Il. ASCENDING ZONES. 17 
saw oats at 1340 feet, barley at 1000 feet, and wheat at 750 and 
800 fect. Turnips and potatoes can be grown up to the extreme 
limit of the oat, or somewhat higher. The highest point at which 
the Pteris has been noted in Yorkshire by Mr. Baker, is about 
1500 feet; and 1680 feeteis the highest point indicated for it in 
the Lake province, at page 337 of the fourth volume of the Cybele 
3ritannica. 
The <Aretic Region is conveniently subdivided into its three 
zones by the upper limits of Erica Tetralix and Calluna vulgaris ; 
the Mid-arctic zone being understood to include any spaces which 
lie between the upper limits of these two heath shrubs on the 
acelivities of the mountains. The spaces situate above the limit 
of cultivation, and below the limit of Erica Tetralix, belong to the 
Infer-arctic zone. Those above the limit of the Calluna, being 
only the upper portions of the highest hills, belong to the Super- 
arctic zone. Of course, it is only in places where the upper limits 
of these shrubs are determined by general climate, that they can 
be correctly taken as tests and indications of the zonal stages. 
On very arid ground, at any elevation, the Calluna might still be 
present, while the Evica Tetralia might be absent owing to lack of 
sufficient moisture ; and in any such case the absence of the latter 
would be uo indication that the ground in question was really a 
portion of the Mid-arctic zone. Neither would the absence of both 
always suffice to show that the surface spaces destitute of them 
were certainly within the uppermost or Super-arctic zone. In 
England, we often find green pasturages, instead of heath-bearing 
moors, quite within the natural limits of those heaths. The test 
derived from the presence or absence of either of those shrubs, 
indeed, is more obvious and more readily applied in the Highland 
provinces, than is found to be the case in those more southward, 
where the ericaceous shrubs are more frequently and completely 
destroyed by fire, in order to the production of a pasturage suitable 
for sheep. The three Arctic zones are further distinguishable by 
the presence or absence, respectively, of several other common 
plants; so that we do not depend absolutely on any one species as 
atest of a zone. Thus, in the absence of Evica Tetralix on dry 
D 
