( 70 ) 
The result of these experiments is therefore that, as in the case 
of phototropic curvatures, so also for geotropic curvatures the product 
of duration of stimulus and intensity of stimulus, is constant. To 
what extent the stimuli of light and of gravity are comparable in 
their action on the plant, cannot be determined by these experiments 
alone. 
Botanical Laboratory , Utrecht. 
Geology. — “The Fens of the Indian Archipelago .” By Prof. 
A. Wichmann. 
In 1778 Franz von Beroldingen propounded the theory, that lignite 
and coal measures are to be regarded as bog strata, of which the 
peat, after it became covered with other soils, was transformed into 
coal. This theory has so far triumphantly maintained itself. Nor is 
there any divergence of opinion concerning a second proposition, via. 
that during the carboniferous period a uniform tropical climate was 
maintained over the whole of the globe, as is deduced by palaeo- 
phytologists from fossil vegetable remains. In disagreement with the 
above-mentioned theory, it is asserted in most, text-books and in other 
publications, that peat formations, at least autochthonous ones, are not 
found in tropical countries. In order to escape from this contradiction 
some geologists assume that coal measures cannot be derived from 
fens, while others seek a solution of the problem by supposing that 
formation of coal, like that of peat, can only take place in a cool 
climate. 
Although the presence of fens in the tropics has been pointed out 
repeatedly, and as far as the Indian Archipelago is concerned, John 
Andersson in 1794 mentioned the peat of Sumatra, these facts have 
been denied the attention which they really deserve. On the contrary, 
it has gradually become a sort of axiom, that fens cannot form a 
part of tropical countries. • The many geographical works on the 
Indian Archipelago are for the greater part silent on the occurrence 
of fens, although they might have been better informed. 
That such formations receive only scant attention, must be attributed 
to. the fact that the vast majority of explorers, and also those who 
are called to occupy themselves with the investigation of the soil, 
possess a wholly inadequate knowledge of the subject. Moreover peat 
is of no economical importance to tropical countries, because the 
latter possess more than enough fire wood for domestic needs and 
because it is of no use for industrial purposes on account of its hig 
