Botany 47 



(c) Pure Sedge (Cladietum). Throughout the hydrarch succession we may 

 reckon that development follows the gradual raising of the peat level up 

 to and ahovc the water level, so that we can utilise the difference between 

 the two levels as a criterion ot successional phase. It then becomes apparent 

 that the artificially steepened banks of lodes and drains, and their artificially 

 raised edges, give no opportunity for the successional stage next following 

 reed-swamp to become easily evident as a zonal community. Drains and 

 trenches, made by removing peat, are now often choked up to form 

 habitats only little drier than those of the reed-swamp: such places 

 support the community we have called "pure sedge". Tliis is a closed 

 community dominated very completely by the prickly sedge, CJadium 

 mariscus. The sedge leaves grow up to 3 m. long, bending over horizontally 

 at about 1*5 m. The luxuriance, the evergreen habit, and the thick 

 "mattress" of dead leaves deposited between the growing shoots of 

 Cladium, prevent the growth of all other plants save infrequent individuals 

 of Phragrnites, Lysirnachia vulgaris, and Salix repens var. fusca. 



Recent studies on the autecology of Cladium mariscus at Wicken have 

 shoviai many features of great interest in the plant itself and the conditions 

 under which it grows. The meristem of the plant lies below ground at the 

 apex of a vertical stock : it is very frost-sensitive, but the wet fen peat has 

 such a small temperature diffusivity constant, that frost seldom or never 

 penetrates deep enough to be harmful. The meristems also offer a barrier to 

 gas diffusion from the growing leaves to the stock and roots, but aeration 

 of these organs takes place through the bases of dead or mature leaves. 

 The drained upper layers of peat show strong seasonal drift in composition 

 of the soil atmosphere determined by changing soil temperature. Values of 

 5 to 6 per cent of carbon dioxide at only 20 cm. depth are reached in 

 summer with correspondingly lowered oxygen values. The soil water 

 below the water table is apparently devoid of dissolved oxygen. 



(d) Bush Colonisation. As the peat level is gradually raised, the pure 

 sedge is invaded by bushes, wliich are dispersed with great rapidity by 

 seeds. The most abundant species is Rhamnus frangula {Frangula ahius) ; 

 next come Rhamnus catharticus, Salix cinerea and Viburnum opulus, and 

 lastly there is a very small percentage of Crataegus monogyna, Prunus 

 spinosa, Ligustrum vulgare and Myrica gale. Both disseinination and estab- 

 lishment are irregular, and early stages of bush colonisation have a very 

 heterogeneous structure, which disappears, however, as the sedge patches 

 are invaded marginally or over their whole extent, and the bushes come 

 to form a complete and uniform cover over the whole area. Birds are 

 probably the most important agents of dispersal of the Rhanmus bushes, 

 especially the large flocks of migratory fieldfares which visit the fen in 



