36 PROF. JULIUS MACLEOD ON TEN 
Having subtracted the breadth of the nerve and the breadth of the two. 
differentiated borders (see § 31) from the total breadth of the leaf, I divide 
the rest by the number of cells (see § 28) : the quotient is the mean breadth 
of the cells. 
It is very difficult to measure exactly the breadth of the nerve, its lateral 
limits being rather indistinct. The error, being divided by the number of 
cells, can be neglected. 
I estimate the possible positive or negative error in the value of the mean 
breadth at 0*5 u. If the number of cells is small (for instance, 25) the error 
may reach 1 w (maximum) *. 
The method here described gives very regular results, and is applicable 
(with the necessary changes) to the measurement of cells, fibres, ete., in 
numerous objects. 
§ 30. NUMBER OF CELLS OF THE BORDER AT THE PLACE OF THE GREATEST 
BREADTH.—In the 10 species of Mniwm which I have studied (and in many 
other Mosses) we find along the margin of the leaves one or several rows 
of cells which constitute the so-called border. The border-cells are differ- 
entiated from the neighbouring cells of the leaf by two properties :— 
1°, they are distinctly elongated, often several times longer than broad, 
whilst the neighbouring cells are about isodiametrical ; 2°, their cell-walls 
are thickened. 
These two characters, however, seem to be to a certain degree independent 
of each other, the differentiation being sometimes limited to one of the two. 
We find, for instance (often in the lowest leaves), border-cells which are 
distinctly elongated, their walls being hardly or not thickened, and other 
border-cells which are hardly longer than broad, but the cell-walls of which 
are thickened. I consider as belonging to the border the cells which are 
differentiated at least by one of both properties. At the inner limit of 
the border we often find cells (fig. 6, e) which are elongated but not 
thickened. Such cells are taken into account in counting the cells of the 
border. 
As the number of border-cells is variable along the margin, I number the 
cells three times in each leaf at each side: 1°, at the place of the greatest 
breadth ; 2°, about 50 to 100 u (according to the dimension of the leaf) 
above this place ; 3°, the same below the mentioned place. I take the 
highest of the three figures. The two maxima (right and left border) are 
* I want to call attention once more to the usefulness of strict conventions. Instead of 
saying, for instance, “in Mnium sp. the diameter of the cells of the leaves is about 20 4,” 
I say, “in Mnium sp. at the place of the greatest breadth of the longest leaf of a fertile 
stem the mean breadth of the cells varies between 17 and 24 p,” and I add the possible 
error, One might say that the disconcerting variation of the character under consideration 
has been caught and confined between two constants. 
