MECHANICS OF DORMANCY IN SEEDS 
107 
hulled seeds concerning hardness throws more light upon the effective- 
ness of the machine than upon the seeds themselves. 
Concerning the biological significance of hard coats it may be 
stated that they place a family of seeds, otherwise extremely 
susceptible to injurious agents among those of the greatest longevity. 
Applying the principles mentioned in our coagulation conception of 
life duration, such seeds in the soil have ideal conditions for longevity 
— low equable temperature and moisture content. Moreover the 
impervious coats protect them against other organisms and the slow 
action of oxygen. 
3. In the light of Miiller's^^ recent work it is not strange that 
some seeds are held in a dormant state because the force of the ex- 
panding contents is not sufficient to rupture the coats. He found 
that in various seeds that germinate readily the outward pressure of 
the contents at the time of rupture was but slightly greater than the 
breaking strength of the water-saturated coat. Both lay in the 
region of 3 to 4 atmospheres. It has been shown that the breaking 
strength of filter paper, leather, Laminaria thallus and most other 
organic materials of colloidal nature rises or falls with a fall or rise 
in water content. Miiller found the same true of seed coats. In the 
cases studied the breaking strength of the dry coats was several times 
the expanding force of the enlarging seed contents. 
Of seeds inhibited in their germination by this method, Alisma 
plantago"^^ and Amaranthus retroflexus'^^ have been most fully studied. 
Water impervious coats play no role here for both absorb water very 
rapidly and reach saturation after about 5 hours' soaking. In the 
saturated condition it is estimated that the embryo of Alisma plantago 
due to imbibitional or osmotic absorption of water is exerting an 
outward pressure on the coats of about 100 atmospheres. In Ama- 
ranthus retroflexus the outward pressure is probably much less, but in 
both seeds the gel-like coats are considerably stretched, and the 
embryo rapidly extends beyond the bounds of the coat as soon as the 
latter breaks. Any time after maturity naked embryos of both these 
seeds are capable of immediate growth, showing no dormant period. 
The embryo of Alisma is a rather slow grower but that of Amaranthus 
very rapid. The latter shows a growth elongation of several hundred 
percent after 12 hours in the germinator under optimum conditions. 
^9 Muller. Jahrb. Wiss. Bot. 54: 529. 1914. 
Crocker and Davis. Bot. Gaz. 58: 285. 1914. 
2^ Unpublished work. 
