, let us revert to 
certain points in the classification we have used in this paper. 
The question may justly be raised, Why not classify lack of light as 
one of the methods of securing dormancy in seeds, just as was done for 
oxygen, rather than treat it as a forcing agent? This question must 
be answered by repeating that the two general topics are closely 
interrelated and that while the present classification has virtue in 
convenience of discussion and in giving aim to future research it must 
be subject to modification with advance in knowledge. Again free 
oxygen is generally spoken of as an essential condition for growth of 
organs of flowering plants while light is generally merely a formative 
condition and only indirectly necessary through food supply. True, 
later work has somewhat modified the clearness of this distinc- 
tion. Free oxygen is hot necessary for considerable growth in seeds 
of various water plants and can be vicariously displaced by other 
conditions for limited growth in certain organs of other flowering 
plants. Even if its main function in maintaining continuous growth 
is the oxidation of toxic products of intramolecular respiration, it 
stands in a much more immediate essential relation to growth than 
does light. Moreover we do not know how general and immediate a 
relation oxygen holds to continuous growth through supply of neces- 
sary energy by normal respiration nor how generally its so-called 
stimulative effects can be temporarily displaced by other conditions. 
For light-sensitive seeds, light has been found capable of displacement 
in one after another until very recently the most persistent case, 
seeds of Gesneriaceae,^^ have yielded to experimentation. I am much 
inclined to believe that future investigation will show that light acts 
through the elimination of one of the other five dormancy producing 
factors mentioned above rather than in such a fundamental and 
essential relation as oxygen. 
Summary 
In closing let us consider a few of the more important general 
bearings of the facts treated in this paper. 
I. Dormancy in seeds results generally from the inhibition of one 
Duvel and Goss. Unpublished Work on buried seeds referred to in the 
Introduction. 
Gassner. Zeitschr. Bot. 9: 609. 1915. 
