10 74 Proceedings of the Royal Irish Academy . 



from the parent plant, will be at once dashed to earth, while another will 

 be whirled a hundred feet up into the air ; and all will be projected forward, 

 and arrested, and flung to this side and that, in a very irregular manner. 



But whether in a steady wind or in the turmoil of a storm, the flying seed 

 is all the time subject to the influence of gravity, and, relative to the air in 

 which it is at any moment suspended, it is falling at a rate which is uniform 

 for each species, and which is easily measured, being (except in certain cases 

 already referred to, see p. 69) the same rate at which the seed would fall in still 

 air. And over and above all accidents of original position or of wind or 

 weather, this rate of fall truly measures the relative capacity of the seeds of 

 different species for dispersal by wind. All the seeds are equally open to the 

 chances and dangers offered by the wind, and these tend in the long run to 

 equalize themselves. As against them, the advantage or disadvantage of 

 original position probably counts for little, especially for long-distance 

 dispersal. But the rate of fall for each species acts steadily throughout, and 

 is thus the determining factor. If the seeds of species A fall at half the rate 

 of those of species B, the chance of the A seeds reaching any boundary that 

 we may set are approximately double those of the B seeds ; or to express it 

 differently, the chances are that double the number of A seeds will be carried 

 to any given distance, as of B seeds. 



As pointed out on previous pages, the characters which render seeds 

 capable of dispersal by wind range themselves under three main heads : — 

 (1) the possession of a plume of hairs or bristles ; (2) the possession of a 

 flattened wing ; (3) reduction in size. Some discussion of these three types 

 has been given already (pp. 67-71); it remains to put certain examples of each 

 to a practical test. Not much experimental work on the fall and carriage of 

 seeds appears to have been carried out. The best discussion of the subject 

 will be found in Dingler's 1 essay. He divides wind-borne seeds into twelve 

 types, varying from powder seed to parachute and winged seeds ; and reducing 

 these to conventional forms, he calculates their theoretical velocities of fall, 

 and compares these with the velocity of fall of actual examples of each type 

 as measured by experiment. But as most of the seeds experimented on 

 belong to foreign species, his results do not directly assist the present inquiry. 



In order to test the relative capacity for wind-dispersal of the seeds of 

 some British plants, an apparatus was devised by which the seeds were 

 allowed to fall freely through still air for a certain distance (12 feet), and the 

 time occupied measured by a stop-watch. At the top, the seeds were 

 liberated by means of a small tilting platform worked from the base of the 



1 HiiKMAXN Dinglek: Die Bewegung der pflanzlichen Flugorgane, ein Beitrag zur Physiologic 

 der passivea Bewegungen in der Pflanzenwelt. Pp. 10 + 342. Taf. i.- viii. Miinchen, 1889. 



