4] DISPERSAL AND MIGRATION IO3 



epiphytes (plants growing on other plants) as Spanish-moss {Til- 

 laridsia umeoides) which get blown to new situations on the trees on 

 which they grow, often abundantly ; seeds which fortuitously stick 

 to or get curled up in dead leaves and are transported with them 

 for considerable distances ; small seeds or fruits which adhere to 

 sticky stalks (for example of Catch-flies, Lychnis spp.) that are blow^n 

 about after detachment ; and soredia of Lichens as well as bulbils, 

 for example of such Grasses as Poa alpina, that may be usefully 

 scattered by the wind. 



(j) Jactitation. This is the slinging of seeds out of fruits such 

 as the capsules of Poppies [Papaver spp.) or Mulleins {Verbascum 

 spp.), which are held aloft on long stalks that are liable to be bent 

 before the wind or jolted by passing animals — often springing back 

 subsequently to jerk out some more of the contents in the opposite 

 direction. Such a ' censer mechanism ' is commonly feeble, barely 

 (or even not at all) removing the seed from the immediate sphere 

 of influence of the parent ; but given the good fortune of a strong 

 wind to carry the seed farther, or a favourable slope down w^hich it 

 can bounce and roll, jactitation may occasionally be quite eflFective. 



Fig. 25 shows a wide range of wind-dispersed disseminules. 



Here it seems reasonable to suggest that the primary objective 

 of a disseminule, so far as transportation is concerned, is to get 

 away from the immediate parental influence and possible competition 

 of seedlings developing from its brothers, which for many small 

 plants is effected by displacement of merely a matter of centimetres. 

 Most dispersal is probably of this relatively minor nature, the long- 

 distance ' saltatory ' dispersal (that may drastically extend the area 

 and ultimately increase the importance of a race) being supposedly 

 much rarer. 



Before proceeding to the next main topic we should give some 

 consideration to the barriers and deterrents to wind dispersal, 

 remembering that it often includes blowing about on the surface of 

 water whose currents may, moreover, carry originally airborne 

 disseminules much farther. Wind dispersal operates chiefly on free, 

 air-buoyant spores etc. in open places — so that it is not unexpected 

 to find that in treeless, high-alpine and arctic regions an unusually 

 large proportion of the native plants have wind-borne disseminules, 

 whereas in dense forests and other sheltered areas wind is little 

 effective. A great deal of wind dispersal, at least of the larger fruits 

 and seeds, is discontinuous, bodies being blown up by a gust of 

 wind and soon alighting to await another gust, the process in some 



