138 PROPAGATION OF PIAKTS. 



greatly, as seen in the difference between that of New 

 York and that of the City of Madrid in Spain, or in the 

 climate of England, and of Labrador. Then, again, the 

 hygrometric conditions may have a very marked influence 

 on vegetation, as seen in the cool, dry, elevated regions 

 of our own and other countries. Plants, native of cold, 

 dry climates, often fail in moist, warm ones, while others 

 are wonderfully improved by the change, and all these 

 varying conditions and results must be duly considered 

 in their propagation by seeds or otherwise. 



When supplied with the requisite amount of heat and 

 moisture, all seeds grow more readily when near the sur- 

 face of the soil than when buried deeply, therefore we 

 should endeavor to so place them in or on the soil that 

 the air and heat can reach them, but at the same time 

 exclude the light, for darkness is favorable to germina- 

 tion. But air, or rather oxygen, is necessary, conse- 

 quently deep planting and exclusion of air retards or 

 wholly prevents growth. Whether seeds should be 

 covered or not with soil to assist or insure germination 

 depends much upon circumstances, as well as upon their 

 size, structure and power of throwing up their stems 

 through the material used for covering them. Some 

 kinds of seeds will germinate far more readily if placed 

 on the surface of the soil and then kept moist and in 

 shade or total darkness. It may be said that we only 

 bury seeds as a matter of convenience, and not because 

 it is actually necessary to insure germination. By cover- 

 ing seeds with soil or other similar material, we are 

 enabled to secure more equable conditions as regards 

 temperature and moisture, as well as the exclusion of 

 light, than if they be left uncovered, and thus we secure 

 better and more uniform results with less attention ; still, 

 roots will generally penetrate to the required depth and 

 position even when the seed has not been buried, pro- 

 vided they fall upon some yielding material or one readily 



