i 9 4 



HINTS AND NOTES 



artificial character of walls requires emphasis 

 in the recording' of the rarer plants that are 

 found upon them. The Wallflower is a plant 

 which is seldom found anywhere but on walls 

 outside gardens, and was certainly introduced 

 in comparatively recent times. But its per- 

 sistence on certain places upon old ruins, &c., 

 is an interesting feature biologically. The part 

 played by walls in the retention of seeds or 

 spores is of importance. The direction of the 

 prevailing winds and the orientation of the 

 walls are points that should be studied in this 

 connection. 



The influence of walls, and of boulders 

 which are dispersed by glacial action up and 

 down the country north of the Thames, as 

 well as of the numerous blocks of stone used 

 to mark the boundaries of roads, is responsible 

 in many areas for the sole persistence of some 

 interesting plants. There are some doubtfully 

 native British plants, as the Oxford Ragwort, 

 which in this country are to be found as a rule 

 only upon walls. 



Conditions of Alpine Rock Habitats. Alpine 

 plants, as has been seen, are largely confined 

 to regions where there are accumulations of 

 mountain-top detritus, or where, as in the 

 case of chomophytes, corries or ravines occur; 

 they are generally found above the limits of 

 tree-growth at altitudes of 2000 ft. and up- 

 wards. 



As a rule there is a decrease in temperature 

 as one ascends, and this is the reason presum- 

 ably for the retreat of the alpines to the moun- 

 tains of this area; in the Ice Age they occurred 

 at lower levels, owing to the lowering of the 

 level of the snow-line, &c. The conditions are 

 therefore much colder, and though the rainfall 

 may be excessive, as at Satterthwaite, in the 

 Lake District, the plants suffer from physio- 

 logical drought, or in many cases transpiration 

 is easy, so that the conditions are similar to 

 those of dry soil. 



The exposure of such elevated tracts to 

 the wind also demands a rosette habit, or a 

 grass habit, whilst other plants form mats or 

 cushions, approximating to the habits of 

 lichens, mosses, &c. The viviparous condi- 

 tion, a vegetative mode of reproduction, is 

 frequent amongst such forms. 



It is extraordinary that amongst the numer- 

 ous rare specialized alpine forms growing at 

 very high altitudes many lowland plants occur, 

 and these must be regarded as descending 

 species. The upright Meadow Crowfoot is 

 found at 3980 ft., Marsh Marigold, Red Cam- 

 pion, Coltsfoot, and Hogweed at about 3500 ft., 

 Bird's-foot Trefoil at 2800 ft., and Bog Violet 

 ranges to 4000 ft. 



In more lowland areas similar dry-soil con- 



ditions are encountered by plants growing 

 upon sub-alpine hills, such as Rock Cress, 

 Alpine Thale Cress, Rue-leaved Saxifrage, 

 Orpine, Rock Rose, Wall Lettuce, &c. 



Siliceous Rocks and Soils. The type of rock 

 termed siliceous is characterized by the close- 

 ness of the texture, as opposed to the open- 

 ness and porosity of sandstones. The rock 

 is denser, in fact, and when wet is pasty. 

 Though plants do not favour bare rock sur- 

 faces, yet some are to be found upon siliceous 

 rocks where there is but little soil formed, or 

 nothing but detritus, and in the clefts of sili- 

 ceous rocks some crevice plants do grow, such 

 as the Rowan Tree, Holly, Ash, Birch, Oak, 

 and occasionally even Willow. 



Many shrubs, especially Brambles, Roses, 

 Hawthorn, Hazel, may be found growing also 

 on siliceous rocks in damp situations, as 

 around slate pits or natural exposures. Of 

 other types may be mentioned such plants in 

 the ground vegetation as Columbine, Carda- 

 mine impatient, which I have observed on 

 talus slopes of older Palaeozoic rocks in Shrop- 

 shire, Bush Vetch, Wild Strawberry, En- 

 chanter's Nightshade, Woodruff, Wall Let- 

 tuce, Wood Loosestrife, Epipactis latifolia, 

 Lusula maxima, Carex pendula, and many 

 ferns in the clefts. 



Where the conditions are drier, Heath Bed- 

 straw, Mouse-ear Hawkweed, Foxglove, Wood 

 Sage, Sheep's Sorrel, Bluebell, &c., are found. 

 Navelwort, Orpine, grow as crevice plants, 

 and so does Golden Rod, and on slopes with 

 little soil Field Mouse-ear (the latter also on 

 marlstone). 



Sandstones. 1 Siliceous soils are fine-grained, 

 whereas sands and sandstones are much 

 coarser and looser, more porous. They are 

 consequently drier than the former. Very 

 little humus is formed upon them, and the 

 soil is deficient in mineral salts as a rule. 

 Such soils are characterized by woodland and 

 heath formations. The woodland is domi- 

 nated by the Oak, both pedunculate and sessile 

 types. The occurrence of Rowan and Birch 

 is characteristic, and also of Bluebell, Bracken, 

 Yorkshire Fog, Tormentil, Foxglove, &c. 

 This is the typical succession where a sandy 

 soil is developed, but frequently such areas 

 may exhibit the rocks themselves in natural 

 exposures, and some plants may be found to 

 grow on the all but bare surfaces, or in 

 crevices, as Orpine, Golden Rod, Sheep's 

 Sorrel. Where flowering plants do not 

 colonize such bare rocks, cryptogams are 

 abundant. 



Limestones. Limestones, as a rule, form 

 bold massifs or hills, and they are not covered 

 i Cf. alo Section IV. 



