18 
Bokorny wrongly asserts for the Crassulaceae. On the 
other hand Czapek believes, that they can occur in the 
cell-sap and in the cytoplasm as, inter alia, may be the 
case in the leaf of Echeveria. 
In 1897 an interesting investigation by Overton !) was 
published. He experimented on Spirogyra with ammonia, 
amines, caffeine, pyridine, quinoline, piperidine, and alka- 
loids. He has no doubt at all that the precipitates which 
are found in the cell-sap are compounds of tannin with 
the above substances. He describes in detail the pheno- 
mena which are brought about by solutions of caffeine 
of different strength, namely, when successively stronger 
or weaker solutions are added. In explanation it is said 
that the compound of tannin and caffeine are in a con- 
dition of hydrolytic dissociation. 
Shortly before the appearance of Czapek’s publication 
quoted above [*) made a preliminary communication on 
the demonstration of tannin in the living plant and on 
its physiological significance. While searching for a method 
of studying the physiological significance of tannin in 
Spirogyra my attention was also drawn to antipyrine and 
caffeine, substances which had not then been used for 
that purpose. 
Like Overton I described the precipitates as tannin 
precipitates and have never for a moment thought of 
regarding them as protein precipitates. All the results 
were in agreement with the view that they were tannin 
1) E. Overton, Ueber die osmotischen Eigenschaften der Zellen in 
ihrer Bedeutung für die Toxikologie und Pharmakologie. Zeitschr. f. 
Physikal Chemie XXII. Bd. 1897, p. 189. 
2?) C. van Wisselingh, Over het aantoonen van looistof in de 
levende plant en over hare physiologische beteekenis. Verslagen der 
Koninkl. Akad. van Wetenschappen te Amsterdam, Maart 1910. On 
the tests for tannin in the living plant and on the physiological signifi- 
cance of tannin. These Proc. XII, p. 685. 
