BRITISH SPECIES OF THE GENUS MNIUM. 7 
-7 mm J 
-6 
Q x} T 2 
i 2°53 ^ — > 8 8 6 
Fie. 1.—Mnium punctatum. Gradation Curve of the length of the leaves of one fertile 
stem, o, Base of the stem; v, Summit of the stem; 1, 2, .. . 10, successive leaves. 
Scale in mm. See the figures in Table I. 
” 
At first sight a gradation curve * recalls by its ascending-descending 
form a variation curve properly so-called ; it is, however, something quite 
different. In the ordinary (individual) variation, the differences between 
the individuals depend on cHaxcE. A variation curve properly so-called 
represents the distribution of tl e observed values of a given property among 
* Percy Groom, * Longitudinal Symmetry in Phanerogamia,” Phil. Trans. Roy. Soc. 
London, ser. B, vol. cc. (1908) pp. 57-115, with numérous figures. 
Percy Groom measured the length of the successive internodes of the stem in a number 
of Phaneroganis ( Rhinanthus, Chenopodiacer, Boraginacec, Solanacer, Lysimachia, Scrophu- 
laria, Ampelopsis, etc.). In his paper one finds numerous internode-curves which represent 
the variation of the length of the internodes along the axis, in the same way as our fig. 1 
represents the variation of the length of the leaves. By the study of what I call Gradation, 
Percy Groom was brought to interesting morphological conclusions—among others, “ that 
the alternate-leaved Chenopodiacee have a phyllotaxis transitional between alternate and 
opposite, and that opposite phyllotaxis is primitive in this family.” (Loe. cit. p. 65.) 
From this example one sees that the study of gradation may render good service in 
a field of research quite different from the subject of the present paper on Mosses. 
B 
