SECTIONAL TRANSACTIONS.—M*. 529 
In theory, the necessity for controlling atmospheric conditions cannot be avoided, 
if only because the metabolism of the fruit is continually changing the composition 
of the atmosphere. By accident if not by design, carbon dioxide must be removed 
and oxygen must be added, and the amounts of these gases present will represent a 
balance between production or consumption and removal or addition. 
Both gases exert characteristic effects on plant metabolism, oxygen as an essential 
for the supply of energy to the living machine, carbon dioxide as a stimulant in small 
doses, a depressant in moderate doses, and in still greater amounts @ poison. 
An atmosphere containing less oxygen and more carbon dioxide than is present 
in ordinary air retards the ripening processes of certain varieties of apples, and is now 
being commercially utilised for their preservation. Caution is necessary in generalising 
this result, since the beneficial effect may easily pass over into a harmful one when the 
percentage of carbon dioxide is further increased, or that of oxygen further diminished, 
beyond a certain optimal amount, which varies with the variety, and is also a function 
of the temperature of storage. ‘ Brown heart’ in commercial shipments of Australian 
apples was one of the damaging effects of gas-storage conditions, inadvertently 
obtained and inadvertently carried beyond a safe limit. 
The growth of moulds is often inhibited by absence of oxygen or by high con- 
centrations of carbon dioxide, and is considerably retarded by concentrations such as 
can be tolerated by apples. 
Water vapour ranks as one of the most important constituents of the atmosphere 
of the cold store, although its actual percentage by volume may be only about 0°5 per 
cent., and its control is difficult because the small amount present is governed by the 
rates of two relatively rapid, opposed processes—evaporation from the fruit and 
condensation on the cooling pipes. In turn, the concentration of water-vapour 
governs the rate of evaporation. Loss of water is to be avoided as far as possible, 
but a high relative humidity favours the growth of moulds. The problem of balancing 
these requirements becomes particularly acute in the storage of soft fruits, where the 
tendency to lose water and the susceptibility to mould attack are both greater. 
The more complicated volatile products of the metabolism of fruits, which are 
largely responsible for their smell and flavour, have in some cases a harmful effect 
when allowed to accumulate in the storage atmosphere. They are probably instru- 
mental in causing ‘scald’ in apples. Acetaldehyde is of particular interest as a very 
simple compound of this class, which probably occurs as an intermediate compound 
in normal metabolism. The respiration of apples and of oranges is considerably 
increased at all temperatures by the addition of small concentrations of the vapour 
acetaldehyde. The germination of mould spores and the growth of moulds, on the 
other hand, are retarded or inhibited. 
Anzsthetics such as chloroform and ether form another group of gaseous sub- 
stances which have been employed to control the metabolism of living plant products, 
and whose physiological effects are moderately well known. Small concentrations of 
hydrogen cyanide greatly accelerate the rate of respiration of potatoes, apparently by 
increasing the rate of starch hydrolysis. 
Ammonia and other gases have recently been shown to retard very considerably 
the development of moulds. Strong claims for ozone, in a low concentration, have 
also been made in this connection, but are not yet well-established, although ozone 
may in any case have a useful application in the oxidation of odorous compounds 
present in the storage atmosphere. 
Ethylene is noteworthy as an accelerating agent for the ripening process in bananas 
and some other fruits: some ethylene derivatives and other hydrocarbons have also 
been shown to exert similar effects. 
1931 MM 
