66 Means of Dispersal 



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

 structural adaptations to flight are of less importance than reduction 

 in weight. The greater the velocity of the wind the more efficient 

 it is as an agent of seed transport. Seeds which are unaffected by 

 the wind in the Swiss plains, or are carried for short distances only, in 

 higher regions, as Vogler has shown, are blown much farther. The 

 upper limit of weight of " light seeds " capable of flight is higher in 

 the mountains where the winds are stronger than in the valleys where 

 lighter winds prevail. Vogler considers that in the case of winds 

 having a velocity of 30 metres [98 ft.] a second, a special apparatus 

 for flight is unnecessary. Winds reaching this velocity are not in- 

 frequent during storms in the Indo-Malayan region. In the Sunda 

 Strait district it is possible, therefore, that light seeds and fruits 

 which possess no special flight-apparatus may be dispersed by wind. 

 Observations in Krakatau demonstrate that for dispersal over wide 

 areas seeds with special structural adaptations (pappus-structures of 

 the Compositae, hair-like flight-organs in the Gramineae, seeds with 

 a membranous border as in the Orchidaceae) have an advantage. 

 Very light seeds specially adapted to wind-dispersal, such as those 

 of Orchids, the spores of Ferns and other cryptogams, are carried 

 by the ordinary monsoon wind, while the introduction of larger and 

 heavier seeds, some of which have no special flight-organs, is the result 

 of more violent storms and cyclones. 



4. The part played by the several agents of dispersal in the 

 development of the present flora of Krakatau. 



It is clear from the above enquiry into the significance of the 

 various means of dispersal of plant-germs in the colonisation of the 

 Krakatau islands, that the classification of the plants so far discovered 

 according to their methods of distribution is by no means easy and 

 cannot in any sense be carried out within sharply defined limits. As 

 regards the five Orchids, which most probably owe their introduction 

 to air-currents, other means of dispersal (as, for example, in crevices 

 in tree stems) cannot be entirely excluded, and this applies to other 

 presumably wind-borne plants, the seeds, or such vegetative parts 

 of which as are capable of reproducing the species, may have been 

 carried in the roots or tangle of branches of floating wood. Grasses 

 and Cyperaceae, which are often found on the beach and in swampy 

 places, may have been introduced on the feet or feathers of swimming 

 and marsh birds. In the case of the strand-plants also, as already 

 stated, in addition to the characteristic method of distribution of the 

 fruits and seeds by currents, which has in several instances been 

 demonstrated as the means of dispersal, there are possibilities of 



