68 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



[Vol. 6 



dependent upon the intensity of color in the test fluids. With 

 a weak beet decoction plus acid or phosphates methyl red 

 was useful from P H 3 to P H 6.8, and phenol red (phenol- 

 sulphonphthalein) from P H 6.4 to P H 9.0. 



Any refinements in the use of the simple indicator method 

 of hydrogen ion determination should find many applications 

 in the wide range of plant physiological studies with both 

 lower and higher organisms. The importance of this factor 

 of active acidity has been repeatedly urged in recent work, 

 yet it is not receiving general consideration. In medicine the 

 value of such determinations has gradually become appar- 

 ent following the interesting development of views regarding 

 neutrality regulation in animal fluids (compare some of the 

 work of Henderson, '08, '09, '09 a , of Henderson and his as- 

 sociates, and others). 



In making determinations of the hydrogen ion concentra- 

 tion of the blood, Levy, Rowntree, and Marriott ('15) have 

 employed a dialysis method, used also in a study of the buffer 

 value (Levy and Rowntree, '16) of this fluid, while a more 

 accurate modification of the method (Marriott, '16) is used 

 to determine the alkali reserve of the blood plasma. In plant 

 studies the matter of neutrality regulation might seem on first 

 thought to be of relatively little consequence, because of the 

 diversity of reaction. The extent of the acid reserve in a gen- 

 eral way is appreciated, but the determination of this has been 

 largely incidental to other considerations. It would be inter- 

 esting to know to what extent an acid reserve is a general 

 characteristic of plant metabolism. 



From the studies reported on animals it would appear that 

 the protoplasm of many organisms is approximately neutral, 

 but the indications would seem to be that plant protoplasm 

 is often far from neutral, frequently exhibiting a relatively 

 high acidity. It is still a question, however, to what extent 

 the P H determined for the juice (as a whole), representing 

 to a large extent the contents of the vacuoles, is an index of 

 the reaction of the protoplasm (Haas, '16). The case of cer- 



tain citrus fruits is. of 



here the 



more acid juice is contained in special sacs. Aside from this 



