206 VEGETATION OF THE PEAK DISTRICT [CH. 
Limestone. Oats are apparently less responsive to soil factors, 
and are grown on all classes of soils. On any given soil, the 
dividing line between the wheat and the no-wheat zone repre- 
sents a limit determined by climatic conditions; and this limit 
varies on the different soils. 
The prevailing views with regard to the climatic factors 
determining the successful limits of wheat cultivation are given 
in the paper on the vegetation of the Leeds and Halifax district 
(Smith and Moss, 1903: 395—8). It is there stated, chiefly on 
the authority of Buchan (1862), that wheat requires an average 
summer temperature of at least 56° F. (13° C.) and a rainfall of 
not more than about 33 inches (84 cm.). Judging by the 
present distribution of wheat cultivation in Somerset and in 
the present district, it would appear that whilst the above 
figures are approximately correct as regards temperature, the 
rainfall figures are too low by about ten inches (25 em.); for 
wheat regularly ripens in Somerset (see Moss, 1907 a or b) and 
in the Peak District, where the mean annual rainfall is over 
40 inches (102 cm.). 
Previous British vegetation maps have indicated the culti- 
vated land by various tints of yellow, irrespective of the 
proportion of arable land to permanent pasture. On future 
maps, it is proposed to make some distinction between cultivated 
land with a high proportion of permanent pasture, as in the 
Peak District, and cultivated land with a low proportion of 
permanent pasture, as in East Anglia; and the maps which 
accompany the present volume are coloured on this plan. 
It has previously been pointed out (Smith and Moss, 1903: 
399; Moss, 1907 a: 61) that the limit of wheat cultivation 
corresponds roughly with the limits of a number of weeds and 
aquatic plants. The following is a list of weeds which have 
been noted in the arable fields of the district; but it should 
be borne in mind that, as there is but little typical wheat land 
in the Peak District, the weeds of the wheat zone are, with 
regard to the district as a whole, either very local or very 
rare :— 
