121 
organic substance was gradually added to, rendering the soil in- 
creasingly suitable for the growth of other plants. In this -con- 
nection it is remarkable that among the pioneers there were 
certain species of orchids, a species of Spathoglottis and one of 
Arondina, plants which in Java also are frequently to be found © 
growing on dry steep banks. Now- orchids are among those 
plants that live in symbiosis with fungi in their roots, these 
fungi being indispensable not only to their ordinary life, but 
-even to their germination. It has been possible to demonstrate 
that many seads of orchids could not germinate when the 
symbiont fungus was not present. Not only therefore must the 
seeds of these orchids have got to Krakatau in great numbers, 
but also the fungi. The symbiosis is not equally intimate in 
all cases; some orchis-fungi penetrate into the soil outside the 
root of the plant. Such masses of fungi within the roots are a 
general phenomenon and Dr. von Faser has been able, as he 
told in his lecture on yesterday-morning, to demonstrate that 
a large number of tropical plants, not only forest-plants but 
beach-plants as well, live in union with fungi, 1. e. possess what 
are called mycorrhizae. Therefore an abundant microflora is a 
necessary condition for the life of the higher plants. 
In the samples of soil collected by Exnsr in 1906, Dr. pz Krvyrr 
was able to demonstrate the presence of 2.200.000 bacteria per 
gramme of earth, which is approximately equal to the propor- 
tion of bacteria in the soil of Buitenzorg. 
Several ‘Leguminosae have been found with their nitrogen- 
tubercles; these plants also help energetically in rendering the 
soil fertile. 
There were however numbers of plants that could not gro 
on Krakatau before the humidity of the surroundings had be- | 
come greater, and this only became possible when the pioneers 
of the arboreal-flora, foremost among which should be mentioned | 
species of Ficus, had begun to build up forests in the ravines. 
In these ravines watery vapours clang more easily and longer 26 
than in the flat open ground; a layer of humus was formed 
suitable to smaller forest-plants, the vege tation also becoming a 
gradually so dense that a moist shade prevailed beneath the . 
