162 A MANUAL OF FORESTRY. 



1. Conditions of Success, 

 a. Sufficient Seeding of the Area. 



The agencies which carry the seed on to the area are 

 air currents, and in some cases water, or the seed may 

 roll by its own weight down a slope. When air cur- 

 rents are the carrying agency it is necessary that the 

 seed should be sufficiently light, and that the wind 

 should blow from the right direction when it falls. In 

 this respect the species, the force of the air current, 

 and the relative position of the regeneration area are of 

 importance. The seed of some species, as Poplar, is so 

 light that it travels for miles, while that of others falls 

 straight to the ground. Gayer gives the following 

 distances for a number of species under the influence of 

 a moderate air current : 



Birch, Elm, Larch, 4 8 times the height of the trees. 



Spruce, Scotch Pine, 



Alder. . 34 



Maple, Ash, Horn- 

 beam . . . 2 3 ,, ,, ,, 



Lime, Silver Fir, 1 2 ,, ,, 



Beech and Oak, scarcely beyond the reach of the crowns. 



In the case of strong winds the distances are .pro- 

 portionately greater ; instances can be seen in Scotland 

 where Scotch Pine seed has been carried to many times 

 the distance given above. The distances are also 

 greater, if the mother trees stand at a higher elevation 

 than the regeneration area. 



The direction of the wind during the fall of the seed 



