32 Plant-life of the Oxford District 



bird-dispersed. So far, it will be noticed that the great majority of indig- 

 enous trees of high-forest are anemophilous (Oak, Beech, Alder), and bear 

 wind-dispersed seeds (Ash, Poplar, Hornbeam, /Vj) : Ulmus and Castanea 

 retain inferior insect-pollinated mechanisms, but may fall back on wind : 

 Tilia, Acer Pseudoplatanus, Aesculus, are later introductions producing bee- 

 pollinated flowers. Many forest-types characteristic of the deteriorated 

 woodland of this country are really underwood forms (Hornbeam, Acer 

 campestre, Hazel, Salix caprea, Holly, Hawthorn), and it is difficult to draw 

 any very exact line. 



As the more characteristic types of underwood may be taken : 



Crataegus monogyna (Hawthorn). Cornus sanguinea (Dogwood). 



Pyrus Aucuparia (Mountain Ash). Salix caprea (Broad-leaved Sallow). 



Euonymus europaeus (Spindle Tree). Salix cinerea (Sallow). 



Prunus spinosa (Blackthorn). Hedera Helix (Ivy). 



Pyrus Malus (Crab-apple). Clematis Vitalba. 



Ligustrum vulgare (Privet). Lonicera Periclymenum (Honeysuckle). 



Ilex Aquifolium (Holly). Viscum album (Mistletoe). 



Sambucus nigra (Elder). Rhamnus catharticus (Buckthorn). 



Viburnum Lantana (Wayfaring Tree). Rosa canina (Dog Rose). 



Viburnum Opulus (Guelder Rose). Rubus fruticosus (Brambles). 



All these have insect-pollinated flowers, and (with the exception of 

 Clematis and Salix) bird-dispersed fruits. The fact that they are con- 

 spicuous and widely known is indicated by their English appellations, as 

 familar woodland forms. As aliens of similar grade may be added Sym- 

 phoricarpus racemosus (Snowberry), Prunus Laurocerasus (Cherry Laurel), 

 Rhododendron, Buxus, and planted species of Salix, Acer, Crataegus, Pyrus, 

 Cotoneaster. Similar types, as general favourites in garden cultivation, are 

 Laburnum, Euonymus japonicus^ Viburnum Tinus, Crataegus Pyracantha, 

 Spiraea (sp.), Rhus typhina, Syringa vulgaris (Lilac), Weigelia^ Buddleia, 

 Philadelphiis, etc., as scheduled in florists' catalogues. 



Such small trees are rarely more than a foot in diameter (Acer cam- 

 pestre, Holly, Pyrus Malus), and range to an inch or less (Rosa, Rubus), in 

 which case new shoots are thrown up with a growth of 1-2 seasons only. 

 Smaller trees follow similar lines of progression to shrubs and woody under- 

 growth, including woody lianas and sub-herbaceous climbers. The empirical 

 criterion of such a shrub-type is taken as one which carries a woody stem, 

 increasing year by year with a definite cambium giving annual rings of wood. 

 Other features of somatic deterioration may be contributory causes ; half a 

 dozen maybe mentioned. The essential feature of the somatic organization 

 of a tree-type of high forest is its strong vertically growing main axis, 

 carrying a great leaf-area to incident light, in competition with its fellows. 

 This involves the co-operation of several distinct mechanisms, as (i) Domi- 

 nance of the main axis over laterals ; (2) Marked negative geotropism ; 

 (3) Active cambial increase ; (4) Effective mechanical tissues ; (5) Inter- 

 calary internodal extension ; (6) Deep-sinking root-system (positively geo- 

 tropic). Under stress of competition for light, water, and substratum, some 

 weak point will develop, and plants may fail in any or in all of these respects. 

 Those falling behind may develop compensatory equipment which may 

 enable the race to carry on in a feebler condition, or the problem may be 

 solved in some other way. 



Elegant applications of the rule are seen in the progression of under- 

 wood shrubs and herbaceous plants. With diminution in the dominance of 

 the main axis, the laterals increase in development, the effect of concentra- 

 tion on the primary stem is lost, a shrubby bush-habit supervenes, and 

 the latter may be effective in xeromorphic habit on a smaller scale ( Broom, 



