336 Transactions. 



The growth-forms are as follows : — 



Shrubs : Creeping and rooting, 2, of Avhich 1 is prostrate loith subterranean 

 stem and 1 bushy and 60 cm. or so tall. 



Herbs : Creeping and rooting, 21, of which 5 are grasslike, 1 rushlike, 2 

 frequently aquatic with floating leaves ; rosette form, 5 ; erect, 3, of which 1 

 is grasslike ; tussock, 3, of which 1 is rushlike (slender) ; cushion form, 

 1 ; prostrate non-rooting, 1. 



At least 70 per cent, of the species are also plants of steppe or other 

 dry habitats. 



Bog at a higher altitude, or exposed to greater snowfall — i.e., to colder 

 water — on an average is more xerophytic, and contains especially a higher 

 percentage of cushion plants. 



Also, pure Sphagnum cushions in vigorous growth, thanks to the non- 

 acid rain-water absorbed by the upper surface, will allow mesophytes to 

 settle down Avhich cannot tolerate acid peat (see Cockayne, 1910, p. 111). 



(^.) ROCK FELL-PIELD SERIES. 



(1.) General. 



The altitude of the upper line of the steppe is very variable, but pro- 

 bably corresponds to a large extent with the line of the ancient valley 

 glacier. It also constitutes the lower limit of the series of plant-associa- 

 tions under consideration, which depends in large measure upon the pre- 

 sence of an easily disintegrated rock, though it is governed to no small 

 degree by altitude and climate. 



Under the influence of frost the much-jointed sandstones, greywackes, 

 and slates become rapidly disintegrated, the stone-fragments (too great in 

 quantity to be removed by rain) accumulate to such an extent that great 

 fields of debris occupy almost the entire mountain-surfaces for hundreds 

 of metres. Here and there jagged masses of much-corroded rock jut out 

 from the stone-fields and break the uniformity of the long grey even slopes. 

 Gullies with more or less precipitous walls seam the mountain-sides, a 

 stream sometimes occupying their floors, the water issuing all on a sudden 

 from the base of a great stone-field at the head of the gully, and perhaps 

 as suddenly disappearing lower down beneath the ever-increasing mass 

 of loose stones. From the above it is plain that the edaphic conditions 

 of the upper subalpine and alpine belts are those of desert, and it is plain 

 also that increase of altitude, irrigation by snow-Avater, strong insolation, 

 high winds, and occasional droughts help to increase the xerophytic character 

 of the plant-habitats during the vegetative period. 



Altitude and the average height of the winter snow-line, above which 

 the snow lies from four to six months.J separates the area into alpine and 

 subalpine belts. 



The three following plant-formations occur, arranged in order of succes- 

 sion : Rock, shingle-slip, fell-field. 



(2.) The Associations. 



{a.) Rock. 



The following are the only special rock-plants : Hymenophyllum villosum 

 (filmy fern), Folypodium pumilwn (very small fern with tufted leaves), 



X This is undoubtedly the outside limit. 



