1915] LAND—MICROTECHNICAL METHODS 399 
In anatomical work with seedlings and sporelings it is necessary to 
have an absolutely unbroken series, extending sometimes over many 
slides, in which the loss of a single section would destroy the value 
of the entire series. 
The well known principle that most colloidal substances, when 
treated with a solution of some salt of chromium, exposed to light 
and dried, become insoluble in water, was utilized with complete 
success. The modern photographic processes, such as printing in 
pigmented gums and gelatin, photogravure, etc., are based on this 
property of bichromated colloids. 
In the Hull Botanical Laboratory the writer and his students 
first tried Le Page’s liquid glue thinned to the consistency of albu- 
men fixative and made slightly yellow by dissolving a small quan- 
tity of potassium bichromate in the thinned glue. The slide was 
smeared with a thin coating of the bichromated glue and dried in 
the light. Later a solution of gum arabic was tried with even better 
results. 
The present practice is to spread a few drops of a 1 per cent solu- 
tion of gum arabic on the slide, taking care to see that every portion 
of the surface is covered, and flood the slide with water made 
slightly yellow by dissolving in it a few crystals of potassium bi- 
chromate. The ribbons are then straightened out on the slide by 
means of heat, the excess solution drained off, and the preparation 
put aside in the light to dry. A very short exposure to light is 
sufficient to render the gum insoluble in water. After the slides 
are thoroughly dry they are treated in the usual manner. 
In heating the slide to straighten out the ribbons no special pre- 
Caution, such as necessary with albumen fixative, need be taken, 
since gum arabic does not lose its adhesive power at temperatures 
below the melting point of the hardest paraffin ever used in 
imbedding. The paraffin in the ribbon may even be melted 
without lessening in the slightest the adhesive property of gum 
arabic. 
When a large number of slides are to be made it is very conye- 
nient to mix the gum arabic and the potassium bichromate solutions 
and flood the slide with the mixture. The solutions should be 
mixed immediately before using, since the mixture does not keep. 
