DORMANCY 
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called ginning; then the seeds are pressed to remove the oil. 
The refuse forms a valuable food for cattle, especially when 
mixed with other foods. The length of the fibers varies 
on different species of cotton, and the uses which are made 
of the fibers depend on their length. 
Linseed oil is obtained from the seeds of the flax. It is 
used in making paint and other substances. 
200. Dormancy. — Most seeds have a rest period, or period 
of dormancy as it is called. This is longer in wild plants than 
in cultivated ones. In most seeds the dormant period is 
only from the summer of one year to the spring of the next. 
In pigweed, however, the seeds of any one year may require 
several years for development, owing to differences in the 
thickness of the testa. Those grow first which have the 
thinnest testa. Years may pass, therefore, before all the 
seeds of a single crop have grown. This fact and the great 
number of seeds produced are the two main reasons why this 
weed is so persistent. Once it has seeded, it has the ground 
supplied with seeds that will grow some one year hence, 
some two, and so on up to thirty years. Other weeds which 
produce seeds that can lie dormant for at least thirty years 
are shepherd's purse, black mustard, chickweed, and curly 
dock. Variations in the thickness of the testa in seeds on 
the same plant is an adaptation which prevents the plant 
from drying out even if unfavorable conditions cause the 
death of all the seedlings of any one year. Closely related 
to the dormancy of a seed is its viability , or power to grow after 
long periods of rest. It has been proved by experiments 
that the seeds of weeds mentioned above are viable or able 
to grow after a dormancy of thirty years. Such facts as 
have been established by experiment lead us to discredit 
stories about the growing of seeds that have lain in Egyptian 
tombs hundreds of years. 
Practical use can be made of this knowledge by any one 
who cultivates crops. For instance, weeds that depend on 
