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 succumb 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. 1 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 farther southward and westward we get 

 in Britain, the higher is the mean winter temperature ; but 

 there are quite local conditions, favourable or unfavourable, 

 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 



1 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. 



