48 Botany 



autumn as the drupes ripen. On the other hand, a very large part of the 

 fruit crop falls to the ground, and, as the stones are exposed by the drying 

 of the fruit, they are taken by fieldmice, which often gather them into 

 stores, where, if forgotten or abandoned, they may germinate. 



(e) Carr {Franguletmn) . As the canopy of the young buckthorn becomes 

 closer there follows, especially where colonisation has been dense, a well- 

 marked phase in which Cladiiim and associated species of the previous 

 stage are killed out; their dead leaves still hang in the crotches of the 

 branches when no living plants persist. The ground becomes almost bare 

 beneath the bushes and a characteristic shade-tolerant flora enters. This 

 usually includes the marsh ferns, Dryopteris thelypteris, Agrostis stolonifera, 

 Urtica dioica, and non-flowering shoots of Lysimachia vulgaris, Symphytum 

 officinale and Iris pseudacorus. Convolvulus sepium is often a conspicuous 

 feature of early phases of scrub formation, twining round the "drawn-up" 

 reed stems, and the sallow, Salix cinerea, is a typical pioneer shrub, 

 succumbing early to the competition of other bushes. 



As the carr ages, the bush density diminishes and it seems certain that 

 dominance passes from Rhamnus frangula to R. catharticns. The mechanism 

 of this displacement is uncertain, but it probably involves an extensive 

 "die-back" disease caused by the fungi Nectria cinnaharina and Fusarium sp. 

 To this, the older R. frangula bushes seem very susceptible; the fungi gain 

 entrance by snags and rapidly kill the bushes. JR.. catharticus, which is not 

 so attacked, increases greatly in the later stages of carr development. At 

 this stage, also, the bushes take on a tree form with central trunk and 

 branches limited to a close crown — a strong contrast to the earlier scrub 

 in which each bush stool has many trunks, and branching is extremely 

 diffuse and extensive. In later stages of development, Viburnum opulus 

 may be of importance: it straggles extensively under the shade of the other 

 bushes, rooting at the places where shoots touch the soil, to form quite 

 impenetrable tangles. 



There is no deciduous fen woodland on Wicken Fen, but in many places 

 birch {Betula alba) is spreading from seed; there are also a few good-sized 

 oaks [Quercus robur), a colony of grey poplar {Populus canescens), and 

 many scattered ashes. It is a remarkable fact that, although the fen peat 

 contains pollen and wood of alder [Alnus glutinosa) in great quantity, the 

 few planted trees now growing on the fen are not able to spread, although 

 they produce abundant viable seed. 



2. Deflected Successions. Over the greater part of the fen the natural 

 succession already described does not take place, for the fen vegetation is 

 cut at intervals, as a rule, of either one year or four years, and the crop is 

 used for litter (cattle-bedding) or for thatching. On these lines, cutting 



