Atkins — Oxydases and their Inhibitors in Plant Tissues. 159 



the blue reaction. Sometimes only the extreme tip is free ; at other times 

 the iuliibitor is limited to the lower two-thirds of the leaf. 



Treatment of similar sections with benzidine results in the appearance of 

 brown in the bast, bundle sheath, and epidermis, also in the walls of the wood, 

 thougli tlie sclerenchyma remains unailected. Indeed it is remarkable how 

 the cell-walls take on a brown colour, especially the thickenings of the guard- 

 cells and epidermis generally. The endodermis of the root is another good 

 instance of this staining of the walls. It should be noted that the mesophyll 

 remains uncoloured, and that the epidermis is frequently inactive, owing 

 to the presence of inhibitors as shown by hydrogen cyanide treatment. 



The a-naphthol reagent beliaves somewhat in the same way, giving purple 

 with the epidermis, guard-cells, bast, wood, but not as a rule with tlie 

 mesophyll, and never with the sclerenchyma. After cyanide treatment the 

 mesophyll reacts positively ; but the sclerenchyma is still colourless. It seems 

 that the different beliaviour with the various reagents may best be explained 

 as due to the varying stability of their oxidation products to reducing sub- 

 stances. If the latter are aldehydes, the action of the cyanide may be readily 

 attributed to a cyanhydrin formation. The coloration of sclerenchyma by 

 guaiacum is probably due to the inclusion of traces of oxydase insufficient 

 to act on the other reagents and to the complete absence of the inhibitor.^ 



Effect of Light and D{trkness upon Oxydase in Leaves. 



To test the effect of light and darkness two plants were brought from the 

 gardens on July 18th ; one was set in a dish in the laboratory dark press, 

 while the other was put in the greenhouse. Both were kept well watered, 

 but it is evident that their external conditions differed in many other respects 

 as v/ell as in the presence or absence of light ; for the temperature, humidity, 

 and circulation of air must have been quite dissimilar. Sections were 

 examined at intervals over two months. These were obtained from the green- 

 house con<-vol, green leaves in the dark, nearly colourless leaves in the dark, 

 and young leaves about nine inches long which had grown from the rhizome 

 during the period of total darkness. Contrary to what might be expected, 

 the oxydase reactions were on the whole but little altered by growth in 

 the dark ; this shows that the inhibitor is certainly not a direct product of 

 photosynthesis. It is true that at times the mesophyll of leaves from the 

 dark press gave stronger reactions than those from the greenhouse, but at 



1 Leaf sections which had heen boiled showed none of these reactions with the oxydase reagents. 

 The cuticle however, appeared green owing to the action of the dissolved chlorophyll upon it. 



2a2 



