288 Oxydases and Pigments of Plants 



with respect to inhibition of pigment formation in the flower (see 

 page 301). Some of the evidence which we possess .as to the nature of 

 the inhibition of pigment formation in the vegetative parts leads us 

 to favour the view that this inhibition applies to the production of 

 chromogen rather than to the activity of oxydase. This evidence is 

 derived from the oxj'dase reaction given by the outermost cortical layer 

 and various tissues of the stele of certain green-stemmed varieties. 

 Instead of no, or at most a weak, reaction, which is to be expected if 

 substances inhibiting oxydase action are present, the tissues just 

 enumerated give a particularly strong oxydase reaction and hence it 

 would appear probable that these cortical and stelar tissues contain an 

 inhibitor which is capable of preventing the production of chromogen. 



On the other hand the cortical tissues of the petioles of certain 

 green-stemmed varieties give no reaction for oxydase although the 

 corresponding tissues of red-stemmed forms are extraordinarily rich in 

 that substance, and in this instance the failure to give the reaction for 

 oxydase may be due to the inhibition of the latter substance. 



Whether inhibition be of the nature of the suppression of oxydase 

 activity, as we know it to be in the dominant white Howers of P. sinensis, 

 or whether it consist in the prevention of chromogen production, it 

 follows that negative results with respect to the oxydase content of a 

 given tissue must be accepted with caution : this applies, of course, not 

 only to qualitative results such as those with which we are dealing, but 

 also and with even greater force to those obtained by quantitative 

 estimates of the oxydase content of plant juices. Elaborate methods 

 are in use for this purpose (cf. Buuzel, 1912) but, unless they are asso- 

 ciated with methods for removing any inhibitors which may be present, 

 the results which they yield must be accepted with reserve. 



To sum up our observations on the distribution of oxydase in the 

 vegetative parts of P. sinensis : We find that the methods described in 

 Section I serve for the faithful mapping out of oxydases in the several 

 tissues of the plant: that this mapping out leads to the conclusion, that 

 although oxydase is more widely distributed than chromogen, the distri- 

 bution is in conformity with that required by the oxydase-chromogen 

 hypothesis and that, owing to the existence of inhibiting substances, 

 caution must be exercised in interpreting the negative results obtained 

 by the use of oxydase reagents as proof of the absence of oxydases. 



