96 METHODS IN PLANT HISTOLOGY 



Green and red make a good contrast, but a section stained with iodine 

 green and safranin would be a failure, because both stains would stain 

 the xylem and neither would stain the cellulose. Both stains are basic. 

 Red lignin and green cellulose could be secured by using safranin and 

 acid green. Green lignin and red cellulose as already indicated, can be 

 got with iodine green and acid fuchsin. 



The time required for the different processes varies greatly, and the 

 time required for a subsequent process is often more or less dependent 

 upon the time given to processes which preceded it. Good mounts of 

 sections of the petiole of A'uphar advena have been secured from ma- 

 terial which had been cut, fixed, stained in safranin and Delafield's 

 haematoxylin, and mounted in balsam, the entire time being less than 

 30 minutes. This is an extreme case, and nothing is gained, except 

 time, and the saving of time is apparent rather than real, for the his- 

 tologist always has something to do while the sections are in the stain. 



Preserved, fresh, and dry material. — If sections are to be cut from 

 material preserved in formalin, the piece should be washed in water, 

 since the odor is annoying and the fumes are injurious to the eyes, and 

 the acid in the formalin interferes with most stains. 



The sections are placed in the stain from water. Sections from alco- 

 holic material are transferred directly to the stain. If the material is in 

 a mixture of alcohol and glycerin, the sections should be washed in 

 water or 50 per cent alcohol until the glycerin has been removed before 

 transferring to the stain. 



Some material cuts well when fresh, but cuts with difficulty when 

 preserved. On the other hand, some material cuts well when pre- 

 served, but hardly at all when fresh. Some material which is too soft 

 to cut when fresh can be cut with ease after it has been in formalin 

 alcohol for a week or more. 



Oak, hickory, maple, and other hard woods need special treatment 

 because they are so hard to cut. It is a good practice to boil the ma- 

 terial in water and treat with hydrofluoric acid before any sectioning is 

 attempted. The following is the usual method when this acid is used: 

 Cut the material into blocks 5 or 6 mm. square and 2 or 3 cm. long and 

 boil in water for several minutes; then transfer to cold water and, after 

 several minutes, repeat the boiling. The alternate boiling and cool- 

 ing, which should be repeated several times, drives out the air. Trans- 

 fer to equal parts of commercial hydrofluoric acid and water. From 1 

 to 3 weeks will be enough for most woods. Some oaks, ebony, apple. 



