THE ECOLOGT OP PLANTS. 433 



408. Chalk Plants. Chalk soils are usually dry, and, in 

 dry continental climates, plants which grow in almost all soils 

 in England may be unable to develop on the calcareous 

 regions of the continent. Thus Birch, Hypericum pulchrum, 

 Stellaria holostea, Galium saxatile, and Broom are all absent 

 from the calcareous Jura mountains, but grow freely upon 

 the Vosges, on which the soil is formed of decomposed 

 granite, and also in marl soils in which the presence of some 

 clay renders the soil more retentive of moisture. 



In the case of chalk and limestone the same rule appears 

 to hold good as with other soils and subsoils, i.e. that the 

 distribution of plants depends much more upon the physical 

 characters of the substratum than on its chemical composi- 

 tion. In our moist climate we get all sorts of plants on chalk 

 downs and limestone hills, but still many of them show more 

 or less decidedly xerophilous characters, and some plants are 

 restricted more or less completely to chalky soils. 



On chalk downs in the south of England we get several Orchids 

 (Bee, Twayblade, Butterfly, Fly, etc.), Traveller's Joy (in hedges), 

 Pasque Flower, Stinking Hellebore, Columbine, Rock-rose (Helianthe- 

 mum, small and shrubby with opp. entire leaves, large yellow flowers, 

 K3, C5, A oo, G (3), ovary 1-chambered), Mouse-ear, Chickweed, 

 Salad Burnet, Dropwort ( Ulmaria jilipendula, Rosaceae), Dyer's Weed 

 (Reseda, a tall plant easily recognised by its greenish flowers, similai 

 in structure to those of cultd. Mignonette), Salad Burnet, Lady's 

 Fingers, Bird's-foot, Trefoil, Hoary Plantain (Plantago media, distd. 

 from other Plantains by having its broad leaves closely pressed to the 

 ground), Hound's-tongue, Thyme, Clustered Harebell (Campanula 

 glomtrata), Goafs-beard, etc. The chief trees and shrubs preferring 

 chalky soil are Ash, Beech, Yew, Juniper, Wayfaring Tree, Guelder 

 Rose, Spindle Tree, Privet, Dogwood, Spurge Laurel. 



On limestone mountains in Yorkshire, Derbyshire, etc., there 

 grow, in addition to most of those just enumerated, such plants as 

 Rock Meadow-rue, Baneberry, Globe Flower, Yellow Violet, Mealy 

 Primrose, Bloody Cranesbill (Geranium sanguineum), Dry as, etc. 



409. Clay-Plants. Pure clay does not form a soil pre- 

 ferred by any flowering plant, and is highly unsuitable to 

 most. When dry a clay soil bakes hard and is almost 

 impenetrable by the roots of plants, while the very fact that 

 when moist it is highly retentive of water renders it difficult 

 for the plant to absorb sufficient water unless the soil contains 

 a large amount. In this case the soil is very badly aerated, 



a. B. 28 



