BRITISH SPECIES OF THE GENUS MNIUM. 35 
occurs : here the base is the broadest part of the leaf. I have considered the 
breadth of such leaves (which are rare) as unmeasurable. 
Sometimes the differentiated border of the leaf is recurved (this happens 
often in M. hornum). Here I measure first the visible breadth b (with- 
out taking the recurved parts into account); then the recurved parts are 
measured separately and their breadth is added to b. 
Iestimate the possible + or — error in the measurement of the breadth 
at 20 u (0°02 mm.). 
§ 27. BREADTH AT THE BASE.—This property is measured in the direction 
of a transverse line passing through the point / mentioned in $ 25 (see fig. 5, 
p.34). If the margin is recurved, I use the method described in § 26. As 
the margins at the base of the leaf are often oblique, a small error in the 
estimation of the position of i may bring about a sensible error in the 
measurement of the character under eonsideration. Therefore the possible 
positive or negative error is proportionally great: I estimate it at 30 u 
(0°03 mm.). 
$38. NUMBER OF CELLS AT THE PLACE OF THE GREATEST BREADTH.—The 
place of the greatest breadth being determined (see $ 26), I count the cells 
in the transversal direction, separately in the right and the left half of the 
leaf, I exclude:—1°, the nerve; 2°, the elongated cells of the differentiated 
border, except the interior one which is taken into account*. In each half 
of the leaf I count the cells four times (proceeding twice from the nerve to 
the margin and twice in the reverse sense) and take the mean value of the 
four figures. Ordinarily these figures differ from each other by one or two 
units. If the differences are greater f, I go on, counting two or four times 
more. The sum of the mean values of the two halves is the figure of the 
leaf. 
I estimate the possible positive or negative error at two cells. This 
estimation is rather too high :when the cells are not very numerous (see 
for instance, the minimal values of serratum and orthorrhynchum in 
Table XXIX.). 
$29. BREADTH OF THE CELLS AT THE PLACE OF THE GREATEST BREADTH.— 
As the form and the size of the cells are very variable, even in one leaf, it is 
practically impossible to obtain useful information about their dimensions 
by measuring them separately. I have tried to surmount this difficulty by 
determining the mean dimension of the cells in the transverse direction 
(in other words, their average breadth) at the place of the greatest breadth of 
the leaf. 
* If the differentiated border consists of one cell (a single row of cells) this is excluded. 
+ If the figures are, for instance, 22, —23, —25, —26 (a rather unsatisfactory result). 
D2 
