Permeability 7 
ion or hydroxyl-ion concentration was o-oi N. In the former 
series and with Browallia petals the order of alkalies was ammonium 
hydroxide, sodium hydroxide and potassium hydroxide, while the 
order of acids was salicylic, benzoic, trichloracetic, formic, hydro¬ 
chloric, nitric, sulphuric, phosphoric, oxalic, tartaric (lactic, citric), 
acetic. The acids included in a bracket penetrated at the same very 
slow rate while the rate of penetration of acetic acid was extremely 
slow. The order of acids in the case of Hyacinthus perianth was much, 
but not quite, the same. In the second series, on the other hand, in 
which the acids were used with the same hydrogen-ion concentration, 
the order of penetration was quite different, acetic acid penetrating 
most rapidly of all the acids examined. 
More recently Brenner (1918) has recorded observations made by 
this method, the tissue used being principally hypodermal cells of 
red cabbage and staminal hairs of Zebrina pendula. His conclusions 
are not identical with those of Pfeffer. Various acids, namely, hydro¬ 
chloric, nitric, sulphuric, phosphoric, citric, malic, tartaric and oxalic 
acids, are stated to penetrate the undamaged protoplasm very slowly 
if they are presented in low non-toxic concentrations, while the some¬ 
what higher concentrations, from which the acids enter more rapidly, 
injure the cell, which apparently accounts for the greater ease of 
penetration. The vitality of the cell after such treatment can be 
tested by plasmolysis and deplasmolysis, and, if the cells are suitable, 
by protoplasmic movement. 
Bethe (1909), Warburg (1910) and Harvey (1911) have extended 
the method to colourless cells by first allowing the intra-vitam stain 
neutral red to penetrate the cells, which are thus stained red. Neutral 
red is an indicator, changing colour to yellow in alkaline medium. 
By this means Harvey showed the rapid penetration of ammonia and 
amines, whereas the strong bases diffused much less readily into 
living cells, and even their entrance might have been due to their 
toxic action rendering the cells more permeable. 
(iii) Salts other than dyes. If a cell contains a substance with which 
a salt reacts to give either a precipitate or a different colour, the pene¬ 
tration of the salt into the cell can be observed by means of the for¬ 
mation of the precipitate or colour. Thus C. Darwin (1875) showed 
the penetration of ammonium carbonate into the root of Euphorbia 
peplus by the cloudy appearance produced in the cells. The aggrega¬ 
tion produced by the same salt as well as by ammonium acetate and 
a number of other nitrogenous substances in the tentacles of Drosera 
was attributed by Darwin to the penetration of the salt or other 
