294 REAGENTS AND PROCESSES 



stained, to saturated chloral hydrate. To stain the leaves transfer 

 them to a deep red solution of safranin in saturated chloral 

 hydrate for a few hours or overnight; then rinse in water and 

 place them in saturated chloral hydrate till all but the tracheal 

 elements of the veins is clear of the stain. Now the leaves are 

 to be put into dilute glycerine and, after this has concentrated by 

 evaporation in a place free from dust, the leaves may be per- 

 manently mounted in glycerine gelatine (see page 269). 



This clearing and staining method may be used to advantage 

 with sections of leaves, stems, etc., and for these a double staining 

 with safranin and Bismarck brown is often desirable. If this is 

 to be done, after clearing out the safranin stain from all but the 

 lignified walls in chloral hydrate, rinse the sections in water and 

 stain for five to fifty seconds in a saturated aqueous solution of 

 Bismarck brown, then rinse quickly in water and transfer the 

 sections to dilute glycerine, from which they can be brought 

 into glycerine gelatine as above suggested. If the brown stain 

 is too intense at first, it must be used for a shorter time. Experi- 

 ence only will show for the specific subject in hand what the 

 time length for this stain should be. 



Clove Oil. — This is an excellent clearing medium, but it has 

 the power of extracting certain stains, and so cannot be used 

 in all cases; it is, however, for this very reason of great advantage 

 in the safranin-gentian violet-orange method of staining. See 

 under this head. 



Collodion. — Used as an imbedding medium (see page 267). 



Congo-red. — This stain is particularly useful in studying 

 the growth of membranes. Old membranes are, as a rule, 

 left unstained by it, while the newly formed membranes are 

 colored red. In a o.oi per cent, solution — that is, i part of the 

 stain to 10,900 of water — algae may continue to live and grow, 

 and they are, therefore, well adapted to the study of the growth 

 of membranes with the employment of this stain. 



Copper Acetate. — Used in the determination of tannins. 

 Small bits of the plant to be tested are placed in a saturated 

 solution of copper acetate, where they remain for eight or ten 



