26 F. C. STEWARD AND J. K. POLLARD 
little was known in the first decade of this century about many of the now familiar 
simple nitrogenous compounds that do occur free in plants. 
The big change in this position came about through the applications of chromatog- 
raphy and of the use of isotopic carbon in the study of metabolism. The technique 
of two-directional chromatography on paper provided the means to separate even 
closely related compounds and such reagents as the ninhydrin and Ehrlich’s reagent, 
or the technique of RYDON AND SMITH??, provide sufficiently general means to detect 
on paper many even unsuspected compounds, and the use of C-labeling techniques 
. 


Collidine-Lutidine —— 

tS Phenol 
Fig. 1. The ninhydrin-reactive, alcohol-soluble nitrogen compounds of the potato tuber. Reproduc- 
tion from an original Kodachrome picture (somewhat scarred) of a chromatogram dating back to 
the period of DENT, STEPKA AND STEWARD! (1947). The spots are designated by number according 
to the convention then adopted, namely: 2, aspartic acid; 3, glutamic acid; 4, serine; 5, glycine; 6, 
asparagine; 7, threonine; 8, alanine; 9, glutamine; 12, lysine; 13, arginine; 15, proline; 16, valine; 
18, leucines; 21, tyrosine; 22, p-alanine; 23, y-aminobutyric acid; 18, leucine and isoleucine, not 
here separated, but cf. Fig. 7. 
also permit the separated compounds to be detected by their carbon skeletons instead 
of by their functional groups. 
The rich harvest to follow was foreshadowed when phenol: collidine—lutidine chro- 
matograms were first made on alcohol extracts of the potato tuber. Although a major 
investigation had been made of the amides of the potato tuber (STEWARD AND STREET?!) 
to show that asparagine and glutamine accounted for all the amide nitrogen present, 
this information and much more could now readily be obtained by chromatography 
in a matter of hours. When all other expected compounds had been accounted for 
on the first potato chromatograms (see Fig. 1), these extracts disclosed other sub- 
stances, the most prominent of which was quickly ascribed to y-aminobutyric acid. 
In rapid succession, other compounds were seen and later identified in extracts of 
References p. 42 
