Means of Dispersal 53 



nitrogen is added to the soil in the form of ammonia, and this can 

 be directly used by several of the green plants ; for others it is made 

 available as a source of nitrogen by the nitrifying action of nitrite- 

 and nitrate-bacteria. 



Another means by which the soil of Krakatau, which originally 

 contained no nitrogen, becomes richer in nitrogenous compounds is 

 afforded by the action of nitrogen-fixing bacteria. The well-known 

 free-living forms Clostridium Pasteurianum and Azotobacter chroo- 

 coccwm, which possess the power of fixing atmospheric nitrogen, 

 were not detected in the samples of soil, but by the employment of 

 Beyerinck's method De Kruyft' 1 succeeded in isolating a new aerobic 

 nitrogen-fixing bacterium, Bacterium Krdkataui, from soil taken 

 from the small forests and from the Pes-caprae zone. 



In addition to this free-living nitrogen -fixing bacterium, Bac- 

 terium radicicola has also been carried to the island by wind-agency ; 

 this species, which lives symbiotically in the roots of Leguminosae, is 

 capable of fixing a considerable amount of atmospheric nitrogen. On 

 the roots of all the Leguminous plants examined, Vigua, CanavaUa, 

 Erythrina, I found the characteristic root-tubercles in abundance. 

 As the result of their symbiosis with Bacterium radicicola, which by 

 the assimilation of atmospheric nitrogen considerably reduces the 

 intensity of the struggle for food on the substratum of pumice and 

 ash insufficiently provided with nitrogenous and phosphorus-con- 

 taining compounds, the Leguminosae have played an exceptionally 

 important role in the colonisation of the island. This family is now 

 represented by 16 species, belonging to 14 genera, partially in the 

 Pes-caprae and Barringtonia formations in the strand-zone ; in 

 the inland flora also the Leguminosae are represented by Vigna, 

 CanavaUa, Caesalpinia, and Mucuna, and in both regions they 

 exceed almost all other arborescent and shrubby plants in the 

 number of individuals. 



V. The relative importance of the different agents of plant- 

 dispersal in the colonisation of the Krakatau islands. 



Under the head of means of dispersal of fruits, seeds, and such 

 vegetative organs as are specially adapted for reproduction, the 

 same agencies were concerned in the colonisation of the Krakatau 

 group, as in the colonisation of new islands in general ; namely 

 transport by birds, by wind, and by sea-currents. 



1 E. De Kruyff. " Sur une bacterie aerobe, fixant l'azote libre de l'atmosphere : 

 Bacterium KraJcataui." Bulletin (lit Dip. de I'agriculture aux hides Neerlandaises, 

 No. iv. (Micro-Biologie n.), Buitenzorg, 1906. 



