The Celloidin Method 87 



the knife-edge and float the section onto the finger, with the concave 

 side ot the section upward. Press the section, with the concave side 

 down, on a sHde flooded with a thin fihn of glycerin-alcohol. Line up 

 successive sections on the slide, where they lie flat; drying of the 

 sections is prevented by the glycerin. When enough sections have been 

 cut, press a dry slide over the sections. Transfer the slides with the 

 sections pressed between them to a dry Petri dish, put lead weights on 

 the top slide and fill the dish with water. As many as four pressed 

 lots can be put into one Petri dish. The water renders the sections 

 flexible and permits them to flatten. The sections are then floated out 

 and stained. For prolonged storage, transfer the slides and weights to 

 a Petri dish of glycerin-alcohol, in which the sections become 

 hardened in a flattened condition and in which they may be kept 

 pressed indefinitely. Sections that have been stored either floating or 

 pressed in glycerin-alcohol are progressively transferred to water and 

 stained. 



Staining 



Sections cut in celloidin on the sliding microtome do not adhere 

 to form a ribbon. They are usually stained as loose sections floating 

 in a watch glass or small evaporating dish. The sections are usually 

 floated off the knife into 95%, alcohol. For staining in an aqueous 

 stain, sections are gradually transferred to water. As the first step, add 

 about one-third as much water as there is alcohol. After 3 to 5 min. 

 pour off half of the liquid, and add an equal volume of water. Repeat 

 the decantation and addition of water two or three times, then drain 

 off all the liquid, and rinse in water. From this point the sequence of 

 operations conforms in general to Staining Chart 111. Drain and cover 

 with hemalum. After 5 min. in stain remove a section with a brush, 

 rinse in distilled water, then in tap water, and examine with a 

 microscope. The intensity of hemalum is correct when the cambium, 

 phloem, cortex, pith, and xylem rays are blue, but lignified tissues are 

 practically colorless. Nuclei should be blue-black. Drain off the stain, 

 and rinse the sections in three to five changes of distilled water and 

 two or three changes of tap water. Overstained sections can be 

 destained in 1/2% HCl, followed by thorough washing in tap water. 

 When the intensity of the blue color is correct, cover the sections with 

 aqueous safranin. Some woody materials take up enough safranin in 

 5 to 10 min. Materials having less highly lignified cell walls may 

 require 12 hr. After the estimated time in safranin, rinse with water 



