Carbohydrate Production in the Higher Plants 3 
of carbonic acid may be assumed to be, initially, a molecule with one 
carbon atom. Formaldehyde is the substance that satisfies all 
theoretical considerations, though its actual occurrence has not been 
proved. For the second stage we jump to sugars with 5 and 6 carbon 
atoms, pentoses and hexoses, while from the latter by doubling we 
get 12-carbon sugars, of which cane-sugar is the most obvious. For 
the third stage we have much condensed polysaccharides like starch, 
inulin, etc., in which the number of carbon atoms may run up to 
several hundreds. 
Now as we are making bold to consider the biochemical vagaries 
of protoplasm with a critical detachment, we may ask why these 
stages occur and not others, why the plant revels in the hexoses, 
makes a certain play with pentoses but does next to no traffic in 
sugars of 3, 4, 7 or 8 carbon atoms—the trioses, tetroses, heptoses, 
and octoses, all of which are preparable in the laboratory. The hexose, 
glucose, is undoubtedly the key-sugar to carbon-metabolism whether 
we consider up-grade processes or down-grade ones like respiration 
and fermentation. Considered purely chemically, glucose is a mole¬ 
cule of remarkable properties, combining a certain stability in pure 
solution with an extraordinary potentiality of varied chemical 
change in presence of what the pure chemist might call impurities. 
Now biochemistry is not the chemistry of pure substances but the 
chemistry of impure substances, indeed of very impure substances. 
It is a mild suggestion that always in the cell, several dozen im¬ 
purities are present with any one substance that we concentrate 
attention upon. 
In considering the important role that various sugars play, the 
biologist sometimes drifts into the teleological way of thinking, and 
regards the cell as having selected these sugars as specially suitable 
for its purposes. 
Let us turn then to the test-tube for illumination on this matter, 
and particular^/ to the wisdom drawn from the test-tube by Nef in 
his elaborate researches on what we may call the purely chemical 
metabolism of sugar molecules. Nef, at Chicago, up to his recent 
death, devoted some fifteen years to studying the spontaneous 
chemical changes undergone by sugars in the presence of impurities, 
such as salts and hydrates of sodium and other metals. He showed 
how every single sugar, be it of 1, 2, 3, 4, 5 or 6 carbon atoms, tends 
to pass over into all the others, so that an equilibrium mixture of 
more than a hundred derivatives may arise in the course of weeks 
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