528 G. T. HARRIS ON MICROSCOPICAL METHODS 



possible to examine the structure satisfactorily in its normal 

 condition, and they are best treated in a solution of caustic soda 

 or potash, as recommended by Dr. Braithwaite. This renders 

 them flaccid and defines the cell structure. They should, of 

 course, be washed well before mounting, and glycerine in some 

 form is desirable with them as with all dense-leaved species. 

 The species of Andreaea are typical of the difficulties that con- 

 front the bryologist in attempting to get satisfactory permanent 

 mounts. They are deep red or black-brown in colour, very dense 

 and cartilaginous in texture, extremely brittle and abominably 

 dirty. It may be remarked that the colour of moss leaves, 

 which is usually some shade of green, is a matter of minor 

 importance to the mounter, hence it can be left out of considera- 

 tion in choosing a medium for mounting. The bryologist mounts 

 the leaves altogether for shape and cell-form, as upon these 

 depend in a great measure the determination of the species. 

 Usually the colour disappears in the course of a few months, but 

 curiously enough in the same medium the green colour of closely 

 allied species will remain fairly good. Very often saprophytic 

 algae will retain their colour perfectly, while the moss has 

 altogether parted with it. 



There appears to be no necessity for " fixing " the specimens 

 before mounting ; but with leaves of comparatively delicate struc- 

 ture it is often advantageous to do so unless the mounting medium 

 contains a fixing agent, and I have had very good results with 

 the complanate branches of such species as Plagiothecium elegans 

 and Plagiothecium depressum by fixing them in picric acid before 

 mounting in glycerine jelly, the picric being washed out previous 

 to mounting. With such a species as Plagiothecium depressum, 

 which bears very characteristic bunches of deciduous flagellae on 

 its branches, every care has to be exercised to avoid detaching 

 the flagellae, whose particular mission it is to become detached 

 with the slightest provocation. Such is the case with many 

 species of Tortula, the leaves of which bear characteristic gemmae. 

 The extremely beautiful bunches of gemmae at the apices of 

 Ulota phyllantha, and the scattered gemmae on the leaves of 

 Orthotrichum Lyellii, are other instances. It is only the novice 

 who will attempt to "clean" such leaves; the confirmed bryologist 

 is too thankful to get the leaves mounted with the appendages 

 adhering to worry over a small amount of adventitious matter. 



