CHAPTER V 
CHEMICAL SIGNS OF LIFE 
We have endeavored to show that the living nerve, 
as long as it is irritable, is chemically active and that 
when it functions this metabolism is accelerated. As the 
irritability of the nerve varies, there are simultaneous 
changes in chemical activity. What characterizes the 
living state is respiration and its increase on stimulation. 
We have come now to our main inquiry, namely, 
whether or not all living matter undergoes respiration 
as long as it is alive, and whether stimulation always 
increases its respiration. In addition, we have to ask 
whether, if this is true, it can be used as a sign of life in 
all living matter. 
Seeds.—It has hitherto been maintained that since 
dry seeds do not respire but are irritable, irritability is 
independent of respiration. The work of Horace Brown, 
Thistleton Dyer, and others indicates that dry seed can 
be kept alive at very low temperatures in conditions 
where no ordinary gaseous exchange is possible. It is 
argued, therefore, that life is possible without any 
metabolic activity. Dry seeds, kept for long periods 
in a closed vessel, have not been found to give any evi- 
dence of this fundamental chemical change occurring 
in living matter, namely, the production of carbon 
dioxide. Such seeds, it is well known, are not really 
dead, for under proper conditions they germinate. 
They appear to live without respiration, but this is but 
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