BRITISH SPECIES OF THE GENUS MNIUM. oo 
forms 7, 2, 3, 4, excluding form 5, which I consider as belonging to the 
perianth *. 
B B B B 
A eli JL shl] 
T 2 3 4 5 
Fia. 4.—1-4, leaves; 5, perianth ; b, base. See text. 
In a fertile stem several leaves near the base are usually destroyed. There- 
fore the number of leaves given in Table XXXVII. (page 56) includes 
merely the leaves which are still in existence when the fruit is ripe or half 
ripe. This error is unavoidable ; it does not prevent the specific figures from 
being comparable with each other. 
The lowest leaves being often concealed among the hairs with which the 
lowest part of the stem is clothed, they are to be searched for under the 
simple microscope, the hairs being carefully removed by means of needles. 
As this is a rather delicate task, owing to the small dimensions of those 
leaves, it may happen that some of them are overlooked. The figures given 
in Table XXXVII. are therefore a little below the reality. I estimate the 
possible error at 3 for the species with smaller figures (punctatum, ete.) and 
at 6 for the species with very numerous leaves (undulatum, ete.). 
Some fertile stems show distinctly two successive periods of gradation f : 
* It may be allowed to make a momentary excursion into a by-path. The breadth of a 
moss-phyllome varies along an axis of gradation, the direction of which is given by the 
nerve. The margin of the phyllome is, on each side of it, a curve of the gradation of the 
breadth drawn by nature itself. Comparing the phyllomes in fig. 4, we see that in leaf Z the 
summit B of the curve is situated between the base and the summit of the leaf; in the 
leaves 2-3 the breadth is invariable from 4 to B (or B' to B), and decreases further towards 
the summit; in phyllome 5 the summit of the curve corresponds to the base ; in leaf 4 we 
find two summits b and D. 
As the development of a phyllome is basipetal, its successive parts from the summit to the 
base have been developed successively, in a similar way as the leaves along the stem 
(although in reverse order). We may thus say that in the phyllomes 1, 2,3, and 5 one 
period of gradation exists, and in leaf 4 two periods are observed. 
The variation of the gradation curves in fig. 4 recalls the variation of the curves in 
Table IX. (page 16). See $ 12, second paragraph. 
T This occurs rather frequently in certain species: for instance, Mnium subglobosum and 
punctutum. It is the ordinary rule in Cinclidium stygium, Sw. 
LINN. JOURN.—BUTANY, VOL. XLIV. D 
