104 APPLICATION OP PRINCIPLES. 



from the first present the sickly appearance of plants 

 suffering from cold (110). Hence arises the impossi- 

 bility of making the seeds of tropical plants germinate 

 when sown in the open air in this country, where the 

 mean temperature of the earth seldom rises to 65°, 

 and that for only short periods of time. It is, there- 

 fore, obvious that all plants require some bottom heat ; 

 but the amount varies with their species, and the only 

 means or power of determining what the amount 

 should be, is afforded by the known degree of warmth 

 of the climate of which a plant may be a native. 



When plants are cultivated in glass houses, there is 

 little difficulty in supplying them with the amount of 

 bottom heat which they may require ; but this can 

 either not be effected at all, or only to a limited degree, 

 by a selection of soils and situations, when plants are 

 cultivated in the open air ; and hence one of the many 

 difficulties of acclimatising in a cold country the spe- 

 cies of a warmer climate. It is true that plants will 

 exist within wide limits of temperature, and, conse- 

 quently, a few degrees of difference in the natural bot- 

 tom heat to which they are exposed may not affect 

 them so far as to destroy them; but it cannot be 

 doubted that the conditions most favourable to their 

 growth are those which embrace a temperature rather 

 above than below that to which they are accustomed 

 in their native haunts. 



The Orange tree is found in perfection where the 

 temperature of the soil maybe computed to rise to 80° 

 or 85°, and never to fall below 58°, as in the Bermu- 

 das, Malta, and Canton. How injudicious, then, ia 



