PRESIDENTIAL ADDRESS 669 
or acquires new ones and progresses. It is open to the Mendelian breeder to 
standardise creation. 
Thus in fulfilling the first part of its task—that of defining the pure-bred— 
the Mendelian method has provided the material for the fulfilment of the 
second part—namely, the investigation of the conditions which make for the 
stability and instability of the organism. I think the time has come when 
this latter task might be undertaken on a large scale and with good prospects 
of success. ; 
So far I have played the part of one of those street-corner watchers of the 
skies who offer a telescope for the inspection of the heavens. I have now to take 
a turn myself, and by means of the binoculars of Mendelism and Physiology 
survey not the celestial bodies, but certain new features of a small and narrow 
terrestrial field which this instrument brings within our ken. My survey has 
reference to the phenomena of the pigmentation of plants, and is confined to 
those presented by the anthocyan or sap pigments to which the colours of many 
flowers are due. 
Until recently knowledge of the processes of pigmentation advanced along 
two main and independent lines. One line of advance—that followed with such 
brilliant success by Bateson and the Cambridge school, as well as by other 
students of genetics—has led to a wealth of exact knowledge with respect to 
the factors and characters which determine coloration. The other line of 
advance, pursued with no less brilliant results by Chodat and Bach and by 
Palladin and his associates, has resulted in a great increase of our understanding 
of the biochemistry of pigmentation. 
The merit of being the first to combine the genetical with the biochemical 
method belongs to Miss Wheldale, to whom, moreover, we owe a good working 
hypothesis of the nature of the processes involved in pigment-formation. The 
work of Palladin and of Chodat and Bach is so well known that I need not 
review it in any detail. To Palladin we owe in large measure the conception 
that respiration consists in a sequence of enzyme-like actions, the later of which 
result in oxidations and are ascribed to oxydases. To the same observer we 
owe also the suggestion that chromogens play a part in the oxidations set up 
by oxydases, and that these colourless chromogens may undergo either alternate 
oxidation and reduction and so take a continuous part in oxydase action, or 
undergo permanent oxidation and so constitute the pigments of the plant. 
Chodat and Bach have given us a serviceable conception of the nature of 
oxydases. According to the Chodat-Bach hypothesis oxydases are of dual 
nature; the complete oxydase consisting of two parts—a peroxydase and an 
organic peroxide. An oxydase reacts with oxidisable reagents, such as guaiacum, 
to produce a characteristically coloured product. Hence these reagents may be 
termed oxydase-reagents. Peroxydases react with oxydase reagents only if 
there be added, as a substitute for the organic peroxide of the complete oxydass, 
a source of active oxygen in the form of hydrogen peroxide. Both oxydases 
and peroxydases occur in the cells of plants, and may be identified in extracts 
therefrom. 
The work of Gortner on the pigments of insects adds confirmation to the 
view that pigments are the product of the action of oxydase on chromogens. 
Thus he has shown that the black or brown melanin of the integuments of 
insects is produced by the action of an oxydase, tyrosinase, on some such product 
of protein-hydrolysis as tyrosin. 
Miss Wheldale’s studies have led her to formulate the hypothesis that the 
anthocyan pigments of plants are the outcome of a series of chemical changes 
of the following order: Glucosides hydrolysed by emulsin yield chromogens 
which, acted on by oxydases, give rise to anthocyan pigments. The difficulty 
in the wav of further advance lay in the unsatisfactory nature of the methods 
for identifying oxydases derived from plant tissues. Hence when we turned 
our attention to this subject Dr. E. F. Armstrong and I made it our first 
task to search for means whereby we should be able not only to identify, 
but also to locate, oxydases and peroxydases in plant-tissues. Clarke had 
tested already numerous oxydase-reagents and found that certain among 
them are adapted for microchemical use. As the result of a considerable 
number of trials of known reagents we have found that a-naphthol and 
