442 THE ECOLOGY OP PLANTS. 



we get chiefly Alder and Willow thickets, while in poor soils (either 

 dry and rocky or moist and peaty) the Birch flourishes. In soils not 

 too moist and poor we get Oak woods and woods of mixed deciduous 

 trees. Beech woods are chiefly found in the southern Half of England 

 on chalk ; elsewhere the tree has usually been planted by man. Ash 

 woods also occur chiefly on limestone. Hazel copses are largely found 

 in places where taller trees have been cleared away, as well as on the 

 edges of Oak and other woods. In boggy places and along streams we 

 get patches of Bog Myrtle ; on gravelly and sandy soils, especially on 

 heaths, extensive "scrubs" of such xerophilous shrubs as Gorse, 

 Broom, etc. ; and so on. 



The underwood varies greatly according to the shade cast by the 

 dominant tree or trees. In open woods there is abundant undergrowth 

 composed largely of Rosaceous plants like Brambles, Raspberry, Roses, 

 Sloe, Rowan, Hawthorn, Crab Apple, Wild Cherry, Gean, Bird Cherry, 

 also Dogwood, Alder, Buckthorn, Spindle Tree, Guelder Rose, Way- 

 faring Tree, Elder, Holly, Privet, etc., besides climbers (chiefly 

 Honeysuckle and Ivy), and sometimes introduced plants like Maples, 

 Lilac, Laburnum, Rhododendrons. The underwood becomes more and 

 more scanty the closer the trees are together or the greater the propor- 

 tion of shade-casting trees like Beech, until in a pure Beech wood it 

 disappears altogether or is represented by Honeysuckle and a few 

 Brambles, besides seedlings of the Beech itself. 



The herbaceous undergrowth of "wood floor" plants varies in 

 abundance and variety with (1) the amount of shade cast by the trees, 

 (2) the nature of the soil, especially as to moisture and depth of humus. 

 The amount of shade in itself has a great effect on the production of 

 humus, and humus increases the water-holding capacity of the soil. 



The chief floor-plants found in fairly well-lighted woods (e.g. Oak 

 woods) include various Grasses (Brormis asper, Melica, Milium, Poa, 

 Festuca, Brachypodium, Aira, Holcus mollis, etc.), Large Woodrush, 

 Bluebell, Twayblade, Arum, Garlic, Daffodil, Bistort Polygonum, 

 Wood Anemone, Goldilocks, Red Campion, Milk Vetch, Bitter Vetch, 

 Wood Pea, Tuberous Pea, Pignut, Trailing St. John's-wort, Sweet 

 Violet, Enchanter's Nightshade, Dog's Mercury, Wood Spurge, Wood 

 Sorrel, Sanicle, Angelica, Primrose, Yellow Pimpernel, Ivy-leaved 

 Harebell, Wood Woundwort, Yellow Archangel, Goldenrod, Wood 

 Hawkweed, Saw-wort, Wood Hawkweed, etc. More local in distri- 

 bution but abundant in many districts are such plants as Solomon's 

 Seal, Star of Bethlehem, Helleborine (Epipactislatifolia), White Helle- 

 borine (Cephalanthera), Military Orchids, and other Orchids. 



Of the Ferns, Bracken occurs chiefly in dry woods, often accom- 

 panied by Ling, Bell Heath, Bilberry, Tormentil, Heath Bedstraw, and 

 xerophilous Grasses (Aira, etc.) ; while moist woods have a great 

 variety of Ferns (Lady, Beech, and Oak Ferns, etc. ) often with Poly- 

 pody growing on the tree branches along with the epiphytic lichens 

 and mosses ; in deeply shaded woods the Bracken disappears. 



As the shade increases the number of floor-plants falls off, until in 

 a dense Beech wood we get, besides Beech seedlings, only a few shade- 

 enduring plants like Sanicle, Woodrush, Wood Sorrel, etc., and in the 



