Hedgerows and Hedgebanks 63 



may tend to regress to woodland shrubs ; but they are subject to destruction 

 and intentional clearing by fire. 



The case of the embankment is more artificial, since the material is 

 largely gravel-ballast, well-drained and exposed to considerable desiccation 

 in summer. The vegetation is usually particularly luxuriant and advanced in 

 early summer ; but is also subject to cutting and intentional or accidental 

 destruction by firing in the dry season, with consequent wholesale renewal. 



(6) As a converse case, the hedge and ditch in the alluvial area tends to 

 be replaced by a ditch only, and the latter subserves drainage as well as 

 acting as a barrier to cattle if deep and wide enough (6-10 ft). The 

 material taken from the ditch is used to increase the height of the sill. Such 

 ditches are then connected in an irrigation system, linked up with the larger 

 streams, and not only take away flood-water, but will retain standing water 

 over long periods of drought. They hence become secondary stations for 

 abundant aquatic types, enriched by surface-drainage from the pastures with 

 animal-droppings, and again give a wide range of flowering plants, as a 

 special meadow-ditch flora of about 100 forms (cf. Iffley Fields, Marston 

 Meadows, Hinksey, Botley), quite distinct from those of the adjacent pas- 

 tures, though subject to a certain amount of overlapping, as typical ditch- 

 plants may be intrusive in the damp pasture and arable land (Phragmites, 

 Eqtdsetum^ Polygonum amphibium) ; while the pasture-weeds may encroach 

 on the ditch margin (Carduus arvensis^ Rumex crispus). Characteristic 

 plants are Spiraea Ulmaria.Epilobium hirsutum, Valeriana(sQ.), Mentka ($$.)> 

 CEnanthe fistulosa, Nasturtium amphibium^ Myosotis palustris, and in or by 

 deeper water, Scirpus lacustris, Sparganium, Sagittaria, Butomus, Sium lati- 

 folium, Alisma Plantago^ Rumex Hydrolapathum^ with numerous large 

 Carices (C. riparia, C. pahtdosa, C. vulpind) ; also Hottonia, Hydrocharis> 

 Lemna and Myriophyllum in the water. 



All these types of Hedge-formation acquire special interest from obser- 

 vation of the relation of the annual succession to their water-supply, which 

 remains the local limiting factor, since temperature and light may be taken 

 as fairly uniform : as also from the standpoint of succession in time, as the 

 particular formation is initiated, and in the course of several seasons attains 

 its special character. It must be noted that the constancy is largely main- 

 tained only by human agency, and that all are subject to the possibility of 

 human interference and destruction at any time ; as s hedges may be trimmed 

 or wholly demolished, and ditches may be cleared out, drained off, or filled 

 up again. The common type of mixed hedgerow is subject to a rotation of 

 a definite number of years (averaging 10), to prevent too great invasion or 

 shading of the field-area, or to supply brushwood for fuel. Hedging and 

 ditching becomes skilled agricultural labour, and is too often a lost art. The 

 inferior types of hedge, as indicative of a deteriorated condition of husbandry, 

 are the stations most let alone ; the examination and observation of the 

 biological factors of a ' hedge ', and the phenomena of its annual succession, 

 progression, or retrogression, afford good exercise in floristic and ecological 

 instruction, and further examples may be taken in detail. 



Regressive Woodland. 



Woodland areas, cleared of all standing timber, and regeneration pre- 

 vented, as by paring and burning, become an open clearing in which the 

 original humus-flora of the woodland-base soon deteriorates, and may be 

 completely destroyed on exposure to the desiccation of summer sun ; the 

 more quickly on clay tracts in which the soil may be badly cracked. With 

 the addition of intrusive weeds of dry pastures, the soil soon reverts wholly 



