F. F. Blackman. 
184 
compound of tannin and caffein, which slowly hydrolyses and 
disappears when left in water. 
Spirogyra gives this precipitate well, except when it is poor in 
tannin, and in Spirogyra it can be seen that the precipitate forms 
partly in the vacuoles and partly in the cytoplasm. 
Certain other bases such as baryta give an analogous compound, 
but soda and potash fail to work because the Na. and K. ions do 
not penetrate the protoplasm. Czapek states that Echeveria gives 
an obvious deposit with ammonia so dilute as 1 mole in 15,000 
liti 'es, so that it is three times as delicate a test for ammonia as 
the much vaunted Nessler’s test. 
In the second paper it is noted that substances which injure 
the protoplasm, as chloroform and acids of sufficient strength 
bring about the exosmosis of the contained tannin, so that the 
cells no longer give the conspicuous myelin-formation when treated 
with caffein. If all the tannin has not escaped, a fine granular 
precipitate may arise in the cells, or perhaps only a brownish 
turbidity. It is clear then that we have here a convenient reaction 
for testing the effect of various water-soluble substances upon the 
permeability of the protoplasm, as exemplified by the exosmosis of 
the tannin. This study Czapek then took up, with important 
results. He tried a great range of the stronger acids and found 
that they all produced exosmosis at the same critical concentration. 
If the acid is more dilute than 1 mole in 6,400 litres, no exosmosis 
results even after many hours, but at this particular strength 
appreciable exosmosis begins and the tannin steadily escapes, so 
that after some eighteen hours in acid, caffein no longer produces 
a precipitate, and, indeed, the tannin can be detected in the 
solution outside the cell. 
An extremely interesting coincidence lies in the fact that 
Kahlenberg and True, in 1896, found exactly this strength as the 
minimum which inhibited the growth of roots submerged in dilute 
acids of various kinds. No doubt the increased permeability of 
the cells of the root-tip inhibited their efficiency as growing units. 
Czapek then tried the effect of other organic solutes upon the 
exosmosis of tannin, using chiefly the mesophyll of Echeveria as 
standard material. Thick sections are laid for twenty-four ho«rs 
in the^solution to be tested, then washed and placed for at least 
one hour in a drop of 0-2% caffein in a damp chamber. At the end 
of this time the cells are examined with the microscope and the 
amount of precipitation noted. 
