i gis ] 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 conve- 

 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. 



