280 Oxydases and Pigmeiits of Plants 



Such forms have been investigated by Bateson and are described as 

 dominant whites. Although they may be identical in appearance with 

 true albinos they, differ fundamentally from the latter in gametic com- 

 position and hence in genetical behaviour. The true albino lacks 

 colour, the dominant white possesses the power of pigment formation, 

 but that power is held in abeyance by an inhibitor. 



Inasmuch as the phenomenon of dominant whiteness is dealt with 

 somewhat fully in the text, we need not devote attention to it here, 

 beyond remarking that attempts to demonstrate by chemical means the 

 truth of the Mendelian hypothesis with respect to dominant whites 

 have not as yet been wholly successful. 



Miss Wheldale (1910), whose researches have contributed so much 

 to our knowledge of this subject, regards inhibition as due to the action 

 of deoxidising substances such as sugars, tannins, and the like, and has 

 brought forward experimental evidence in favour of this opinion. In 

 cases of partial inhibition, which is illustrated in P. sinensis by the 

 dominance of pale over more deeply coloured varieties, it is suggested 

 that the effect is due to a reductase. 



Methods. 



The methods in general use for detecting the presence of oxydase in 

 plants depend on the addition of a colourless chromogen to the solution 

 or extract obtained from the plant. If the result of the operation is to 

 produce a pigment it is concluded that oxydase is present. If the 

 pigment be produced only after the addition of hydrogen peroxide the 

 oxidising substance is described as a peroxydase. The method has 

 many disadvantages. Certain of the chromogens used as tests for 

 oxydases undergo oxidation with more or less rapidity when exposed to 

 the air. The extract or solution may contain reducing or inhibitory 

 substances which interfere with the reaction, and in any case the 

 localisation of the oxydase in the tissues of the plant is wellnigh 

 impossible by this method. 



Clark (1911) has carried out recently an elaborate series of tests with 

 different chromogens and finds that certain of them may be used for the 

 microchemical determination of oxydase in plant tissues. Among the 

 reao-ents used by Clark are guaiacum, phenolphthalein and a-naphthol. 

 Pyrogallol in the presence of glucose is employed by Chodat. Schreiner 

 has obtained valuable results with respect to the oxydases of living 

 roots by the use of benzidine. 



