564 Cultivation of Plants. 
scarcely a difference. In practice, it is well known that the 
various plants employed for the summer decoration of the 
parterre have each and all of them clearly defined constitutions. 
One or two degrees of frost for the shortest period will kill 
some plants outright, and the same species will suecumb to a 
continued temperature many degrees above the freezing point; 
whilst other species will bear five, ten, fifteen, twenty or more 
degrees respectively without sustaining any permanent injury. 
The common Groundsel, and several other early-flowering some- 
what succulent plants, will bear as much as forty degrees of 
frost for a short time without receiving any permanent injury. 
From the existence of these natural laws, it will be apparent that 
only those plants from countries enjoying a similar climate to 
our own will flourish in the open air all the year round with- 
out protection.! Plants, it has been observed, will bear a 
lower air-temperature, under certain favourable conditions, than 
that to which they are normally exposed. These modifying con- 
ditions are, the nature of the soil and the situation, governed by 
the quantity of moisture in the soil and atmosphere. But we 
shall return to the consideration of this question when we come 
to treat of soils. It is evident from what we have said, that 
the average annual rainfall, winter and summer temperatures, 
and the extremes of heat and cold of different parts of the 
country, are, to a limited extent, a guide to the gardener 
as to what plants will succeed in his particular locality. 
Speaking generally, the further southward and westward we get 
in Britain, the higher is the mean winter temperature ; but 
there are quite local conditions, favourable or unfavouralle, 
that render calculations based entirely upon the temperature 
and rainfall of a district almost valueless. These are chiefly 
dependent upon the nature of the soil and subsoil, and the 
altitude and inclination of the ground. One great modifying 
influence on the winter temperature of the south-western coast, 
especially of the British Isles, is the warm ocean stream that 
flows from the Mexican Gulf and washes our shores. To a 
smaller extent, this holds good for the whole country. As com- 
pared with the same latitudes on the continent, our winters are 
milder, and our summers some degrees colder. And, by way 
of compensation for our cloudy skies and frequent rains, we 
' In the Introduction, a few remarks on the Geography of Plants hardy in Britain 
will be found, with indications of the principal countries whence we obtain our 
hardy exotics, 
