Atkins — Factors affecting Hydrogen Ion Concentration of Soil. 399 



turned methyl orange-red, and gave a slaty blue with brom phenol blue, 

 corresponding to pH 4-1. Further on, near Two Bridges, an excavation 

 exposed a granite subsoil. At a depth of three feet this was pinkish in 

 colour, and at pH 5-4, whereas the blackish soil above it at a depth of 4-6 in. 

 was at pH 5'2. In this region were upland pastures, and Digitalis 2norpurea 

 was plentiful. More than a mile up this valley lies Wistman's Wood, a 

 small grove of stunted oaks, gnarled to an extraordinary degree, and coA^ered 

 with lichens. The trees grow between large granite boulders, among which 

 Vacmmcm Myrtillus and Seduvi anglicum abound. The soil reaction here 

 was pH 5'1. In the approaches to the wood Pteris aquilina is dominant, and 

 the pH value is, doubtless, closely similar, but was not tested. 



The flora of sea cliffs was studied mainly at Polzeath and Pentire Head, 

 on the north coast of Cornwall. The coast-line soil is on slaty rock, except 

 at the extremity of the head, where the rock is " pillow lava." In reaction 

 it varies from pH 6-6 to 7'0 ; the agencies which prevent the development 

 of greater acidity have already been mentioned. The elevation reaches 

 300 ft. as a maximum, and in the more rocky portions are found Silene 

 viaritima, Armeria vulgaris, SjKrgularia rupestris, S. ruhra, Sedum anglicum. 

 The sloping banks, on which the grass had been withered by the drought, 

 when seen at a small angle were blue with Scilla autumnalis; bushes of 

 Ulex europaeus were found on the banks also, together with Galium verum, 

 and very small plants of Plantago Coronopus. Nearest the sea, and apparently 

 splashed by it at times, were found Crithmum maritimum, A. vidgaris, and 

 S. rupestris, the reaction of the soil in the crevice being pH 8"2. At Youghal 

 the two latter plants were similarly located at pH 7'2-7*6, but spray would 

 at once raise the alkalinity. 



In gullies, where streams ran in, the blackish-looking soil was found to 

 be mixed with sea sand, and the boggy parts were at about pH 7'6, prominent 

 plants being Oonvolvulus sepium, Mentha aquatica, and Epilohium parvifiorimi. 



Along the coast at Pentire and Polzeath Sedum anglicum abounds, but 

 S. acre could not be found. As previously mentioned, this region is at 

 pH 6.6-6'8 mostly. Inland, however, about half a mile away, S. acre was found 

 on a wall, the reaction being pH 7'8. Since ^S'. acre occurs on sand dunes 

 also, and S. anglictmi on Dartmoor, neither proximity to the sea nor eleva- 

 tion can be considered as limiting factors. One is forced to the conclusion 

 that the hydrogen ion concentration is in this case undoubtedly the factor 

 which determines which of these two species occupies a situation. 



Another rather striking instance of such a limitation is shown by 

 Centranthios ruber. This has not been observed at less than pH 7"4 on a 

 cliff, but on embankments round limestone quarries it is often the dominant, 



