ridges, which, however, were almost parallel and so close to each 

 other, that a person shouting on the one ridge was heard and under- 

 stood on the other. Penzig who was accompanied by Treub (who 

 had found the plant 1 1 years before) expressly states that Onychium 

 was not found back '), R a c i b o r s k i on the contrary declares of 

 the same trip and the same plant : Very common in the island Kra- 

 katao 2 ). Doubtless both were, in some way, right; on the one ridge 

 the fern may have grown copiously, on the other it will have been 

 absent or very scarce. Quite analogous phenomena may be obser- 

 ved everywhere in the Javanese mountains and hills and often also in 

 the plains 3 ). 



Since the vegetation of two adjacent ridges of Krakatao differed 

 at the same time so much that a species absent or very rare on 

 the one ridge was copious on the other, much caution is required in 

 comparing the results of consecutive excursions. If one is not absolutely 

 sure of having investigated the same locality under the same clima- 

 tological conditions as one's predecessor and with sufficient accura- 

 cy, one cannot possibly know whether in the time elapsed between 

 two investigations a given species has disappeared or made its first 

 appearance, whether it has become more common than formerly or 

 otherwise. The two first explorations of Krakatao exclusively took 

 place on the northwestern side, consisting for a large part of basaltic 

 rocks, partly bare; the three subsequent investigations were made not 

 only there but also on the /ou-cv eastern and southeastern slopes of the 

 island, which almost everywhere are covered by a thick layer of ashes 

 and pumice. Considering that two adjacent ridges differed already in 

 vegetation, it is extremely probable that the south and south-eastern 

 slopes, lying wide apart form the north-western side and differing 

 moreover geologically, have shown important differences in the flora. 

 But until now, authors on the flora of Krakatao have simply lumped 

 together the results obtained in different localities and at different 



x ) Ann. |ard. Bot. Buitenzorg XVIII (1902), p. 108. 



2 ) Raciborski. Die Pteridopliyten der Flora von Buitenzorg (1898), p. 164. A spe- 

 cimen collected in Krakatao by Raciborski is present in the Buitenzorg Herbarium 

 and there is another one collected in March 1905 by Vale ton in the same island. 



3 ) A remarkable instance of this fact was observed by Dr. L. den Berger and 

 the author in April 1920 south of the village Bantoor in Eastern lava. From this 

 village two roads run in a southern direction at a small distance from each other. 

 Along the eastern road Priva lappulacea Pers,, a herb introduced form tropical America, 

 was growing in innumerable specimens; along the other one, apparently presenting the 

 same conditions, we only found a single specimen after having sought a long time. In 

 some inundated ricefields of lava Xyris indica L. is so very numerous that the whole 

 field is tinged with yellow whilst in adjacent fields of apparently the same condition 

 no specimen is to be seen. The same experience is often made with Hydrolea zeylanica 

 L. and Ammannia octandra &**!>. Many more instances might be added, 



