16 THE SCOTTISH BOTANICAL REVIEW 
and the vegetation may form no index to the underlying 
limestone except by a luxuriance of deep-rooted species due 
to the effect of lime in the ground waters. Where, on the 
other hand, the climate is uniformly moist and cold, the 
limestone surface may become thickly covered with peat 
supporting a moorland flora. In either case, with the 
development of underground drainage channels, the ground 
waters will sink to a greater depth, and the soil or peat will 
come to suffer drought in comparison with that of the 
surrounding country. This will be specially marked if the 
area undergoes elevation relatively to sea level. The plant 
associations will gradually come to differ from others of the 
surrounding districts, not as a result of the chemical effects 
of lime, but from drought induced by the soluble nature of 
the rock in its physiographic relations to drainage. A 
further stage is reached when the soil or peat is gradually 
removed by the wind or through pipes and sinks into the 
underground drainage channels, and the surface is reduced 
to the barren condition geologically known as limestone 
pavement. 
The actual chemical effect of lime on plant association is 
seen where the plants grow directly on limestone rock, where 
the soil overlying such rock is very shallow owing to its slow 
accumulation or constant removal, or where lime in solution 
affects the ground or surface waters. 
(To be continued.) 
REFERENCES. 
(1) Scuimper, Dr. A. F. W.—Plant Geography upon a Physio- 
logical Basis. Eng. trans. ; 
(2) GraEBNER, P.—Die Heide Norddeutschlands. 1got. 
(3) MEMBERS OF THE CENTRAL COMMITTEE FOR THE SURVEY AND 
Stupy oF British VEGETATION.—Types of British Vegeta- 
tion. Edited by A. G. Tansley, M.A., F.L.S. 1911. 
(4) CowLes, Henry C.—‘‘ The Physiographic Ecology of Chicago 
and Vicinity: a Study of the Origin, Development, and Classifi- 
cation of Plant Societies.” Botan. Gazette, I1go1. 
(5) CLements, F. E.—‘The Development and Structure of Vegeta- 
tion.” Botan. Survey of Nebraska, 1901. 
(6) Moss, C. E.—“The Fundamental Units of Vegetation.” The 
New Phytologist, vol. ix., Jan. and Feb. 1910. 
