10 72 Proceedings of the Royal Irish Academy. 



Hie Efficiency for Dispersal by Wind of tJic Seeds 

 of some British Plants. 



The mechanics of wind-transport, or, at least, of so much of them as have 

 a practical bearing on the question of seed-dispersal, are very simple. If the 

 motion of the air were uniform, then a seed once started on a wind-journey 

 would have no motion relative to the air hut the vertical motion due to 

 gravity. In all seeds efficient for wind-dispersal this motion is uniform 

 practically from the moment of liberation of the seed, the resistance of the air 

 neutralizing almost at once the acceleration due to gravity. 



Were there no wind the course of the seed would be downward from the 

 point of detachment from the plant, either in a straight line, or in some zig- 

 zag or spiral course, according to the shape of the appendages. "Were the 

 wind a uniform horizontal air-current, the course of the seed would be a 

 straight line (or zigzag, &c, as aforesaid"), sloping from the place of detach- 

 ment of the seed down to the ground, the angle of slope depending upon the 

 strength of the wind and on the rate of fall of the seed, the resulting course 

 of the seed and its actual velocity being displayed graphically by the diagonal 

 of a " parallelogram of velocities." This being so, it is clear that for any given 

 kind of seed, with a fixed rate of fall, the distance to which it will be 

 transported depends on the velocity of the wind, and on the presence or 

 absence of upward or downward eddies that may prolong or curtail its flight. 

 And the majority of seeds fall so fast in comparison with the velocity of 

 any wind which they are likely to encounter that this question of eddies 

 becomes of great importance, as it is only by their aid that any kind of long- 

 distance dispersal can be hoped for. 



While the motion of the air over rough land, especially near the ground, 

 is very irregular and full of eddies, occasioned by irregularities of the surface 

 see p. 67, ante), it does not follow that the same holds when the wind is 

 blowing across uniform surfaces such as water. On the contrary, there seems 

 every reason to believe that at sea upward or downward eddies are unimpor- 

 tant under usual conditions. Experiments on this point which I attempted 

 by liberating small parachutes at sea from the mast of the Fisheries steamer 

 " Helga " were unsuccessful, owing to the eddies caused by the sbip, which 

 drew the parachutes at once down into the water. But observation of the 

 passage of the smoke of steamers, and also of the dense smoke of kelp-burning 

 across many miles of sea, in both light and strong winds, gave but slight 

 indication of eddies. On another occasion thistle-down was watched half a 

 mile from the shore blowing seaward, and in every case its course was 

 practically a straight line. 



Mr. B. G. K. Lempfert, of the Meteorological Office, has kindly given 



