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 Jungermannise, 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 Jungermannise 
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 
alternate ; 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 row® 
