237 
tree ; and the layers from such trees are much 
more hardy than the layers from myrtles reared 
within doors. 
The arbutus, probably originally from similar 
cultivation, has become the principal ornament of 
the lakes of the south of Ireland. It thrives even 
in bleak mountain situations ; and there can be 
little doubt but that the offspring of this tree, 
inured to a temperate climate, might be easily 
spread in Britain. 
The same principles that apply to the effects of 
heat and cold will likewise apply to the influence 
of moisture and dryness. The layers of a tree 
habituated to a moist soil will die in a dry one ; 
even though such a soil is more favourable to the 
general growth of the species. And, as was stated 
page 17<5., trees that have been raised in the centre 
of woods are sooner or later destroyed, if exposed 
in their adult state to blasts, in consequence of the 
felling of the surrounding timber. 
Trees, in all cases, in which they are exposed in 
high and open situations to the sun, the winds, 
and the rain, as I just now noticed, become low and 
robust, exhibiting curved limbs, but never straight 
and graceful trunks. Shrubs and trees, on the 
contrary, which are too much sheltered, too much 
secluded from the sun and wind, extend exceed- 
ingly in height ; but present at the same time 
slender and feeble branches ; their leaves are pale 
and sickly, and in extreme cases they do not bear 
fruit. The exclusion of light alone is sufficient to 
produce this species of disease, as would appear 
from the experiments of* Bonnet. This ingenious 
