124 Methods in Plant Histology 



each block with the celloidin adhering to it and harden it in chloro- 

 form for 12 hours. Then transfer to a mixture of equal parts of 

 glycerin and 95 per cent alcohol, where the material should remain 

 for a few days before cutting. 



Cutting, Staining, and Mounting. Although 10 ju is usually 

 thin enough, sections are readily cut as thin as 5 y, by this method. 

 Remove the celloidin before staining by treating 10 to 15 minutes 

 with ether; then wash in 95 per cent alcohol and transfer to water, 

 and then to the stain. Stain to a fairly dense purple in an aqueous 

 solution of Erlich's haematoxylin; wash in dilute aqueous solution of 

 calcium or sodium carbonate, and then in two changes of distilled 

 water. Add a few drops of alcoholic solution of equal parts of 

 Griibler's alcoholic and aqueous safranin, and stain to a rich red. 

 A dilute stain acting 1 to 2 hours is better than a more concentrated 

 stain acting for a shorter time. Transfer directly to absolute alcohol, 

 clear in xylol, and mount in balsam. 



Haidenhain's iron-haematoxylin is a very satisfactory stain for 

 photographic purposes. 



The celloidin method has its disadvantages as well as its advan- 

 tages. It is extremely slow and tedious, and it is rarely possible to 

 cut sections thinner than 10 ju, while, on the other hand, it gives 

 smoother sections. 



Succulent tissues, which are usually damaged by the paraffin 

 method, are easily handled without any injttry in celloidin. The 

 fact that the method may be used without heat is often a further 

 advantage. Stems and roots which cannot be handled at all in 

 paraffin cut well in celloidin, and much larger sections can be cut than 

 in paraffin, but most material of this kind can be cut without any 

 imbedding. 



When material is to be imbedded, use celloidin as a last resort. 

 Use paraffin when you can, celloidin when you must. 



