STAINING ' 67 



The time required for staining varies with the material 

 and the stain employed, and proper conditions can 

 only be determined by experiment. The general 

 statement may be made that most vegetable sections 

 will be sufficiently stained in ten minutes. For ready 

 manipulation of specimens attached to slides there 

 are several forms of special staining jars available. 

 These jars have a decided advantage in that the slide 

 is placed on edge during the staining process, thus 

 avoiding deposition of foreign materials. They are 

 also economical in that less staining fluid is required. 

 These jars are usually provided with covers and may 

 be used as laboratory containers for stains in frequent 

 use. In many instances better results and more 

 exact control may be had by overstaining and sub- 

 sequently washing out the excess stain with dilute 

 alcohol. 



Combinations of two or more stains may be used 

 for the purpose of obtaining differences in color in 

 different parts of the cell. This process is termed 

 double staining. Double staining may be. effected 

 by using combinations of different dyes in one solution 

 or by using a separate solution of each stain. Certain 

 cell substances, notably those of many micro-organ- 

 isms possess the power of retaining the stain after 

 immersion in dilute hydrochloric or sulphuric acids. 

 This firm combination of stain and cell substance is 

 termed acid-fast. Although this character is of minor 

 importance in vegetable histology it is of great value 

 in differential work on organisms. Specimens should 

 be well rinsed in water after all staining procedures. 



Stains for Cell Walls. — Differences in chemical 

 composition of cell walls and contents are rendered 



