628 
Notes. 
for example, shows diagrams of several species as if characterized 
by constant differences of structures. Observation shows that in some 
species (e.g. P. Convolvulus) variations are comparatively infrequent and 
slight, but that in most (e. g. P. Persicaria and aviculare) they are 
extremely frequent, and lead to very great changes in floral structure. 
Often it is scarcely possible in such species to find two flowers alike 
on the same branch, or even on the same plant. Within a species 
individual plants show wide differences in the frequency and extent of 
variations. 
A comparison of different species shows that while each varies, so 
as in the more variable species to cover almost the whole range 
observed in the genus, each shows a tendency to certain lines of varia- 
tion. These tendencies are more alike usually in the more nearly 
allied species, so as to correspond in the main with the groups based 
on habit, and they lead from group to group. 
The modes of variation commonly observed include almost all the 
recognized modes of departure from floral symmetry. They affect all 
the whorls. The perianth in some species is very constant. In 
others it habitually shows cohesion of two or more segments, or 
abortion in different degrees, or suppression of one or two (usually 
the inner) segments. Chorisis of a segment is less frequent. Enations 
from one or more segments are frequent in certain species, rare or 
absent in others. The outer stamens often show cohesion of the two 
in each >air, varying from the slightest union of the bases of the 
filaments to absolute union of even the anthers. Abortion (in all 
degrees to complete suppression) of one or more stamens is not rare, 
frequently reducing this whorl to 3 (less often to 2) in aviculare. 
Chorisis is not rare, especially of the unpaired stamen. The inner 
stamens seldom show cohesion (except in aviculare and its allies) with 
stamens of the outer whorl. Abortion (in all degrees to complete 
suppression) is very frequent, and in certain species ( amphibium ) this 
whorl has completely disappeared. In aviculare and allied species the 
inner whorl shows abortion less than the outer. Chorisis in the inner 
whorl most frequently shows itself in the posterior stamen. Adhesions 
of stamens to perianth segments and petalody of stamens are not 
frequent. 
(In P. amphibium , the land form near Aberdeen very generally has 
the anthers very small or abortive, and the stamens hidden within the 
perianth, while the form growing in water has the anthers well 
