AS AIDS IN THE DETEEMINATION OF SPECIES. O 



The sections for the permanent type-slides were cut with a microtome, after 

 embedding in parafBn in the usual way. To clear the sections for these mounts, a 

 mixture of glycerine and caustic soda solution was generally used. Warming in 

 nitric acid was in some instances attended with brilliant results, while in other cases 

 the sections were ruptured and distorted by its action. 



Most of the sections were photographed without any preparatory staining. 

 Indeed, in some instances, especially in the case of petioles kept for many mouths in 

 ordinary tank rain water (and some kept for over three years were not in the least 

 decomposed or affected by fungus growths), the colour acquired by the specimens — a 

 rich yellowish-brown tint — was itself a stain not to be surpassed for the purposes of 

 actinic contrast. "When this tint was not acquired, and in the case of fresh material, 

 the sections were, if necessary, bleached by the action of free chlorine, and then 

 faintly stained with the stain first proposed by Dr. Frances Hoggau.* It is prepared 

 by adding pyrogallic acid to tincture of steel. This was found more easy of 

 application than the iron stain, composed of pyrogallic acid and protosulphate of iron, 

 recently recommended in the Journal of the Royal Microscopical Society,! which, 

 however, gave equally good results to the eye and on the photographic plate. 



The Hoggan iron stain was originally recommended as especially suitable for the 

 treatment of cartilage, in w4iich the desired depth of colour could be produced in a 

 few minutes. 



We found that the time required for the proper treatment of the sections of 

 petioles varied from five minutes to half an hour, the stain here taking a longer time 

 to penetrate. 



The source of illumination was a clock-work petroleum lamp with a one-inch 

 wick, and the light was all that could be desired, but a blue glass modifier was used 

 in every case, whether the object was stained or not. 



Of the mounting media tried, glycerine jelly was found the most suitable. 

 Balsam was used in a few cases, but it was invariably found to render the epidermal 

 layers too transparent, and consequently somewhat indistinct. It may be mentioned 

 that it was found to be essential for purposes of comparison and identification that 

 the sections should be quite thin, showing little more than one layer of cells in the 

 cortical ground tissue. 



• Marsh, "Microscopical Section-cutting," 2nd Ed., p. 131. 

 + Journ. Eoy. Micro. Soc, 1888, p. 157. 



