GENERAL METHODS. 5- 



solution of potassium nitrate (saltpeter) rendered Treub 

 (I, 9) good service. 



The observation of uninjured chromatophores can be 

 very successfully conducted in many cases, as Bredow (I) has 

 stated and the author can confirm, by placing the fresh 

 sections directly in oil (pretty fresh olive-oil). The cells 

 not only remain alive in this for a long time, but the oil 

 acts as a clearing agent, from its strongly refractive prop- 

 erty, and usually excludes the air from the intercellular 

 spaces pretty completely. 



When using other fluids for observation, however, one 

 must commonly use a filter-pump for removing the air ; and 

 sections on the slide can be treated, or larger pieces of 

 tissue can be filled with fluid before cutting. The pieces 

 are placed in small crystallizing dishes and are attached by 

 filter-paper to their bottoms so as to remain wholly covered 

 with fluid during the pumping. 



The same object can be attained by placing the sections 

 containing air in boiled water ; but in general the pump 

 will accomplish the desired end sooner. 



II. The Investigation of Dried Plants. 



6. For the investigation of dried Algce and Fungi, Lager- 

 heim (I and II) recommends that they be first softened in 

 water and then placed in concentrated lactic acid, in which 

 they are to be heated until they show small bubbles. The 

 organisms thus treated, having completely resumed their 

 original forms, can then be directly studied in the lactic acid. 



7. Herbarium material of the higher plants may often be 

 made suitable for sections by simply soaking in water. In 

 many cases the dried parts may be treated with dilute am- 

 monia or caustic-potash solution, which should be washed 

 out before cutting. The strength of the solutions and the 

 time of their action must be governed by the nature of the 

 objects, and must be determined for each separate case. 

 Small or friable objects are best cut with the microtome 

 after being imbedded in paraffine or similar substance (cf. 



