CHAPTER IV 



GENERAL REMARKS ON STAINING 



The function of a stain is to make structures visible which cannot be 

 seen without staining, or to bring out clearly structures which are 

 only faintly visible. A filament of Spirogijra shows the chromatophore 

 clearly if merely mounted alive in a drop of water; the nucleus is visi- 

 ble and the pyrenoids can be distinguished. Such a study is necessary 

 if one is to understand anything about the plant and, for an elemen- 

 tary class, this much is sufficient ; but a drop of iodine solution applied 

 to the edge of the cover would emphasize certain details, e.g., the 

 starch would appear blue, the nucleus a light brown, and the cyto- 

 plasm a lighter brown. This illustrates at least one advantage to be 

 gained by staining; it enables us to see structures which would other- 

 wise be invisible, or almost invisible. Much of the recent progress in 

 morphology and cytology has been due to the development of critical 

 methods of staining. Some of the combinations and methods recom- 

 mended by various workers are good in themselves, while others, not 

 so good, have yielded results because they have been so skilfully used. 



CHOOSING A STAIN 



Some stains which are excellent in differentiating certain structures 

 are worthless for others; but the worthless stain may be the best one 

 for something else. Beautiful and instructive preparations sometimes 

 result from some happy chance, as when a shde is passed through al- 

 cohols which have become tinged with various stains. Such slides may 

 show four or five stains well balanced ; but uniform success demands 

 skill and judgment in manipulation, and also a knowledge of the struc- 

 tures which are to be differentiated. Let us take a vascular bundle for 

 illustration. Safranin stains the xylem a bright red, but, with judicious 

 washing, is entirely removed from the cambium and cellulose elements 

 of the phloem. A careful staining with Delafield's haematoxylin now 

 gives a rich purple color to the cellulose elements which were left un- 

 stained by the safranin, thus contrasting sharply with the lignified 

 elements. If cyanin and erythrosin be used, the xylem takes the blue 

 while the cambium and phloem take the red. 



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