44 Biological Stains 



dinary chemical reactions tend to continue until one of the com- 

 ponents of the reaction is exhausted. Although these facts are 

 not incompatable with chemical explanations, the original expon- 

 ents of the physical theories regarded them as full refutation of the 

 possibility of chemical action. 



The original dispute concerning, physical and chemical theories 

 of stain action is now rather obsolete. Much of it was based on 

 a priori argument or on insufficiently understood evidence. Since 

 1945 there have been various investigations of a more scientific 

 nature, due largely to the growing importance of histochemistry, 

 which are continually bringing out new facts concerning the 

 mechanism of staining. The most recent pubUcations on the sub- 

 ject do not try to differentiate between physical and chemical 

 theories. 



The Physical Theory. It was formerly pointed out that all or- 

 dinary dyeing or staining phenomena can be explained on a physi- 

 cal basis. There are three factors, all purely physical in nature, 

 which together or separately may explain many, if not all, stain- 

 ing phenomena. In the first place, as nearly all substances stained 

 are more or less porous, simple physical forces such as capillarity 

 and osmosis can account for penetration of the dye into the interi- 

 or of the tissue. In the second place, the action of acZsorption 

 can account for many staining phenomena, even for much of the 

 selective staining with which the biologist is familiar. In the 

 third place, a dye may penetrate some cellular element by absorp- 

 tion, remaining there in a state of solid solution. Any one or 

 all of these three forces may operate in any given instance; and 

 even the exponents of the physical theory of staining differ in 

 the amount of weight given to each of them. The penetration 

 of the dyes into the tissue by osmotic action is generally ad- 

 mitted; but some students of the subject favor adsorption, others 

 absorption, as the primary explanation of staining phenomena. 

 For a concise discussion of the physical explanations, especially 

 the solution theory, see Holmes (1929 b). 



The absorption or solution theory is very simple and the action 

 of some dyes on some kinds of cellular tissue in the presence of 

 mineral salts suggests that this factor is important, at least in some 

 instances. It is also supported by the fact that a dye causes the 

 tissue to become the same color as the dye shows in solution, but 

 not necessarily the same as it shows in its dry form. Dry fuchsin, 

 for example, is green; its solution, however, is red, and so are 

 tissues stained by it, no matter how completely they may be dried. 

 It is nevertheless difficult to explain all staining phenomena, par- 

 ticularly differential staining, on the basis of solution. There are 

 admittedly some special staining phenomena, like fat-staining, 

 where simple solution is the obvious explanation; these, however, 

 form a special case. 



