On Fences. 
201 
which was cut clown last year to about four feet high, they are very 
little the worse for the weather." — Vol. I., p. 332. 
7. Our present fences are injurious on account of their shade. — 
The shade of fences is a point not often insisted upon, but I consider 
its importance entitles it to be reckoned as one of the chief evils of 
the present system of hedging. High screens set around a corn 
field must have a very hurtful tendency both in spring time and 
during the season of ripening. In spring the range of the sun is 
low, and consequently the shade of fences and trees is extended a 
considerable distance over the fields which they surround. Towards 
autumn, again, when his influences are most wanted, and at a 
time, too, when he declines so rapidly from his vertical course, the 
interruption of the sun's rays by high trees and hedges must be 
very injurious. The quantity of secretion in a plant is exactly in 
proportion to its exposure to the light and air, for if a plant be 
grown where there is no light, it ceases in every constitutional 
respect to be any longer a type of the species. Plants which are 
naturally poisonous are in the absence of light and air no longer 
hurtfijl; and it is equally true that a potato grown without sun- 
shine is void of flavour, and almost void of amylaceous or nutritive 
matter. Of course, no hedgerow screens can shut out all, or even 
any great proportion of the sun's rays, but that they do so to a 
certain extent is abundantly proved by the meagre returns obtained 
from land in their immediate neighbourhood. 
8. Our fences are injurious on account of the great exhaling sur- 
face they present. — The effect of so many hedgerows upon our 
climate is a consideration which should not be passed over in an 
essay of this description. From the large space of vegetable sur- 
face exposed to the heavens, humid exhalations arise which go in 
a certain measure to cause the cold and vaporous atmosphere ex- 
perienced throughout England. From its having few or no hedge- 
rows or trees intersecting its fields, France has a much less exhaling 
surface, and many have thought, and (1 doubt not) correctly, that 
its drier atmosphere is partly consequent upon that fact. It is, at 
any rate, clear that in open countries, svich as France, or in similar 
districts throughout England, those diseases in corn known by the 
names of blight, smut, rust, and mildew, are not so prevalent as in 
those places where a calm close atmosphere is produced by artifi- 
cial shelter. 1 have ascertained that during spring-time the leaf 
of the common elm sends oft' vapour at the rate of three grains 
daily ; that of a beech tree two grains ; an oak two and a half 
grains; a hawthorn, which is a very small leaf, one grain; a sprig 
of Scotch pine scarcely any thing, and a sprig of holly, yew, and 
laurel about a third of a grain each. The hawthorn, therefore, 
exhales more than any other tree of a similar-sized leaf, so that it 
is evident, where practicable, that the holly hedge should be pre- 
