56 Botany 



plant in our woods and its relation to the primrose (Primula vulgaris) are 

 still extremely uncertain, though the hybrids are both abundant and 

 fertile. 



The moss flora is not extensive, the commonest species being Thuidium 

 tamarisciiiuin, Brachytheciuin rutahuhim, Catharinea undulata, Hylocomium 

 triquetrum, H. squarrosum, Fissidens taxifolius, Eurhynchium praelongum, and 

 Porotrichum alopectirum. Elin woods on the Boulder Clay, mostly Ulmus 

 minor, can probably be taken as plantations, such as that at Knapwell in 

 which a field system is stiU recognisable. 



II. Scrub 



In the south-west of the County, the Boulder Clay cover over Gault has 

 proved so intractable, on account of its very high clay content and deficiency 

 in phosphate, that large areas were allowed to go out of cultivation.^ Exten- 

 sive areas of hawthorn scrub of different ages, in consequence, now occupy 

 the ground, and it is possible to make out the main stages of a secondary 

 succession towards woodland. Besides the dominant Crataegus monogyna, 

 the young scrub shows frequent Rosa canina and Prunus spitwsa ; while the 

 following are either occasional or rare: Rosa arvensis, Rubus fruticosus, 

 Ligustrum vuJgare, Rosa micrantha, Rubus caesius, Acer campestre, Rhamnus 

 catharticus and Viburnum lantana. Along with these, scattered trees of oak 

 or ash are found, and Ulmus minor often extends by suckering from nearby 

 hedges. The early stages of bush growth show a remarkable flora of 

 ruderal and pasture species, strongly influenced by very heavy rabbit- 

 grazing and by the local water-logging that follows clogging of the field 

 drains. As the bush canopy closes, this ground flora becomes sparser, and 

 internal competition between the bushes grows, until there is produced a 

 dense scrub of pure Crataegus monogyna bushes 5 to 6 m. high, and well 

 spaced apart. The ground is practically bare, but there may be present a 

 very few weakly plants of Viola hirta, Mercurialis perennis, Urtica dioica, 

 Brachythecium pin-tim, Eurhynchium praelongum, Fissidens taxifolius, Hyloco- 

 nuum triquetrum and Milium undulatum. It has recently been demonstrated 

 that this scrub shows stages in the development of a new natural soil 

 profile from the old puddled clay surface, and, with further thinning of the 

 old hawthorns, further entry of trees might be expected. 



Throughout the succession, animal factors seem to be of great importance : 

 rabbits, mice, woodpigeons, and magpies are present in very great 

 density, while caterpillars wreak great havoc at times below the hawthorn 

 canopy. 



' See pp. 131 and 150 below. 



