Wicken Fen. 
75 
largely determined by the water content of the soil; as a given 
plant often has more or less vertical roots when growing in a fairly 
dry soil, while in a wetter one its roots grow much more horizontally. 
Compare the relative positions assumed by the roots of Lysimachia 
vulgaris, when growing at different depths in a moderately damp 
soil (Text-fig. 14). This phenomenon is doubtless correlated with 
the paucity in oxygen of the more waterlogged soils. 
Text-fig. 14. Lysimachia vulgaris. These two plants were found growing 
near each other in moderately damp soil. X (September). 
As pointed out above, many plants possess a very wide moisture 
range. Hydrocotyle vulgaris for example, may occasionally be 
found submerged, at other times it will grow on fairly dry exposed 
peat banks. Cladium again may grow (as commonly in the Norfolk 
Broads) actually in water, though at Wicken it is generally found 
under intermediate moisture conditions. On one occasion I found 
a dwarfed specimen of this plant growing quite contentedly on the 
top of a dry mound of peat, some two feet above the surrounding 
soil. Again, I have succeeded in growing quite vigorous specimens 
of Spiraea Ulmaria, which is a dry-Fen plant, with their rhizomes 
submerged in water; while Lythrum Salicaria, a relatively “ wet 
plant,” will grow luxuriantly in ordinary garden soil. 
But although individuals can readily adapt themselves to 
considerable variations of habitat, yet in nature one seems to rarely 
meet with these plants in anything like the extreme positions which 
they are capable of occupying. For instance, although Spiraea will 
grow and even flower, with its rhizome submerged, yet I do not 
remember finding a single specimen naturally growing in really 
