162 APPLICATION OF PRINCIPLES. 



sither exhaust the soil of its organisable matter, or 

 overshadow each other so as to hinder the requisite 

 quantity of light, some will die in order that the 

 remainder may live ; and this, in the case of rare 

 seeds, should, of course, be guarded against very 

 carefully. 



With regard to the temperature to which a seed 

 should be subjected, in order to secure its germina- 

 tion, this, undoubtedly, varies with different species, 

 and depends upon their peculiar habits, and the tem- 

 perature of the climate of which they are native. So 

 far as general rules can be given upon such a subject, 

 it may be stated that the temperature of the earth 

 most favourable for germination is 50° to 55° for the 

 seeds of cold countries, 60° to 65° for those of "green- 

 house plants," and 70° to 80° for those of the torrid 

 zone. No seed, however, has been known to refuse 

 to germinate in the last mentioned temperature, al- 

 though those to which such a heat is necessary will 

 not, in general, grow in a healthy manner in a lower 

 temperature. We have no exact experiments upon 

 this subject, except in a few cases recorded by Messrs 

 Edwards and Colin, by whom there is a very valu- 

 able set of observations upon the temperatures borne 

 by certain agricultural seeds {Annales des Sciences, 

 new series, vol. v. p. 5), the result of which may be 

 thus stated : — 



At 44'6°, Wheat, Barley, and Kye could germinate. 



96°, in water, for three days, four-fifths of the Wheat and Kye, 

 and all the Barley, were killed. 



104°, in sand and earth, the same seeds sustained the tempera- 

 ture for a considerable time, without inconvenience. 



