530 G. T. HARRIS ON MICROSCOPICAL METHODS 



simplest way is to cut the sections in an ether-freezing microtome, 

 and extremely thin sections are not demanded, but as the sections 

 must be cut at right angles to the axis of the leaf, some care 

 in orientating the leaf has to be taken. Sections of stems are 

 also required in determining species of Sphagnaceae, but these 

 are, of course, a simple matter. 



In dealing with the capsule of the moss two methods of 

 mounting may be employed for the study of the peristome. It 

 may be mounted dry as an opaque object, or in some medium 

 as a transparent one. Mounted dry and illuminated with 

 reflected light the peristome is the only concession the moss 

 world makes to spectacular effect. For purposes of study, how- 

 ever, the limitations of this method are great, and the student is 

 forced to adopt transparent mounting. At the same time dry 

 mounts of some peristomes are quite useful to the bryologist in 

 getting at the general appearance ; the sulcae of the Orthotrichum 

 capsule and the cilia of its endostome are quickly and satisfactorily 

 exhibited as an opaque object under the binocular microscope. 

 For anything like detailed study, however, the peristome must be 

 mounted as a transparent object. A good deal depends upon 

 getting the capsule in the right condition. It should be quite 

 ripe, so that the touch of a needle at the junction of the operculum 

 with the capsule will liberate the lid and enable the peristome 

 to unfold without injury. If the lid has to be forced away some 

 of the teeth of the peristome usually go with it. To prepare it 

 for mounting, the capsule may be severed transversely about the 

 middle, then a longitudinal slit made through the annular ring 

 and that portion of the capsule wall adhering ; this permits of the 

 peristome being spread out flat. The spores require washing 

 away, though it is an advantage if some of them adhere to the 

 teeth, as they often afford valuable specific characters. If the 

 capsule has already shed its spores when gathered, but is not 

 dilapidated, it is often in a very favourable condition for mount- 

 ing, and in those mosses with a double peristome the endostome 

 can often be detached and mounted separately from the exostome ; 

 this enables the cilia so often present in double peristomes to be 

 studied to the best advantage. When the cut peristome has been 

 laid flat and judiciously cleaned it should be subjected to a cover 

 slip and spring clip for a day or two, glycerine and water being 

 run under by capillary attraction. The annular cells are usually 



