ii8 
WM. CROCKER 
Soil as a substratum^^' 31, 48 frequently forces the germination of 
dormant seeds. It also produces secondary dormancy. Here a 
number of reagents may play a part, each acting either upon the coat 
or upon living structures: changed oxygen supply, salts, acids, bases, 
and other substances. Kidd has shown that mustard seeds are 
thrown into secondary dormancy by the high carbon dioxide content 
of certain soils. Other narcotizing and inhibiting substances may also 
be present in the soil. 
High temperature^^ is frequently effective. In some cases it acts 
through coat restrictions to oxygen or water absorption. These 
restrictions give a germinative temperature minimum far above that 
of the naked embryo. Independent of coat restrictions, however, the 
minimum temperature for the germination of seeds of many wild forms 
is very high, being near 20° C. This embryo character accentuated 
by coats plays a very great role in dormancy in nature. Alternation 
of temperatures"' is also an effective means of forcing many seeds. 
It is extensively used for practical testing in the seed laboratory of 
the U. S. Bureau of Plant Industry. The method of action of alter- 
nating temperatures is not known. 
Finally light^^ is an important condition in determining dormancy: 
it forces the germination of many seeds that would otherwise be 
dormant and inhibits many that would grow in darkness, while the 
greater number of seeds are indifferent to it. In certain seeds Kinzel 
has connected the effect of light with mobilization of reserved materials 
while Lehmann and Ottenwalder^^- on the basis of inferential and 
philosophical considerations give it a role in protein hydrolysis. The 
assumption that it is always effective through modification of living 
substances alone is quite gratuitous. It may modify either the living 
structures or the coats, each in a variety of ways. It must be recog- 
nized that the latter are in the more exposed position. There is great 
need of a scrutinizing chemical-physical study of the effect of light 
upon seed germination. 
We must point out a biologically significant matter concerning 
light requiring seeds: in the soil they lack this one condition for 
germination and are held in a dormant state. Some seeds (Nicotiana 
Pickholz. Zeitschr. Land. Versuchs Oesterr. 14: 124. 181 1. 
^' Lehmann and Ottenwalder. Zeitschr, Bot. 5: 337. 1913. 
48 Ottenwalder. Zeitschr. Bot. 6: 785. 1914. 
