Clare Island Survey — Phanerogamia. 1 73 



rue information on this subject. He emphasizes the fact that sea winds are 

 much more steady than those over land, instancing the diagrams bearing on 

 this question given in a recent report by W. N. Shaw on " Details of Wind 

 Structure," &C. 1 He adds that it is unlikely that the wind will become 

 absolutely steady over the open sea ; and remarks, " Of course every upward 

 motion must be compensated somewhere by a downward motion ; but I 

 suppose it is quite possible that an individual seed might by good luck 

 make use of more upward motions than downward ones, and so get 

 transported over a considerable distance." 



It is to be noted that eddies, therefore, will tend equally to shorten 

 and to lengthen flight, bringing in a possibility of extended flight, but leaving 

 the average the same. Also that the longer the flight the greater the 

 chance of upward and downward eddies equalizing each other, tending to 

 make the length of flight approximate to the theoretical distance which 

 would be attained in a horizontal air-current. 



Again, the narratives of airmen, as recorded in the press, refer to strong 

 downward eddies experienced when travelling seaward over steeply-shelving 

 shores, which suggests a high mortality among seeds drifting seaward. All 

 of these facts point to the conclusion that without a high initial elevation 

 attained by means of eddies over the land, it is not probable that flying seeds 

 will, under any kind of ordinary conditions, succeed in crossing considerable 

 stretches of water. We are again driven towards the " occasional " theory, 

 and have to call in whirlwinds and tempests to give what assistance they 

 can. 



The actual distance which a wind-borne seed of any species will travel thus 

 depends on several factors, some of them determinate, others indeterminate. 

 The conditions under which a seed is launched forth include the height above 

 ground at which it is liberated, but just as important are the exact situation and 

 surroundings of the particular plant bearing the seed. The greater the height 

 above ground at which a seed is borne, the further will it probably be carried ; 

 but particular conditions affecting local air-currents may place a low herb in 

 as favourable a position for seed-dispersal as an adjacent forest tree. 



The next factor, and one which is determinate, is the rate at which a seed 

 falls. 



The most important factor to be reckoned with, and one which is in each case 

 quite indeterminate, is the wind itself. During those stormy periods when 

 seed-dispersal .is most likely ta reach its maximum of efficiency, the motion 

 of the air — at least over rough land — is most turbulent. One seed, liberated 



1 Advisory Committee for Aeronautics. Reports and Memoranda, No. 9. 1909. 

 B.I.A. PK0C, VOL. XXXI. K 10 



