72 
R. H. Yapp. 
the result that the general water level is fairly constant. Except 
after heavy rains it rarely varies, in all probability, either in winter 
or summer, more than some three to six inches. Only when the 
Cam itself is in flood, and rises above the level of the apertures in 
the Upware sluices, is there any danger of the Sedge Fen being 
inundated. At most times of the year the soil is firm, and one can 
walk dryshod over the greater part of the Fen. Apart from the 
more marked elevations and depressions, the actual water level is 
found, say in the summer time, at depths varying on different parts 
of the Fen, from some four inches to about a foot below the surface. 
But as a rule the water-content, even at the surface, is high, owing 
to the great water capacity of peat. 1 
These differences of level, by affecting the amount of soil 
moisture, exert a profound influence on the distribution of the Fen 
plants. For instance, at certain times of the year it is perfectly 
easy to pick out, even from a distance, all those parts of the Fen 
which are raised above the surrounding level, by means of the 
plants growing on them. Thus towards the end of July, many of 
the banks and raised patches dotted about the Fen, are easily 
recognised by the masses of flowering Spiraea Uhnaria, which at 
the present time is exceedingly abundant at Wicken. Where the 
Fen in general is comparatively dry, the patches are less distinct, 
but where they are raised above a fairly wet part, these Spiraea 
“ islands ” are sharply defined. Associated with Spiraea on the 
“ islands ” are frequently Eupatorium cannabinum (which flowers 
later than Spiraea ) and Symphytum officinale ; sometimes also 
Convolvulus sepium, Agrostis canina, &c., and, if the soil is dry 
enough, a few aliens such as Urtica dioica, Centaurea nigra, &c. 
But as Wallis points out, 2 the “ wet plants” frequently invade 
the dry places, while the “ dry plants ” are not so usually found 
encroaching on the wetter parts. Thus Phragmites (or occasionly 
ffuncus obtusijlorus) may often be found on a Spiraea “ island,” and 
indeed commonly occurs on the drier banks of the lodes (Text- fig. 11). 
This is no doubt due in large measure to the habits of growth of 
the subterranean parts of these plants. 
It is a well known fact that the underground parts of marsh 
plants are much more extensively developed than those of bog 
plants. 3 Creeping subterranean stolons and rhizomes are very 
’ Cf. Hall, The Soil. 1904. p. 66. 
2 Wallis, l.c., p. 221. 
Frah & Schroter, l.c,, pp. 14 and 15. 
