HircucockK: DIFFERENTIAL STAINING 379 
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membrane; yet, clearly, the dye is taken from the dilute, outer 
solution and concentrated within the walls. 
With the printer’s proof at hand I would like to add that the 
vacuole contents may be strongly colored by reason of an acid 
reaction. Ordinary water, or distilled water, usually is slightly 
alkaline, slowly changing neutral red to yellow or orange. When 
Nitella is grown in water slightly acidified with acetic acid, the 
vacuole becomes much more deeply colored than the culture 
solution. 
