30 BULLETIN 907, U. S. DEPARTMENT OF AGRICULTURE. 
THE PHYSIOLOGICAL CONDITION OF THE PLANT. 
The influence of the physiological condition of the plant on injury 
has until recently received scant attention by writers on fumigation. 
It is evident that plant injury from hydrocyanic acid is influenced by 
the chemical condition of the cells at the time of treatment, for other- 
wise how could the fact, well known to every fumigator, be explained 
that tender growing citrus plants are less resistant to gas than those 
in a dormant and hardened condition, as during the winter. This is 
equally true with the young leaves as with the mature ones, which 
indicates that In becoming resistant young growth passes some sort 
of maturation process. In fact it appears that the condition of citrus 
plants which renders them hardy or resistant to frost injury likewise 
develops increased resistance to hydrocyanic acid. 
Harvey (8) has shown that in the case of cabbage the hardening 
process results in an increase in the glucose and sucrose content over 
that present in nonhardened plants and quotes Lidforss as authority 
for the statement that this is a common transformation in plants 
generally during the cold season. Chemical changes increasing or 
reducing the percentage of other substances are also shown to be a 
result of hardening tissues. It is further stated that the hardening 
of plants which results in an increase in the cell-sap concentration is 
an accommodation brought about by low temperature. Plants can 
also be rendered resistant by growth in a dry soil. 
Stone (18), working with cucumber plants grown under different 
light and soil-moisture conditions, showed that the development of 
tissue is influenced by these factors as well as their susceptibility to 
burning with hydrocyanic acid. The weaker tissue produced by 
inferior light or excessive moisture was decidedly more injured than 
that grown under full ight conditions or in dry soil. 
More recently Clayton (1) experimented with tomato plants and 
similarly observed that resistance to hydrocyanic-acid gas was modi- 
fied by the conditions under which the plants were grown. Slow- 
growing plants with a high chlorophyll content per unit area were 
found to be more resistant to hydrocyanic acid than plants grown 
rapidly with low chlorophyll content per unit area, and his conclu- 
sions that the water supply was the underlying cause of these differ- 
ences is in full accord with the prior work of Stone. Chemical 
examination of the two sets of plants gave results in agreement with 
those of Harvey (8) and others for hardened and nonhardened or 
actively growing plants, that the more resistant forms have greatly 
increased carbohydrate content, especially of the reducing sugar 
calculated as dextrose. Experiments conducted by this writer with 
plants infiltrated with dextrose showed that resistance to hydrocyanic- 
acid gas was developed by this procedure and the conclusion was 
reached that glucose in a plant acts as a protective agent against 
injury by cyanid. 
