122 J. H. Priestley and Edith E. North 
very convincing although indirect, of the presence of fatty substances 
in the Casparian strip. 
For some time the hypothesis had been entertained that these 
fatty substances were the condensation products of unsaturated fatty 
acids, whilst the staining reaction with dimethylaminoazobenzene, 
described later (p. 127), suggested that these substances remained 
acid in reaction. 
An examination of the fatty acids present in the germinating 
seeds of Vicia Faba shows the presence of two unsaturated acids, 
oleic and linoleic, but not in sufficient quantity to condense readily 
to a solid product as in a drying oil, so that their accumulation in 
the Casparian strip was not accounted for. 
Fortunately, at this point, attention was directed to the work of 
Crocker and his colleagues (Botanical Gazette —references will be given 
in a separate communication) upon the toxic action of illuminating 
gas or smoke from smouldering organic substances, upon etiolated 
pea seedlings. 
In a later paper of this series (Priestley and Ewing, vi), it will 
be shown that the stems of etiolated pea seedlings contain a functional 
primary endodermis, absent from the stems of the normal seedlings. 
The description given of the effects produced by the toxic agent 
immediately suggested that the endodermis failed to form in the 
presence of the toxic constituent of the gas or vapour and that this 
structural change was mainly responsible for the visible effects noted 
such as increase in stem girth, irregular stem curvatures, etc. 
A repetition of Crocker’s experiments has confirmed this sugges¬ 
tion. Crocker supplies convincing evidence that the toxic substances 
are the unsaturated hydrocarbons in the gas, such as ethylene. It 
therefore appears probable that the presence of the ethylene prevents 
the normal accumulation of unsaturated fatty acids in the region 
potentially capable of forming a Casparian strip. The reason might 
obviously be because some constituent of the plant membrane, that 
normally collects the unsaturated fatty acid, is instead reacting with 
the ethylene gas. Ethylene could only react by virtue of the un¬ 
saturated linkage it contains, presumably therefore in the absence 
of the ethylene the unsaturated fatty acid is held at this point by 
reaction with its similar CH = CH linkage. In this case it is easy to 
understand how the unsaturated acid becomes sufficiently concen¬ 
trated in this region to condense to a hard resistant varnish in the 
presence of oxygen and why the unsaturated acids in this strip 
retain their acid reaction. 
Further evidence in support of this explanation is supplied by 
