V. LOWER LIMITS. 357 



Explanations relating to the Lovjer Limits. — The lower limits of plants 

 are more variable, and are less readily determined than the upper limits ; 

 those of one species comparatively with, or relatively to, those of another. 

 With decreasing height the surface to be examined becomes wider and 

 more diversified. The modifying influences of maritime proximity, of 

 shelter and exposure, of character or configuration of surface, of agricul- 

 tural and other human operations, seem all to act with increasing force 

 on the lowest limits of plants. Those limits are in consequence more 

 difficult to determine, and seem less clear or satisfactory when brought 

 into a series arranged on paper. A first attempt at such an arrangement 

 was printed in the first volume of the ' London Journal of Botany,' pages 

 241 to 253 ; but restricted to the plants of the Grampian mountains. 

 The more extended series above given is founded on the same notes and 

 observations, with many additions since made ; but the localities are 

 here reduced to one or two for each species, in order not to exceed a 

 single line. 



The names are placed in series according to the supposed temperature 

 of the lowest spots, relatively to the latitude, at which the species have 

 been noted ; the altitudes being represented by centigrade temperature, 

 in accordance with the scales given on pages 164, 165 of this volume. 

 The indication of temperature may thus be said to show the highest 

 (warmest) climate which the plant appears adapted to sustain, so far as 

 its places of growth in Britain may suffice to show this. But such 

 climatal indications can pretend only to be approximations, more or less 

 close for each plant. The lower is the name in the list, the less certain 

 is its posiliou, as a general rule. The first locality or altitude stated for 

 each species, is the one with which the stated temperature is supposed to 

 correspond; and it as at present the lowest ascertained, relatively to lati- 

 tude and temperature. For instance, Draha incana being found at the 

 sea-level in Islay, southward of latitude 56, where the sea-level tempe- 

 rature should exceed 8.50 centigrade, it is placed below Alchemilla 

 a/pina ; this latter not having been noled anywhere exactly on the shore 

 or sea-level, and probably not existing below 100 yards anywhere to the 

 southward of latitude 56. 



The succession of names in the list thus constitutes a climatal series, 

 chiefly determined by altitude, and in so far representing simple alti- 

 tude ; but this latter condition being also read in relation to latitude ; 

 and not absolutely, unless under equal latitude. As a general rule, 

 always liable to various exceptions in its local applications, plants may 



