OF THE MOSSES AND THEIR ALLIES. 147 



the leaf (especially in the Hepaticse) can only be properly 

 seen with a microscope whose powers are not less than 200 

 diameters. For this purpose a leaf must be separated, by 

 means of a pair of forceps, quite close to the stem, or the 

 stem itself may be divided above and below the point of 

 attachment, and the whole section submitted to the micro- 

 scope. This last is perhaps the better arrangement, be- 

 cause it often happens that the base of the leaf is furnished 

 with peculiar cells, which are of service in discriminating 

 species. A drop of water should be added to the leaf, when 

 it is laid on the slide, as this renders the delicate network 

 of cells more pervious to light. 



I have found the following plan bring out the form of 

 the cells of the Jungermanniae, and indeed of many of the 

 Mosses, very clearly. First, let the leaflet be warmed to 

 ebullition in a solution of caustic potash, rinse it in soft 

 water, and then add a drop of a solution of chloride of zinc 

 and iodine. By this means the cell walls, after a while 

 (though sometimes not for hours), assume a blue tint, and on 

 being slightly pressed under the covering glass, the layers 

 of cellular tissue exhibit themselves to great perfection. 



The arrangements, however, of the leaf cells may be best 

 seen in vertical sections, made by means of a divided cork 

 in the way recommended at p. 129. In many cases the 

 necessity for preparing these thin sections is not called for, 

 while in others (as Fissidens, Polytrichum, Sphagnum, &c.) 

 the true construction of the leaf cannot be made out without 

 them.* 



* The following observations on the leaves of the Jungermanniee 

 are of value : — ' The leaves are remarkably varied in their form and 

 arrangement, and usually afford excellent guides in the discrimina- 

 tion of one species from another. A glance at the figures which 

 follow will show their great variation in this particular. . . . In 

 all cases the leaves are without footstalks, and in each British species 

 lternate; that is, 'they are not arranged in pairs at the same level on 

 the stem, but one is always a little above or below its nearest neigh- 

 bour in its attachment. In some cases they are ranged in two rows 



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