Clare Island Survey — Phanerogamia. 1 69 



series of species whose seeds bear every gradation of pappus, from an elaborate 

 parachute down to an almost microscopical ring of bristles; yet as many 

 common and widespread species will be found at one end of the series as at 

 the other (see also p. 63). Neither does any phylogenetic relationship exist 

 among the pappus-bearing species. " Constant or nearly so in each species, 

 with very few exceptions," says Bentham, 1 " the pappus will often, in a most 

 natural genus, so vary from species to species, as to make it a most difficult 

 task to decide whether it should be neglected altogether " as a diagnostic 

 character. 



As already mentioned, many of the plume-seed plants rely on structures 

 simpler than the pappus to aid them in wind-dissemination — such as the long 

 feathery awn in Clematis Vitalba and Dry as oetopetala ; but there is no need 

 to go into the forms displayed in different species, or then- morphological 

 significance. Their relative efficiency, which is the point that mainly concerns 

 us, is discussed later on. 



Winy Seeds. 



Eegarding these not much need be said. Even when a very expanded 

 wing is present, as in Pinus sylvestris, Heracleum, Sphondylium, or TJlntus 

 montana, its efficiency as an agent of dispersal cannot compare with the 

 pappus. This is shown by the results given below (p. 77). Except in 

 very high winds, winged seeds cannot be carried more than a short distance 

 from the parent (see also p. 66, supra). 



In one important respect the behaviour of these flat bodies in wind is 

 different from that of the plumed seeds. In accordance with mechanical 

 laws, they tend to turn their flat surface towards the direction of greatest 

 resistance. When falling in still air, therefore, or in air moving imiformly, 

 while the course of the seed is usually a zigzag or spiral, the flattened surface 

 tends to maintain a horizontal position all the time— which is also the position 

 which is most effective in restraining the velocity of fall. But if the wind is 

 gusty, and the seed is subjected to forward and backward jerks, then each of 

 these puffs will tend to cause the flattened seed to present its surface to 

 the direction in which it is impelled. In consequence, its edge will tend to 

 be directed towards the ground, and the velocity of fall will be increased 

 accordingly. When the motion of the air is irregular, then, the efficiency of 

 such seeds for dispersal is reduced. 



Povjder Seeds. 

 "In the dispersal of seeds and fruits by wind over long distances," 

 remarks Ernst, 2 "structural adaptations to flight are of less importance 

 1 Compositae, supra cit., p. 354. 2 A. Ernst : The New Flora of Krukatau, p. 66. 



