466 THE AMERICAN NATURALIST. [Vor. XXXV. . 
it is present in amounts insufficient to accomplish the whole 
result. The union is accomplished by and in the living cell; 
whether with a more readily oxidizable substance first formed 
from sugar, or with sugar itself, is not now known. All that is 
known is that sugar, or some similar substance, and oxygen 
unite, forming as end-products mainly carbon-dioxide and water. 
The following reaction, without indicating what, if any, inter- 
mediate stages there may be, shows the material results : 
n ((CsH1206) + 6 O2 + Aq.) = n (6 CO, + 6 H20 + Aq.). 
(Ag. representing the water in which the sugar is dis- 
solved in the cell, does not enter the reaction. indicates 
the unknown multiple of the minimum proportional formula 
C6H1206, which stands for the sugar molecule. The 6 H20 
produced in the course of combustion may unite with the 
solvent water (Ag.), or may pass off as vapor, diffusing the 
faster from the cell by reason of the heat liberated). 
Since other substances than sugar are also oxidized physio- 
logically in the cell, other products will be formed, the kinds 
and the quantities of the latter varying according to the former. 
The commoner of these minor products are oxalic, malic, and 
citric acids, which accumulate in considerable quantities in 
certain plants (e.g. in the leaves of Oxralis acetocella, in the 
Crassulaceg, in apples, etc., and in the citrous fruits, lemons, 
limes, oranges, etc. or are converted into salts (e.g., calcic 
oxalate, crystallizing out of the solutions in which it is formed 
in the cell), or undergo other changes (e.g., further oxidation). 
In all organisms the oxidation of nitrogenous compounds, as 
well as non-nitrogenous, occurs in normal respiration. The 
proportional amounts of the two groups of compounds phys- 
iologically oxidized vary with different organisms. In the 
majority only organic and highly complex compounds are 
made to yield the needed energy, but in some much simpler 
inorganic compounds suffice, and in a few organisms already 
known the carbon compounds are not used at all. 
The nitro-bacteria, as first shown by Winogradsky,! oxidize 
! Winogradsky, S. Recherches sur les organismes de la nitrification, An 
nales de Inst. Pasteur, tomes iv, v (1889-91), and other papers. 
